Plants and immunity

DEVELOPMENTAL AND COMPARATIVE IMMUNOLOGY, Vol. 3, pp. 571-589, 1979. 0145-305X/79/040571-19502.00/0 Printed in the USA. Copyright (c) 1979 Pergamon Press Ltd. All rights reserved.

PLANTS AND IMMUNITY

Adrienne E. Clarke and R. Bruce Knox School of Botany, University of Melbourne, Parkville, Victoria 3052, Australia

ABSTRACT The nature and development of recognition systems in plants is reviewed and compared with the parallel systems in animals. Higher plants have not retained the phagocytic capacity characteristic of certain lower plants and of animal c e i l s . They have, however, developed the capacity to discriminate self from non-self in t h e i r mating systems, defence reactions and somatic cel] interactions. The biology of these systems is described and the available information conerning the interacting surfaces is reviewed and interpreted in terms of the i n i t i a l recognition event and the response. CONTENTS INTRODUCTION DIFFERENCES BETWEEN PLANT AND ANIMAL CELLS AND ORGANISMS Presence of a cell wall Immobility of multiceI1ular plants Other differences PLANT PHYLOGENY AND THE EVOLUTION OF THE ANIMAL IMMUNE SYSTEM Phagocytic a b i l i t y Capacity to discriminate between self and non-self

Plant defence reactions Plant graft acceptance or rejection Plant chimeras and graft hybrids Cultured ca]lus cells Plant pathogens Plant parasites and symbionts

Mating systems Mating systems in algae and fungi Pollination in cycads and gymnosperms Pollination systems in flowering plants CONCLUSIONS The i n i t i a l recognition event The response REFERENCES 571

INTRODUCTION Plants, l i k e animals, have adapted successfully to survival in an environment harbouring many potential pathologic hazards. These hazards can arise from external sources and also i n t e r n a l l y by aberrant cell divison leading to malignant growth. Animals have responded to these threats by evolving several defence systems e s p e c i a l l y the immune system, u l t i m a t e l y expressed by the capacity of the organism to discriminate between s e l f and non-self (1). This capacity implies the existence of a functional system for recognizing and responding to foreign organisms and molecules. Plants have also responded e f f e c t i v e l y , but in d i f f e r e n t ways, compatible with t h e i r adaptation to an immobile, autotrophic existence. In this article we will consider the possible parallels and differences in the development of recognition systems in animals and plants, and review the few plant experimental systems which have been explored in sufficient depth to give an insight into the molecular basis for the discrimination between self and non-self in plants. We have attempted to present an overview which may be of interest to readers primarily concerned with animal cell biology and have indicated sources for more detailed information for those interested in plant cell biology. DIFFERENCES BETWEEN PLANT AND ANIMAL CELLS AND ORGANISMS Before we can attempt any comparison of immunity in animals and similar processes in plants there is a need to point out structural differences which may have influenced the evolution of recognition systems. Presence of a Cell Wall At the cellular level a major difference between plants and animals is the presence in plant cells of a cell wall overlaying the plasma membrane. In cells of growing regions this is the primary wall and consists of a gellike matrix of charged and neutral polysaccharides given rigidity by a network of cellulose microfibrils (2,3). As the cells mature they become thickened and strengthened by deposition of other material forming secondary walls which are more resistant to potential pathogens. Epidermal cells usually secrete protective plates forming a waxy cuticle on their outer surface. The cell wall presents a potential barrier to intercellular communication on the membrane-to-membrane basis operative in animal cells. However, intercellular plasma-membrane contact is achieved between certain groups of plant cells via plasmodesmata (4) which are channels through walls of adjacent cells lined with extensions of the plasma membrane. This could allow some intercellular contact and may involve the desmotubule, which is apparently an extension of the endoplasmic reticulum, forming a tubule within some plasmodesmata. Plant cells also differ in their cytoplasmic organelles, especially in the possession of a large vacuole which contains organic acids, sugars and polyphenols (5). Immobility of Multicellular Plants Higher plants are characteristically attached to their substrates, usually by some kind of root system, or haustorium. In response to this immobility the organism has developed special means of gamete transfer and a vascular system for transport of carbohydrates formed in the leaves as a result of photosynthesis, and of water and minerals absorbed from the soil via the root, to the other plant organs. Functionally, the vascular system, through which the fluids may pass, consists of rigid xylem and phloem vessels and their associated living cells. The movement is controlled by pressure gradients and other physical forces (6). Plants have no circulatory system of cells analogous to that of either invertebrates or vertebrates. Although

Vol. 3, No. 4

PLANTS AND ~ I T Y

573

the vessels of the xylem and sieve tubes of the phloem do not contain c i r c u l a t i n g c e l l s , i t is possible that certain macromolecules can be transported w i t h i n t h i s system (6,7,8,9). In some plants, there is a f u r t h e r network of canals associated with the vascular system; these secretory canals are usually f i l l e d with mucilage (10) and may provide a f u r t h e r conduit for movement of macromolecular material around the plant (10). There is also the p o s s i b i l i t y of macromolecular movement between r e s t r i c t e d groups of c e l l s via the plasmodesmata or d i r e c t l y through ceil wails. Other Differences There are several other major differences between plant and anima] c e l l s . One of the most p e r t i n e n t to the problem of plant cel] recognition is that p]ant c e l l d i v i s i o n and growth is meristematic, that i s , from speciali:zed clusters of c e ] l s p r i m a r i l y at the shoot and root t i p s . These provide for t h e i r r e l a t i v e l y unlimited growth pattern and a b i l i t y to regenerate new organs. Also, plant c e l l s from a v a r i e t y of organs may be taken into s t e r i l e c u l t u r e , and may retain the capacity to divide and d i f f e r e n t i a t e into callus c e l l s or embryoids, and under some circumstances into whole p l a n t l e t s (11). Thus, somatic plant c e i l s retain a greater potent]a] f o r d i f f e r e n t i a t i o n than do animal c e l l s . There are other major differences related to the p]ant photosynthetic capacity. These include the chloroplasts as major cell organelles, the pigment systems and metabolic pathways. These and other differences such as hormone action are apparently not immediately relevant to short distance c e l l c e l l communciation and are not discussed here. PLANT PHYLOGENYAND EVOLUTION OF THE ANIMAL IMMUNE SYSTEM The progenitors of immunity in p r i m i t i v e animals include the phagocytic capacity and the a b i l i t y to discriminate between s e l f and non-self (12,13). These properties can be detected in the simplest s i n g l e - c e l l e d protozoa. At t h i s stage in e v o l u t i o n , there is a crossroad with plants and animals evolving along d i f f e r e n t pathways. The common ancestral ]inks are believed to be the Chrysophyta ( p h y t o f l a g e l i a t e s ) (14) which are photosynthetic p r o t i s t s capable of phagotrophy. In the development of the animal immune system, phagocytosis is considered to be the oldest of a11 the responses (12). The c e l l u l a r and humoral immune systems developed higher in the evolutionary sequence: i n i t i a l l y a c e l l u l a r response detected experimentally by the a b i l i t y to r e j e c t an a l ] o g r a f t in the Coelenterata, and l a t e r in the cyclostomes, humoral immuni t y , detected by the production of c i r c u l a t i n g antibodies. The phagocytic capacity has been :retained throughout the animal kingdom. U l t i m a t e l y , a l l these responses rely on the capacity of a p a r t i c u l a r c e i l type to perceive a s p e c i f i c molecular configuration on the surface of another c e i l or molecule and to i n i t i a t e a reaction to t h i s signal (15). For a few systems, both components in s p e c i f i c cell recognition systems have been defined. The best known is the i n t e r a c t i o n between antigen and the surface immunogIobulin of a B-lymphocyte which results in production of s p e c i f i c antibodies (16). In the other systems investigated, a common theme seems to be that of an i n t e r a c t i o n between s p e c i f i c monosaccharide sequences in complex carbohydrates of the plasma membrane, and protein or glycoprotein receptors on other c e l l s (17,18,19). These receptors are then l e c t i n - l i k e molecules, In that they bind s p e c i f i c carbohydrates, and whether they might in fact be glycosyltransferases (20) is a question that has not yet been resolved. Several clear examples of t h i s type of reaction have been defined f o r animal systems - f o r example, many viruses bind via i e c t i n s which intereact with c e l l surface carbohydrates ( 2 ] ) , bacteria such as Escherichia coli apparently adhere to c e l l surfaces via surface carbohydrates''(22); and lectins

574

PLANTS AND IMMUNITY

Vol. 3, No. 4

have be~n found at the surface of mammalian c e l l types, f o r e x a m p l e T-lymphocytes (23), liver parenchymal cells (24), myoblasts (25) and a number of other vertebrate cell types and tissues (19). In liver parenchymal cells these membrane lectins are specific for the penultimate sugar of circulating glycoproteins and are believed to function in controlling the half life of glycoproteins and possibly other cells (24). As we might expect, the best examples come from simple organisms, and perhaps the best defined is the surface lectin of the slime mould Dictyostelium discoideum which appears during aggregation (26). Whether the slime moulds should be classified as plant or animal is a matter of debate, but in several lower plant systems, a similar mechanism is clearly operative. For example, the mating interactionsof both Chlamydomonas and the yeast Hansenula depend on this type of reaction (18). There are indications that similar systems exist in higher plants, thus sugar cane.leaves have a membrane receptor for the galactoside toxin produced by the fungal pathogen Helminthosporium sacchari (27). Also, adhesion of the zoospores of the pathogen Phytophthora cinnamomi to the root surfaces of host plants is apparently mediated by carbohydrates of the root slime (28). The occurrence of lectins in high concentrations in the seeds of many plants has been a great spur to investigations of these reactions. Undoubtedly, they have been a most useful model for establishing the nature of proteincarbohydrate interactions, but although they have been implicated in a number of plant physiological roles, their function in the plant is still largely a mystery (29, 30, 31). One other group of carbohydrate containing macromolecules which occur in both higher and lower plants has been considered as a possible mediator of recognition reactions in plants. These are the arabinogalactan-proteins, a group of proteoglycans (9) which are located at the cytoplasm-wall interface of a variety of plant tissues, as well as being found in secretory ducts (lO). They are closely related chemically to the gums which are also secreted, in many cases into special canals. Thus, they are strategically located for a recognition fucntion. In addition to their location, the carbohydrate components of thse molecules have a common structural core, a 1,3-8-galactan which is branched through C(0)6 to a variety of terminal substituents. The analogy between these proteoglycans and the blood group substances has been drawn (9), where the core of the molecules is essentially constant, with variations in the terminal saccharide sequence giving the molecule antigenic identity (32). As there is some relationship between the taxonomic grouping of the plant and the terminal saccharide sequences of these proteoglycans (33), they may well be involved in expression of identity. These molecules may be secreted in a number of situations where a recognition function is implicit: in infection, at cut surfaces of grafting partners and on surfaces of receptive female stigmas of flo~eringplants (9,34). It may be that their adhesive properties are more impo~ant, and that they are involved i-n the relatively non-specific stage of cell aggregation, in a way analogous to that of the surface glycoprotein known as fibronectin or LETS protein (35), which is ubiquitous in mammalian cells. Carbohydrate is undoubtedly an important recognition marker for specific interactions and probably also for relatively non-specific adhesive interactions in both animal and plant cells, and the question of how such surface carbohydrate-protein reactions elicit specific signals is under active consideration in many laboratories. We will now consider whether there are any parallels in the plant with the animal immune system.

Vol. 3, No. 4

PLANTS AND I~/~UNITY

575

Phagocytic A b i l i t y Phagocytosis has not been observed in higher plant c e l l s although these c e l l s possess l y r i c compartments in the vacuoles (5,36). The major function of plant cell vacuoles is probably in the maintenance of high turgor pressures tnrougn the storage ot metabolic intermediates such as organic acids, amino acids and sugars. This function Hatile (36) has claimed precludes any c o n t i n u i t y between the vacuolar f l u i d and the ce|l surface. The c e l l s of certain lower p l a n t s , however, are known to have a vacuolar system continuous with the e x t r a c e l l u l a r space, for example, the myxamoebae stage of the slime moulds and the oospheres of certain 0omycetes (36). The presence of a cell wall may also present a b a r r i e r to endocytosis. Heterotrophic algae of the Dinophyceae are capable of both photosynthesis and phagocytosis; although they are covered by a c e l l u l o s i c w a l l , there is a specialized w a l l - d e f i c i e n t area of the c e l l surface which is adapted for the phagocytic function (37). Capacity to Discriminate Between Self and Non-self This capacity is manifested in a number of plant systems. We will now consider each in turn in an attempt to find any parallels that may exist with the cellular and humora] immune systems of animals.

Plant defence reactions Plant graft acceptance or rejection: Grafts between plants, like animal transplants, are uncommon in nature but they provide an experimental system in which the a b i l i t y to discriminate between self and non-self can be assessed by the reaction between the donor and recipient (38,39). Experimental data on plant grafting are largely oriented to horticultural practice, for example, in the breeding and improvement of f r u i t tree cuitivars. None of the published experiments have been oriented to the problem of whether there is any specificity or memory in the system. Indeed, design of such experiments is fraught with d i f f i c u l t i e s inherent in the systems. For example, serial grafting of woody plants is a long term project whose result may not be known for many months or even years. During this time the experimental material must undergo its normal seasonal growth and dormancy patterns and also encounter the usual hazards of plants grown in field conditions, as growth of numbers of large trees in controlled environments is not often practicable. For these reasons, the soft herbaceous plants" offer more amenable experimental systems, and i t is with these plants that some useful data are becoming available. Plant stems can be cut and placed onto stems of the same or related species or genera, and under favourabie conditiDns adhesion w i l l occur and vascular connections develop soon after. However, in grafts between more distantly related genera, vascular connections may not develop and the graft w i l l wither. The situation usually is that autografts and allografts are accepted, many xenografts w i l l also be accepted but most wi11 be rejected. This is well-illustrated in experiments on the cohesive strengths of grafts of tomato, Lycopersicon esCulentum (Fig. 1). Self grafts show complete union within 7 days (39), while xen0grafts with the thorn apple, Datura stramonium, are accepted but f a i l with Nicandra physaloides (39), both of ~ i c h are also genera of the family Solanaceae. Responses in these cases were completely reciprocal. Polarity was also evident, and inverted stem grafts failed to produce successful unions even in self grafts. When self grafts were broken between I and 3 days after i n i t i a l placement, the new graft achieved cohesive strength more rapidly than the i n i t i a l grafts. Graft union is dependent on the development of cohesion between the two pieces of stem. The initial reactions at both cut surfaces form part of the wound response, which is evoked immediately after cutting. This results in the activation of many metabolic processes in the cells adjacent to the cut

576

PLANTS AND IMLMi~ITY

Vol. 3, No. 4

surface, including increased RNA and protein synthesis, increased uptake of phosphate, o x i d a t i v e phosphorylation and synthesis of phospholipids and "wound r e s p i r a t i o n " (40) as well as the deposition of the cell wall polysaccharide callose ( 4 ] ) . These events lead to cell d i v i s i o n , and a layer of callus c e l l s d i f f e r e n t i a t e at each cut surface, providing the f i r s t cel. l - c e l l contact between the wounded surfaces. The events of g r a f t union in the tomato have been delineated (38,39): (1) adhesion mediated by a surface exudate secreted amongst the wounded c e l l s , (2) d i f f e r e n t i a t i o n of callus c e l l s by d i v i s i o n of c e l l s adjacent to the cut surface, (3) recognition between compatible g r a f t i n g partners, presumably by i n t e r a c t i o n of callus c e l l s , (4) g r a f t union achieved by fusion of vascular systems. Rejection apparently occurs by a f a i l u r e at step 3 (Fig. 2). Another most i n t e r e s t i n g finding is that the response of seedling plants to g r a f t i n g may d i f f e r from adult m a t e r i a l . Grafts of peach stems from 2-year old trees on wild cherry plum, cv. Mirobolan stocks are unsuccessful (42). However, i f g r a f t s are i n i t i a t e d at the seedling cotyledon stage, successful union may be achieved. This suggests that the c e l l u l a r recognition systems which t r i g g e r g r a f t r e j e c t i o n may not be expressed in embryonic tissues (43), a s i t u a t i o n which is perhaps analogous to the acceptance of a l l o g r a f t s by mammalian foetal tissues (1). Plant chimeras and g r a f t hybrids: Another system which demonstrates the mutual tolerance of g e n e t i c a l l y closely related c e l l s is that of chimeras and g r a f t hybrids. They are found in many h o r t i c u l t u r a l n o v e l t i e s which e x h i b i t striped f o l i a g e , flowers or f r u i t , and arise as a r e s u l t of somatic mutations for pigment production in the p l a s t i d s of the outer dermal layers of the organs. They can also be produced by g r a f t i n g (44); in t h i s case, copious out-growths of callus c e l l s from both g r a f t i n g partners are formed at the s i t e of g r a f t union, and in some cases new shoots incorporating c e l l s from both parents are organized from the mass of c a l l u s . These give rise to a new plant which is a mosaic of somatic c e l l s of the two parents. The somatic c e l l s appear to be p e r f e c t l y integrated in the tissues of the chimera. The case of allophenic mice provides an apparently p a r a l l e l s i t u a t i o n (91). When the partners belong to d i f f e r e n t c u l t i v a r s or species, g r a f t hybrids are produced. Two i n t e r e s t i n g cases have been reported in the breeding of the tomato, Lycopersicon esculentum. Seedling shoots of cv. Golden Trophy were successfully grafted onto mature stems of cv. K a r t o f e l i s n i , and the Golden Trophy flowers s e l f - p o l l i n a t e d . A r i s i n g from these Golden Trophy seeds were plants with leaf and f r u i t characters t y p i c a l of the g r a f t rootstock, cv. K a r t o f e l i s n i (45). These were considered to be g r a f t hybrids since they could be obtained only by s e l f - p o l l i n a t i n g grafted but not non-grafted plants. I t is possible that the stems of Golden Trophy that produced the flowers were mosaics that may have arisen from the mixed callus c e l l s at the g r a f t union, and so could contain gametes of both types. Such an hypothesis has yet to be tested. Secondly, i n t e r s p e c i f i c hybrids between L. esculentum and L. peruvianum were successfully produced from g r a f t hybrids, when d i r e c t c r o s s - p o l l i n a t i o n f a i l e d (46). Successful g r a f t i n g between stems of the two species was achieved at both the seedling cotyledon stage and in mature vegetative plants. Hybrid seeds were obtained only from stems grafted at the seedling cotyledon stage, and not from those grafted with adult m a t e r i a l . Several explanations of these results can be given: a l l assume some a l t e r a t i o n in the c h a r a c t e r i s t i c s of the grafted stem when i t develops on the foreign rootstock. The differences between embryonic and adult plant tissues in graft responses are supported by the experiments with peach and wild cherry plum (42). Taken together with evidence that the antigenic cellular determinants of seedlings differ from those of adult plants, there is the possibility

Vol. 3, No. 4

PLANTS AND ~ I T Y

577

that experiments using t r a n s p l a n t s at d i f f e r e n t stages in the l i f e of the p l a n t may give some i n s i g h t i n t o the mechanism c o n t r o l l i n g the somatic i n t e r actions o f p l a n t c e l l s . Cultured c a l l u s cells= The c a l l u s c e l l s which a r i s e at wounded surfaces are part of the g r a f t i n g program and can a l s o be produced by explants of various p l a n t organs taken i n t o s t e r i l e c u l t u r e ( l l ) . They w i l l grow as clumps of c e l l s on s o l i d media and may be t r a n s f e r r e d i n t o l i q u i d media to give a suspension of small aggregates of c e i l s , i f the c u l t u r e s are continuously shaken. Like animal c e l l s in c u l t u r e they shed a n t i g e n i c determinants i n t o the c u l t u r e f l u i d (47). Also, they r e t a i n some of the a n t i g e n i c c h a r a c t e r i s t i c s o f the parental t i s s u e , but lose some a n t i g e n i c determinants on successive s u b c u l t u r e s . The i n t e r a c t i o n of c a l l u s c e l l s appears to be the determining p o i n t of acceptance or r e j e c t i o n in stem g r a f t s . The capacity of these c e l l s in c u l t u r e f o r r e c o g n i t i o n has been examined. Blocks of s o l i d c a l l u s o f the same parental o r i g i n grow together c o n f l u e n t l y forming a s i n g l e s o l i d mass, w h i l e blocks of c a l l u s from l e s s - c l o s e l y r e l a t e d genera f a i l to fuse, and some cases produced an a c t i v e r e j e c t i o n response manifested by a n e c r o t i c region at the surfaces o f adjacent c e l l types (48,49). These experiments i n d i c a t e t h a t c a l l u s c e l l s , having a c e l l u l o s i c c e l l wall but being devoid of a c u t i c l e and being derived from somatic c e l l s of a d i v e r s e range of organs, have the capacity to d i s c r i m i n a t e between s e l f and n o n - s e l f . Callus c e l l s derived from tobacco (50), somatic c e l l s of corn roots (51) and Gladiolus leaves, corms and petals (52), a l s o d i s p l a y both species- and organ s p e c i f i c antigens as well as determinants common to a range of p l a n t organs. The r e l a t i o n s h i p s between parental antigens from leaves, stems, p i s t i l s and anthers and t h e i r derived c a l l u s c e l l s and p r o t o p l a s t s have been examined using s i m i l a r methods iq the sweet c h e r r y , Prunus avium (47). In the cherry l e a f , f o r example, a s p e c i f i c antigen also occurred in leaf c a l l u s , while other antigens were held in common w i t h other organs. In a d d i t i o n , the c a l l u s c e i l s possessed t h e i r own s p e c i f i c antigens t h a t were not expressed in the parental organs. Evidence t h a t the antigens were located at the plasma membrane and in the periphery of the cytoplasm was obtained by immunofluorescence. Further evidence f o r a plasma membrane l o c a t i o n o f the antigens was obtained using a p r o t o p l a s t r o s e t t i n g technique (53). These r e s u l t s indicate t h a t there are s t r i k i n g p a r a l l e l s between animal and p l a n t c e l l s in t h e i r possession of a n t i g e n i c determinants located at the plasma membrane. But whether such determinants a r e involved in any r e c o g n i t i o n reactions is one of the c h a l l e n g i n g questions of p l a n t c e l l biology today. Plant pathogens: Of the great number of p l a n t diseases caused by v i r a l , b a c t e r i a l , fungal and animal pathogens, few can y e t be described in molecular terms. Nevertheless some progress in c e r t a i n systems is being made by " a few brave and p e r s i s t e n t i n d i v i d u a l s " (54). Plant species, l i k e animals, show great v a r i a b i l i t y in t h e i r s u s c e p t i b i l i t y to disease; in both systems, p a t h o g e n i c i t y is t h e e x c e p t i o n r a t h e r than the rule (66) and t h i s implies the existence of r e c o g n i t i o n and defence mechanisms (92). As f o r animals, there seem to be a number of general as well as some h i g h l y s p e c i f i c defence mechanisms. The f i r s t b a r r i e r to i n f e c t i o n of a p l a n t i s , as f o r an animal, the o u t e r integument. For p l a n t s , the epidermal c e l l s of a e r i a l surfaces are covered w i t h a c u t i c l e , and woody stems may be covered w i t h bark. However, j u s t as the skin of animals may not be an impenetrable b a r r i e r to i n f e c t i o n , so p l a n t surfaces can a l s o be breached by pathogens. In leaves, f o r example, t h e stomates are openings in the epidermis e s s e n t i a l f o r gas exchange but through which c e r t a i n pathogens f i n d a ready e n t r y p o i n t . Many plants have a generalized defence mechanism which depends on the presence of microbial

578

PLANTS AND IFhCUNITY

Vol. 3, No. 4

i n h i b i t o r s such as p h e n o l i c s , t e r p e n o i d s and a l k a l o i d s , in the t i s s u e s ( 5 6 , 5 7 ) . Some p l a n t t i s s u e s a l s o c o n t a i n m i c r o b i a l t o x i n s in an i n a c t i v e form, p a r t i c u l a r l y as g l y c o s i d e s . M i c r o b i a l i n v a s i o n causes enzymic h y d r o l y s i s o f the t o x i n to an a c t i v e form which then a r r e s t s the i n v a s i o n . The p l a n t may also respond to c e r t a i n fungal i n f e c t i o n s by producing enzymes, f o r example, 1,3B - glucanase and c h l t i n a s e , which may lyse the hyphal c e l l w a l l s (93). A defence mechanism o f p a r t i c u l a r i n t e r e s t in p l a n t s is t h e i r a b i l i t y to s y n t h e s i z e f u n g i t o x i c substances, the p h y t o a l e x i n s , in response to i n t e r a c t i o n w i t h c e r t a i n pathogens. These p h y t o a l e x l n s are g e n e r a l l y f l a v o n o i d s or i s o p r e n o i d s , and although the s t r u c t u r e s o f many of these a n t i f u n g a l agents have been e s t a b l i s h e d , t h e i r r o l e as n a t u r a l a n t i b i o t i c s in the mechanism o f r e s i s t a n c e is not p r e c i s e l y d e f i n e d (58). However, the system is amenable to a molecular l o g i c : the response o f the p l a n t to i n f e c t i o n implies a mutual r e c o g n i t i o n o f p l a n t and pathogen which t r i g g e r s p h y t o a l e x i n synthesis. I t seems reasonable to suppose t h a t such r e c o g n i t i o n r e a c t i o n s would occur between the p l a n t and funga] c e l l s u r f a c e s , and t h a t t h i s c o n t a c t may i n i t i a t e the i n t e r a c t i o n . To t e s t t h i s s u p p o s i t i o n , fungal c e l l wall p r e p a r a t i o n s and the s e c r e t i o n s o f fungal c e l l s in c u l t u r e have been f r a c t i o n ated to e s t a b l i s h the components a c t i v e in e l i c i t i n g the p h y t o a l e x i n response, the e l i c i t o r s . There appear to be two types o f e l i c i t o r s : p r o t e i n s or g l y c o p r o t e i n s , and p o l y s a c c h a r i d e s . A polypeptide elicitor o f the p h y t o a l e x i n p h a s e o l l i n produced by the red kidney bean, Phaseolus v u ] ~ a r i s , was i s o l a t e d from the fungal pathogen M o n i l i n i a f r u c t i c o l a in 1969 (59). Since then s e v e r a l other elicitors h~ve been i s o l a t e d and d e s c r i b e d . A glycoprotein elicitor has been d e t e c t e d in Rhizopus s t o l o n i f e r a (60) which t r i g g e r s p h y t o a l e x i n f o r m a t i o n in c a s t o r bean. G]ucan e l i c i t o r s have been shown to be produced by Phytophthora megaspermae var. sojae which are e f f e c t i v e e l i c i t o r s in soybean (61); f o r C o l l e t o t r i c h u m lindemuthianum and red kidney bean (62); Phytophthora i n f e s t a n s and the p o t a t o (63); Phytophthora cinnamomi, the cinnamon fungus, and e u c a l y p t systems (64). In each case, the e l i c i t o r was a branched 1 , 3 - g l u c a n , a s t r u c t u r e which is present in many fungal cell walls as part of the structural polysaccharide. The simplest structure effective experimentally was an oligosaccharide containing 3-1inked, 3,6-1inked and terminal glucosyl linkages in a ratio of I:1:1, MW~I0,000 (61). The precise structure of the e l i c i t o r has not yet been established (Fig. 3). The effectiveness of this small, branched carbohydrate in specifically e l i c i t i n g its response is remarkable; not only do minor modifications of the structure destroy its capacity to e l i c i t the response, but i t is also structurally related to components of the yeast cell wall which e l i c i t a range of responses in animal cells, including stimulation of the reticuloendothelial system i'n mammals, and defence reactions in invertebrates (69). The significance of these effects is not apparent, but these 1,3-glucans and related oligoglucosides do have a remarkable range of specific biologic a c t i v i t i e s for such apparently simple oligosaccharides. Such responses do imply the existence of receptors for the molecule on the surfaces of both plant and animal cells. For animal ceils, there is presumably a membranebound receptor; for the plant cell, i t is presumably located at the surface, associated with the cell wall, or with the underlying plasma membrane. The precise nature of the receptor is not known. The e l i c i t o r s do have interesting practical possibilities as environmentally-acceptable fungicides (61). Thus plants treated with e l i c i t o r s would build up phytoalexin levels prior to the seasonal appearance of the fungus. When a plant cell encounters a potential pathogen which i t recognises and to which i t responds by phytoalexin production, i t may undergo a general "hypersensitive" response (65). Not only are phytoalexins produced but other metabolic changes occur in the region of infection. These may be reminiscent

of the wound response, resulting in the death of the tissue, its walling off

Vol. 3, No. 4.

PLANTS AND I ~ U N I T Y

579

and f i n a l i y removal o f the whole i n f e c t e d area. For i n s t a n c e , w a l l i n g o f f r e a c t i o d s have been described in the course o f b a c t e r i a l i n f e c t i o n s of p l a n t s such as tobacco (66,67) and red kidney bean (68). In the l e a f of the red kidney bean, attachement o f the bacterium Pseudomonas p u t r i d a is f o l l o w e d by i t s i m m o b i l i z a t i o n and e n c a p s u l a t i o n by f i b r i l l a r m a t e r i a l o r i g i n a t i n g from the p l a n t c e l l wall (68). The attachment and e n c a p s u l a t i o n processes were both in some way a p p a r e n t l y mediated by the bean l e c t i n s . This kind o f m o d i f i c a t i o n o f e x t r a c e l l u l a r m a t e r i a l to p h y s i c a l l y i s o l a t e p o t e n t i a l pathogens from u n i n f e c t e d t i s s u e seems to be a defence mechanism which has been conserved in both p l a n t and animal systems. For animals, a m o d i f i c a t i o n of the c o n n e c t i v e t i s s u e , p a r t i c u l a r l y c o l l a g e n , is i n v o l v e d ; f o r p l a n t s , i t is the e x t r a c e l l u l a r wall m a t e r i a l t h a t is a p p a r e n t l y m o d i f i e d to g i v e the fibrillar m a t e r i a l o f the capsule. One mechanism for "walling off" is of particular interest: this is the deposition of callose at sites of wounding and fungal infection in plants. It is also produced in pollen grains and pistils following incompatible matings and is laid down and removed seasonally in the phloem as part of the induction and breaking of dormancy (69). It has been identified mostly on the basis of its specific staining with aniline blue, but in several situations it has been shown chemically to consist predominantly of a 1,3 B-glucan. The relationship, if any, between the mechanism of deposition of this material and of that which allows recognition and response to chemically similar sequences in the glucan elicitors is not known. The evolution of relationships that ensure survival of both plant and pathogen has resulted in development of precise relationships between host resistance and pathogen specificity. These are genetically controlled, so that host resistance to a pathogen race depends on a particular gene, the gene product being able to inhibit infection by that pathogen race (65). Other pathogen races are not inhibited by that particular gene product, so that the host is susceptible to them. In these cases the host does not react to the presence of the pathogen which grows through the tissues without biochemcial obstruction to exert its pathogenic effects, such as occlusion of the vascular system, etc. Thus there is said to be a gene-for-gene relationship between host variety and pathogen race. At this stage, the gene products involved in this type of defence and their mode of mutual recognition are not known• The picture that emerges is that plants have evolved mechanisms to defend themselves against pathogens, some general in nature and specificity, and others highly specific. There is some specificity in the phytoalexin response, but not of the order which is expected from the host-pathogen genefor-gene relationship. At the present state of knowledge, the closest parallel to the antigens recognized in the animal immune system would be the elicitors, but these apparently lack the specificity of antigens. The question of whether there are any cells in the plant equivalent in function to lymphocytes in being specifically adapted for receipt of information from antigens is easily answered: there are no known circulating cells in plants - but whether circulating macromolecules could act as transducers of information or whether the epidermal or hypodermal cells whose walls contact the pathogens have specialized recognition facilities is not known. A bacterial infection of plants of particular interest is crown gall disease induced by Agrobacterium tumefaciens. This bacterium induces tumors in the plant host by transmission of plasmids which are maintained and transcribed by the host. In this case, as with the fungal pathogens, innoculation of susceptible plants with a related non pathogen confers some immunity on the host (89). The two most characteristic features of the animal immune response are its specificity and memory. We have considered the question of specificity in the plant defence system and now we can consider whether or not there is

580

PLANTS AND II~N/NITY

Vol. 3, No. 4

any "memory" in the system - or can a plant be "immunized"? The answers, as we might expect, are not clear. It is well established that many plants can be protected from pathogenic invasion by prior infection either with a pathogen or a non-pathogen. For example, infection of the first true leaf of cucumber plants with the fungal pathogen Colletotrichum lagenarium systemically protects the plant against further infection with either the same fungus or the bacterial pathogen. Pseudomonas lachrymans; the effect is reciprocal, that is inoculation with the bacterial pathogen gave protection towards infection by both the fungal and bacterial pathogens. The molecular basis of this induced resistance has not been established, but apparently living cells were the agents of resistance, as neither autoclaved cells nor culture filtrates were effective in providing protection (94). Induction of the phytoalexin response by infection with a non-virulent strain of pathogen also will provide protection against infection by a virulent strain (61); similarly, application of elicitors will evoke the phytoalexin response in treated plants, and provide the equivalent of broad spectrum antibiotic cover in animals, but whether this is really memory, or whether there is a heightened response with the same pathogen, remain to be determined. Certain empirical horticultural practices are believed to increase resistance to disease, for example, wounding the trunks of fruit trees to induce gum formation is said to increase the tree's resistance to disease in the following growing season, but again is memory involved in such a practice? The answers to a lot of these questions will undoubtedly emerge in the next few years as a result of the molecular approaches to plant disease which are now being actively pursued. Plant parasites and symbionts: Parasitism between flowering plants is confined to a fe-w--families (70). Semi-parasites, which are not wholly dependent on the host for nutrition, attack the aerial parts of plants and occur in the families Loranthaceae (mistletoes), Santalaceae and Scrophulariaceae. Holoparasites, which depend completely on the host for both photosynthate and nutrition are found in the Orobanchaceae (broom rape, root parasites) and Convolvulaceae (dodder and other stem parasites). Host specificity of these parasites varies widely. In Viscum, the European mistletoe, a few species have only one known host, while others occur on a wide range of hosts (70). The genetic specificity of the relationship is demonstrated by the presence in the host of genes for resistance to the parasite, for example, in sunflower strains resistant to broom rape, 0robanche (71). Despite the diversity of angiosperm families containing parasitic genera, there is a remarkable uniformity in the parasitic organs, the haustoria, which arise within an adhesive disc, the endophyte, and penetrate the host after attachment of the endophyte to the host surface. Parasitism is initiated at the seedling stage, and results in most cases in some degree of vascular connection between host and parasite (Fig. 4). Continuity between xylem elements has been demonstrated in many mature haustorial systems ensuring transfer of water, minerals and root-derived growth substances to the parasite. Direct connections between the phloem sieve tubes of host and parasite have not been demonstrated (72). In all cases studied, an interface of parenchyma cells, perhaps analagous to callus cells, link the two phloem systems, for example, in Orobanche ramosa (73), providing for the transfer of phloem-transported sugars and other host substances. The endophyte which makes contact with the host and adheres to its surface displays a remarkable diversity of ultrastructural form. The mistletoe, Arceuthobium, has contact cell walls which fuse completely with those of the host (74). In another mistletoe, Phthirusa, they consist of a layer of octopusshaped cells with the nucleus in the head and radiating arms whose tips burst, releasing their adhesive contents on the host cell surface. Analogous fingerlike processes have also been reported at the endophyte interface in Orobanche

Vol. 3, No. 4

PLANTS AI~ I I ~ I T Y

581

and Cuscuta. In Orobanche ramosa, c e l l w a l l s are absent on many endophyte contact c e l l s which appear amoeboid, making d i r e c t plasma membrane contact possible (75), as the c e l l w a l l s have been degraded. In the dodder, Cuscuta odorata, Dorr (76) has demonstrated the remarkable f i n g e r - l i k e '~hyphae" of the endophyte which grow In and among the host c e l l s , and even clasp the host phloem sieve tubes. U l t r a s t r u c t u r a l studies i n d i c a t e t h a t contact between host and symbiont f r e q u e n t l y involves fusion of c e l l w a l l s or even plasma membrane contact a f t e r wall degradation. C e r t a i n l y , adhesion and r e c o g n i t i o n are implicated in the interactions. Of especial i n t e r e s t is the presence at the host/symbiont i n t e r face of c a l l u s - l i k e c e l l s , perhaps d i f f e r e n t i a t e d i n i t i a l l y as a r e s u l t of a wound response as in stem g r a f t s , which may once again be implicated in c e l l cell recognition. As p o t e n t i a l ~ystems f o r studying c e l l recognition, the h o s t - p a r a s i t e systems are e s s e n t i a l l y unexplored models. A s t a r t has been made by T s i v i o n (77) who has demonstrated the role o f the hormones, c y t o k i n i n s , in inducing h a u s t o r i a ] growth and host p e n e t r a t i o n in the twining stems of Cuscuta campestris. Another system which is a t t r a c t i n g considerable a t t e n t i o n is the h i g h l y s p e c i f i c i n t e r a c t i o n between the n i t r o g e n - f i x i n g Gram-negative b a c t e r i a Rhizobium and t h e i r legume hosts. An i n t e r a c t i o n between the ] e c t i n of the legume and the b a c t e r i a l surface polysaccharides has been considered as a basis f o r the i n t e r a c t i o n , but whether an i n t e r a c t i o n a c t u a l l y mediates the s p e c i f i c r e c o g n i t i o n is s t i l l under debate (17,88).

Mating system8 In lower p l a n t s , the mating systems are s p e c i a l l y amenable to experiment and provide model systems f o r i n v e s t i g a t i o n of p l a n t r e c o g n i t i o n mechanisms (78,79). The production o f gametes can often be induced in v i t r o , and the gametes may be s i n g l e c e l l s which lack the t h i c k c e l l w a i l s c h a r a c t e r i s t i c of somatic c e l l s , so that r e c o g n i t i o n may be mediated via membrane-membrane contact w i t h o u t i n t e r f e r e n c e o f the c e l l w a l l . Furthermore, a p o s i t i v e r e c o g n i t i o n can o f t e n be simply assayed by the a g g l u t i n a t i o n and subsequent fusion of compatible mating p a r t n e r s . The mating systems o f f l o w e r i n g plants are more complex experlmenta] models since the fusion of compatible gametes occurs w i t h i n the embryo sac deep w i t h i n the p i s t i l . I n i t i a l contact is made by pollen g r a i n s , 2- or 3c e l l e d s t r u c t u r e s c a r r y i n g the male gametes (80), and which germinate on the receptive surface of the p i s t i l , the stigma, to produce a pollen tube. This t r a n s p o r t s the male gametes to the egg c e l l which is held w i t h i n the embryo sac, thus the actual surfaces of the egg and sperm are r e l a t i v e l y inaccessible to experimental manipulation. In f l o w e r i n g p l a n t s , male-female r e c o g n i t i o n is t h e r e f o r e c u r r e n t l y being explored in molecular terms at the f i r s t stage: contact between the pollen g r a i n and the stigma surface. Both c e l l s have extremely complex w a l l s o v e r l a y i n g the membrane, and i t is also not always possible to e s t a b l i s h l a b o r a t o r y assays f o r successful p o l l i n a t i o n , f o r example by measuring seed s e t , which is both time consuming and subject to many environmental v a r i a b l e s . We w i l l now consider the progress that has been made in understanding • the molecular basis o f p l a n t c e l l r e c o g n i t i o n in the various mating systems. Mating systems in algae and f u n g i : For the u n i c e l l u l a r green alga, Chlamydomonas, the sexual process involves fusion of b i f l a g e l l a t e gametes. The f i r s t step in the a g g l u t i n a t i o n is contact between the f l a g e l l a t i p s of mating p a r t n e r s , and c e l l u l a r r e c o g n i t i o n is s p e c i f i c a l l y between d i f f e r e n t mating types of the same species (Fig. 5). Wiese has shown that f l a g e l l a t i p s •from one mating type w i l l a g g l u t i n a t e c e l l s of the opposite type and p r e l i m i n ary evidence suggests that the r e c o g n i t i o n is mediated by i n t e r a c t i o n between a g l y c o p r o t e i n on the male gamete and a p r o t e i n receptor on the female gamete

582

PLANTS AND IMMUNITY

Vol. 3, No. 4

(8~). The yeast mating i n t e r a c t i o n is a p p a r e n t l y mediated in a s i m i l a r way. Crandall and Brock (92) have shown t h a t s u r f a c e g l y c o p r o t e i n s from one mating type w i l l a g g l u t i n a t e the o p p o s i t e type. B a l l o u and coworkers (79) have c h a r a c t e r i z e d the f a c t o r s r e s p o n s i b l e f o r a g g l u t i n a t i o n o f mating types 21 and 5 o f Hansenula w i n g e i . The type 21 f a c t o r is e s s e n t i a l l y a p r o t e i n which has lectin-like a c t i v i t y and binds a complementary r e c e p t o r on the type 5 f a c t o r , which is a mannose-containing g l y c o p r o t e i n . P o l l i n a t i o n in cycads and gymnosperms: Hating i n t e r a c t i o n s in many algae i n v o l v e m o t i l e sperm which bind to the female gamete w i t h i n an aqueous e n v i r o n ment. These e s s e n t i a l f e a t u r e s are r e t a i n e d in the more p r i m i t i v e h i g h e r p l a n t s , the cycads, such as the screw p i n e , and gymnosperms, such as Ginkgo b i l o b a , the maidenhair t r e e . P o l l i n a t i o n in these p l a n t s occurs between separate male and female t r e e s ; a i r b o r n e p o l l e n is r e c e i v e d in a chamber above the female gamete where i t germinates producing a t u b e - l i k e anchor; the m o t i l e sperm are released i n t o a pool o f f l u i d which bathes the female gamete, where r e c o g n i t i o n between sperm and egg occurs. Pettitt (83) has shown t h a t t h i s f l u i d contains g l y c o p r o t e i n s that w i l l bind to the l e c t i n concanavalin A, nons p e c i f i c e s t e r a s e a c t i v i t y and a h a e m a g g l u t i n i n . There is l i t t l e evidence f o r specificity in the e a r l y events o f p o l l i n a t i o n , d i s c r i m i n a t i o n being c o n f i n e d e s s e n t i a l l y to sperm/egg i n t e r a c t i o n s (Fig. 5). In the h i g h e r gymnosperms, such as Pinus, the sperm are n o n - m o t i l e , and, as in the f l o w e r i n g p l a n t s , are t r a n s p o r t e d to the ovule by means of a p o l l e n tube. P o l l e n is r e c e i v e d at the r e c e p t i v e s u r f a c e o f the female cone, and germinates very s l o w l y , f o r example in A u s t r i a n p i n e , Pinus n i g r a , the p o l l e n tubes may take twelve months to reach the egg. I n t e r s p e c i f i c p o l l i n a t i o n may r e s u l t in r e j e c t i o n o f the p o l l e n tubes as they grow towards the egg, w i t h p o l l e n tubes o f d i s t a n t l y r e l a t e d species showing l e a s t growth (84). In Pinus, a r e c o g n i t i o n event associated w i t h the p o l l e n tube has been imposed be'5"~irore the sperm/egg i n t e r a c t i o n (Fig. 5). P o l l i n a t i o n systems in f l o w e r i n g p l a n t s : In f e r t i l i z a t i o n of flowering plants, the p o l l e n tube assumes a more complex r o l e , in t h a t i t s growth may be h a l t e d in incompatible p o l l i n a t i o n at several p o i n t s (Fig. 5). In a compatible pollination, the p o l l e n h y d r a t e s , s w e l l s and germinates to produce a p o l l e n tube (43). This tube u s u a l l y p e n e t r a t e s the stigma s u r f a c e c u t i c l e , and passes between the c e l l s o f the s t y l a r t r a n s m i t t i n g t i s s u e to the embryo sac, where the tube e n t e r s a s y n e r g i d c e l ] and the sperms are d i s c h a r g e d , one f u s i n g w i t h the egg c e l ] , the o t h e r w i t h the primary endosperm nucleus in the double f e r t i l i z a t i o n t h a t is c h a r a c t e r i s t i c o f angiosperms. Foreign p o l l i n a t i o n s do not u s u a l l y lead to f e r t i l i z a t i o n , and the f u r t h e r a p a r t g e n e t i c a l ] y the mating p a r t n e r s are the less l i k e l y is f e r t i l i z a t i o n to occur. The sequence o f events may be h a l t e d at any o f the p o i n t s i n d i c a t e d (Fig. 5). There are more than 300 f a m i l i e s of f l o w e r i n g p l a n t s and in these most species are s e l f compatible - t h a t i s , s e l f p o l l e n w i l l e f f e c t fertilization and seed s e t . However, c e r t a i n species in n e a r l y 200 f a m i l i e s have evolved s e l f i n c o m p a t i b i l i t y systems to encourage o u t b r e e d i n g w i t h i n the species - in these cases a r r e s t o f the events of f e r t i l i z a t i o n can a l s o occur at any o f the p o i n t s i n d i c a t e d , but each species has a c h a r a c t e r i s t i c cut o f f p o i n t in t h i s s e r i e s . The system is u s a l l y based on a s i n g l e g e n e t i c locus, the S gene, which has many a l l e l e s (up to 150 in c l o v e r ) (85); systems w i t h more than one s e l f - i n c o m p a t i b i l i t y locus have a l s o been d e f i n e d . The presence o f d i f f e r e n t a l l e l e s in p o l l e n and stigma a l l o w s f e r t i l i z a t i o n w h i l e the presence of the same a l l e l e precludes f e r t i l i z a t i o n . I t is not known whether t h i s major locus c o n t r o l s a l l the s e q u e n t i a l s e r i e s o f events of p o l l i n a t i o n or o n l y one i n d i v i d u a l step. However, i t appears to be a m u l t i g e n e f a m i l y

Vol. 3, No. 4

PLANTS AND ~ I T Y

583

s i m i l a r to the genes in mammalian systems c o n t r o l l i n g ribosome blood group substance and immunog]obulin synthesis (86). However, in contrast to these, the gene product is not known. One of the most i n t e r e s t i n g features of the p o ] l i n a t i o n system is the production of ca]lose as a r e s u l t of incompatible matings. This seems to occur always in e i t h e r the pollen or the pollen tube in an incompatible system, and also occurs in the female stigma in incompatible matings in at least two f a m i l i e s , the daisy fami]y (Compositae) and the cabbage family (Cruciferae). I t is localized in the pol]en and stigma wail at the s i t e of contact with the mating partner, and contains a 1,3 8-glucan: i t can be considered as a true active r e j e c t i o n response (87). Glycoproteins and proteins from the pollen wa]] have been implicated in the induction of t h i s response in stigmas. Experimental manipulation of both pollen and stigm surfaces has demonstrated t h e i r determinate role in f e r t i l i z a t i o n . Wal] proteins and glycoproteins held w i t h i n e x t r a c e l l u l a r s i t e s in the pollen grains apparently act as the male recognition substances (87). In airborne pollen types, the al]ergens that may cause hayfever and seasonal asthma in man, together with a range of enzymes, are also present in the pollen grain walls. Stigma surfaces have been c ] a s s i f i e d into a number of types, p r i n c i p a l l y those with a dry, adhesive surface, and those with a wet pool of exudate (87). This surface secretion is characterized by i t s n o n - s p e c i f i c esterase a c t i v i t y , by the presence of glycoproteins that bind to the l e c t i n con A, and by the presence of arabinogalactan proteins (9). Adhesion of pollen at the stigma surface appears to depend on components of both i n t e r a c t i n g cel] surfaces (95) and a q u a n t i t a t i v e assay of adhesion for t h i s system has recent]y been developed (96). CONCLUSIONS The idea that concepts of immunity in animals may have p a r a l l e l s in p]ants was f i r s t formally considered nearly f i f t y years ago by Chester (90). During the following period, the c e l l u l a r and molecular basis of the immune system has been established. We can now state that although a plant is able to recognize and defend i t s e l f against p o t e n t i a l invaders, i t does not have an immune system d i r e c t l y comparable with that of an animal. The a v a i l a b ] e evidence reviewed here suggests that "immunity" in plants is a multistep process in which two major components can be defined: (1) the i n i t i a l recognition event, (2) the response. The I n i t i a l Recognition Event This event f o r plants, as for animal c e l l s , is considered to depend on complementary i n t e r a c t i o n between surface macromolecules of the partners. The only examples of plant c e i l i n t e r a c t i o n s where the nature of both surface reactive molecules is known are the mating systems of lower plants. In these systems, as for many animal cell recognition systems, the basis is an i n t e r action between surface carbohydrates and protein or glycoprotein receptors on the surface of the partner c e i l . There is also one examp]e of t h i s type of i n t e r a c t i o n in a p a t h o t o x i n - p l a n t receptor system. For the i n t e r a c t i o n s of higher p]ants with fungal pathogens, saccharide sequences on the fungal c e l l surface w i l l e l i c i t a response, which is, however, non-specific in nature. The plant c e l l receptor for these fungal surface polysaccharides is unknown. For higher plant mating systems, the complexity of macromolecules at the surfaces invo]ved is known and some progress in analyses of the major components has been made. In plant somatic c e l l s , antigenic determinants which specify the organism, tissue and cell type are known and may, as in animal c e l ] s , be imp]i'cated in cell recognition reactions. As for animals, there is a marked difference in the antigenic determinants of embryonic and adult plant c e l l s . They may be involved in the d i f f e r e n t responses of seed-

584

PLANTS AND ~ I T Y

Vol. 3, No. 4

ling and adult stem grafts, and graft hybrid formation. Two important features of the animal immune system are the specificity and memory in recognition of non-self. Attempts to make comparisions with plant systems are fraught with difficulty because of the paucity of information. Many of the reactions described, especially the sexual interactions, are highly specific, but whether there is any memory in either sexual or somatic interactions has seldom been examined. The Response There are three responses which have an animal counterpart: (1) Walling-off of infected from healthy host tissues. In both animals and plants the mechanism of walling-off by modification of extracellular materials is retained. In vertebrates it is dependent on invasion of fibroblasts; in plants there are no circulating cells and the molecular basis for the reaction is not understood. (2) Response to infection. In plants a common response to infection is produciton of phytoalexins which are relatively non-specific in their microbial toxicity, and can be induced by surface oligosaccharides of the pathogen. In animals, antibodies are produced which are highly specific for a particular micro-organism or molecule. The molecular basis for highly specific genetic control of plant disease resistance remains to be established. (3) Rejection of xenografts. In both animals and plants xenografts of somatic tissues are usually rejected. In plants much wider genetic differences between successful grafting partners are tolerated. Whether there is an accelerated rejection to a second challenge with an incompatible graft in plants is not known. (4) Humoral responses. There is no definitive evidence for humoral responses in plants. However, macromolecules with some informational potential, such as gums, are often secreted at the site of infection and may be transported within the plant,

ACKNOWLEDGEMENTS We are most grateful to Dr. Gretna Weste and Dr. Doug Parbery, of the Univers i t y of Melbourne and to Dr. Tom Mandel of the Waiter and Eliza Hall I n s t i t u t e of Medical Research, Melbourne, for t h e i r helpful discussion of some of the issues raised in this review, and to the Australian Research Grants Committee for financial support. REFERENCES I .

2.

.

.

.

6. 7.

BURNET, F.M. Self and Not Self. MelbourneUniversity Press. 1972 NORTHCOTE, D.H. The synthesis and assembly of plant cell walls. In: The Synthesis, Assembly and Turnover of Ceil Surface Components. G. Poste and G.L. Nicolson (Eds.) Elsevier North Holland. 1977 ALBERSHEIM, P. Concerningthe Structure and Biosynthesis of the Primary Cell Walls of Plants in: Biochemistry of Carbohydrates I I , Volume 16 (Ed.) D.J. Manners, University Park Press, Baltimore. 1978. GUNNING, B.E.S. Age-related and o r i g i n - r e l a t e d control of the numbers of plasmodesmata in cell walls of developing Azolla roots. Planta. 143, 181, 1978. GUNNING, B.E.S. and STEER, M.W. Ultrastructure and the Biology of Plant Cells. Edward Arnold. ]975. CANNY, M.J.P. Translocation. Cambridge University Press. 1976. SLOAN, R.T., SABNIS, D.D. and HART, J.W. The heterogeneity of phloem exudate proteins from different plants, a comparative survey of ten plants using polyacrylamide gel electrophoresis. Planta (Berl.). 132, 97, 1976.

Vol. 3, No. 4

8. 9. lO. II. 12. 13. 14. 15. 16.

17. 18. 19. 20.

21. 22. 23. 24. 25. 26.

27. 28.

PLANTS AND II~'IINITY

585

KEEN,N.T., WANG, M.C., BARTNICKI-GARCIA, S. and ZENTMYER, G.A. Phytot o x i c i t y of mycolaminarans-B-l-3 glucans from Phytophthora spp. Physiol. Plant Pathol. 7, 9 l , 1975. CLARKE, A.E., ANDERSON, R.L. and STONE, B.A. Form and function of arabinogalactans and arabinogalactan-proteins. Phytochemistry. 18, 521-540, 1979. CLARKE, A.E. GLEESON, P.A., JERMYN, M.A. and KNOX, R.B. Characterization and localization of B-lectins in lower and higher plants. Aust. J. Plant Physiol. 6, 707, 1978. STREET,H.E. Tissue Culture and Plant Sciences. Blackwell: Oxford. 1975 COOPER,E. Comparative Immunology. Prentice-Hall Inc. 1976. I~ARCHALONIS,J.J. Immunity in Evolution. Harvard University Press: Cambridge, Mass. 1977. AARONSON, S. Molecular evidence for evolution in the algae: a possible a f f i n i t y between plant cell wails and animal skeletons. Ann. N.Y. Acad. Sci. 1970, 53l, 1970. BURNET,F.M. Self recognition in colonial marine forms and flowering plants in relation to the evolution of immunity. Nature (Lond.) 232, 230, 1971. NOSSAL,G.J.V., PIKE, B.L., STOCKER, J.W., LAYTON, J.E. and GODING, J.W. Hapten-specific B lymphocytes: enrichment, cloning, receptor analysis and tolerance induction. Cold Spring Harbor Symp. Quart. Biol. 4l, 237, 1976. ALBERSHEIM,P. and ANDERSON-PROUTY, A.J. Carbohydrates, proteins, cell surfaces and biochemistry of pathogenesis. Annu. Rev. Plant Physiol. 26, 31, 1975. CURTIS,A.S.G. Cell-ceil recognition. Symp. Soc. Exp. Biol. 32. Cambridge University Press 1978. HUGHES,R.C. and SHARON, N. Carbohydrates in recognition. Trends in Biochemical Science. 3, 275~ 1978. SHUR,B.D. and ROTH, S. Ceil surface glycosyltransferases. Biochem. Biophys. Acta. 415, 473, 1975. MARCHESl, V.T. and ANDREWS, E.P. Glycoproteins: isolation from cell membranes with Lithium Diiodosalicylate. Science. 174, 1247, 1971. OFEK, I., BEACHEY, E.H. and SHARON, N. Surface sugars of animal cells as determinants of recognition in bacterial adherence. Trends in Biochemical Sciences. 3, 159, 1978. BOLDT, D.H. and ARMSTRONG, J.P. Rosette formation between human lymphocytes and sheep erythrocytes: inhibition of rosette formation by specific glycopeptides. J. Clin. Invest. 57, I068, 1976. ASHWELL, G. and MORELL, A. Membrane glycoproteins and recognition phenomena. Trends in Biochem. Sci. 2, 76, 1977. NOWACK, T.P., KOBILER, D., ROEL, L.E. and BARONDES, S.H. Developmentally regulated lectin from embryonic chick pectoral muscle - purification by affinity chromatography. J. Biol. Chem. 252, 6026, 1977. SIMPSON, D.L., ROSEN, S.D., BARONDES, S.H. 1974. Discoidin, a developmentally regulated carbohydrate-binding protein from Dictyostelium discoideum. Purification and characterization. Biochemistry 13: 3487-93. SINENSKY,M. and STROBEL, G. Chemical composition of a c e l l u l a r fraction enriched in plasma membranes from sugar cane. Plant Sci. Lett. 6, 209, 1976. HINCH,J. and CLARKE, A.E. Adhesion of fungal zoospores to root surfaces is mediated by carbohydrate determinants of the root slime. Physiol. Plant Path. (in press) 1979.

586

29. 30. 3]. 32. 33. 34.

35.

36. 37. 38. 39. 40.

41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 5]. 52.

PLANTS AND IMMI~ITY

Vol. 3, No. 4

GOLDSTEIN, I . J . and HAYES, C.E. The l e c t i n s : carbohydrate binding protein of plants and animals. Adv. in Carb. Chem. and Biochem. 35, "127, 1978. LIS, H. and SHARON, N. Lectins: t h e i r chemistry and a p p l i c a t i o n to immunology. In: The Antigens. M. Seia (Ed.). 4, 429, 1977. BROWN,J.C. and HUNT, R.C. Lectins. I n t . Rev. Cytology, 52, 277, 1978. WATKINS, W.M. Genetics and biochemistry of some human blood groups. Proc. R. Soc. Lond. B. 202, 31, ]978. ANDERSON,D.M.W. and DEA, I.C.M. Chemotaxonomic aspects of the chemistry of Acacia gum exudates. Phytochemistry. 8, 167, 1969. KNOX, R.B., CLARKE, A.E. HARRISON, S., SMITH, P. and HARCHALONIS, J.J. Cell recognition in plants: Determinants of the stigma surface and t h e i r pollen i n t e r a c t i o n s . Proc. Nat. Acad. Sci. (U.S.A.). 73, 2788, I976. VAHERI, A. and MOSHER, D.F. High molecular weight, cell surface-associated glycoprotein (Fibronectin) lost in malignant transformation. Biochim. Biophys. Acta. 5]6, ], 1978. HATILE, P. The L y t i c Compartment of Plant Cells. Springer-Verlag: B e r l i n , New York. ]975. DODGE, J.D. and CRAWFORD, R.M. The morphology and fine s t r u c t u r e of Ceratium h i r u n d i n e l I a (Dinophyceae). J. Phycol. 6, 137, 1970. LINDSAY, D.W., YEOMAN, M.M. and BROWN, R. 1974. An analysis of the development of the g r a f t union in Lycopersicon esculentum. Ann. Bot., 38: 639-646. YEOMAN, M.M. and BROWN, R. Implications of the formation of the g r a f t union for organization of the i n t a c t plant. Ann. Bot. 40, ]265, ]976. LATIES, G.G. Solute transport in r e l a t i o n to metabolism and membrane permeability in plant tissues. In: H i s t o r i c a l and Current Aspects of Plant Physiology: A Symposium Honoring F.C. Steward. P.J. Davies (Ed.) Corne]] Univ. Ithaca. ]975, P. 98. LIPETZ, J. Wound healing in higher plants. I n t . Rev. Cyt. 27, 1, 1970. HERRERO,J. and TABUENCA, M.C. I n c o m p a t i b i l i t y between stock and scion. X. Behaviour of the peach/myrobolan combination when grafted at the cotyledon stage. An. Au]a Dei. lO, 937, 1969. CLARKE, A.E. and KNOX, R.B. Cell recognition in flowering plants. Quart. Rev. B i o l . 53, 3. 1978. KNOX, R.B. and CLARKE, A.E. Discrimination between s e l f and non-self in plants. In: Contemporary Topics in Immunobiology, 1979 (in press). GLAVINIC, R. Vegetative h y b r i d i z a t i o n of tomatoes. Agrobiologiya. 1956, 86, 1956 (in Russian). NIRK, H. Interspecific hybrids of Lycopersicum. Nature. ]84, 1819, 1959. RAFF,J.W., HUTCHINSON, J.F., KNOX, R.B. and CLARKE, A.E. Cell recognition: Antigenic determinants of plant organs and their cultured callus ceils. Differentiation. 12 179-186 (1979). BALL, E.A. Growth of-plant tissues upon a substrate of another kind of tissue. Zeitschr. Pf1. Phys. 65, 140, 1971. FUJII, T. and NITO, N. Studies on the compatibility of grafting of f r u i t trees. I. Callus fusion between rootstock and scion. J. Jpn. Soc. Hortic. Sci. 41, I, 1972. BOUTENKO,R.G. and VOLODARSKY, A.D. Analyse immunochimique de la d i f f e r enciation cellulaire dans les cultures de tissus de Tabac. Physiol. Veg. 6, 299, 1968. KHAVKIN,E.E., MISHARIN, F . I . , IVANOV, E.N. and DANOVICH, K.N. Embryonal antigens in maize caryopses: the temporal order of antigen accumulation during embryogenesis. Planta (Ber].). 135, 225, 1977. CLARKE, A.E., KNOX, R.B., HARRISON, S., RAFF, J. and MARCHALONIS, J. Common antigens and male-female recognition in plants. Nature (Lond.) 265, 161, 1977.

Vol. 3, No. 4

53. 54. 55. 56. 57.

58. 59. 60. 61. 62.

63.

64. 65. 66.

67. 68. 69. 70. 71.

72.

PLANTS AND ~ I T Y

587

RAFF, J . , HUTCHISON, J., CLARKE, A.E. and KNOX, R.B. Natue of determinants of plant cells and t h e i r derived caI1i. Proc. Aust. Biochem. Soc. 11, 34, 1978. SMITH, H. Recognition and defence in plants. Nature 273, 266, 1978. YARWOOD,C.E. Some principles of plant pathology. 11. Phytopathol 63, 1324, 1973. HARBOURNE,J.B. Introduction to Ecological Biochemistry. Academic Press: London, New York. 1977. SCHONBECK, F. and SCHLOSSER, E. Preformed substances as potential protectants. In: Encyclopedia of Plant Physiology, New Series, 4, 653. Physiological Plant Pathology. R. Heitefuss and P.H. Williams (Eds.) Springer-Verlag: Berlin. 1976. KUC, J.A. Phytoalexins. In: Encyclopedia of Plant Physiol, New Series, 4, 632. Physiological Plant Physiology. R. Heitefuss and P.H. Williams (Eds.) Sprinqer-Verlaq: Berlin. 1976. CRUIKSHANK, D. and PERRIN, D.R. The isolation and partial characterization of H o n i l i c o l i n A, a polypeptide with phaseollin-inducing a c t i v i t y from Monilinia fructicO]a. Life Sciences. 7 (11), 449, 1968. STEKOLL, M. and WEST, C.A. P u r i f i c a t i o n and properties of an e l i c i t o r of castor bean phytoalexin from culture f i l t r a t e s of the fungus Rhlzopus s t o l o n i f e r . Plant Physiol. 61, 38, 1978. ALBERSHEIH,P. and VALENT, B. Host-pathogen interactions in plants. J. Cell Biol. 78, 627, 1978. ANDERSON-PROUTY,A.J. and ALBERSHEIM, P. Host-pathogen interactions V I I I . Isolation of a pathogen-synthesized fraction rich in glucan that e l i c i t s a defense response in the pathogen's host. Plant Physiol. 56, 286, 1975. CHALOVA,L . I . , OZERETSKOVSKAYA, O.L., YURGANOVA, L.A., BARAMIDZE, V.G., PROTSENKO, M.A., D'YAKOV, Y.T. and METLITSKII, L.V. Metaboiites of phytopathogenic fungi as e l i c i t o r s of defense reactions in plants (in the potato - Phytophthora infestans i n t e r a c t i o n ) . Doklady Biological Sciences. 226, 433, 1976. " WOODWARD,J.R., KEANE, P.J. and STONE, B.A. Structures and properties of w i l t inducing polysaccharides from Phytophthora species. Physiol. Plant Path. (in press) (1979) DEVERALL,B.J. Defence Mechanisms of Plants. Cambridge Monographs in Experimental Biology No. 19. Cambridge University Press. GOODMAN,R.N., HUANG, P.Y. and WHITE, J.A. Ultrastructural evidence for immobilization of an incompatible bacterium, Psuedomonas pis.i, in tobacco leaf tissue. Phytopathology. 66, 754, 1976. SEQUEIRA,L., GAARD, G. and DEZOETEN, G.A. Interaction of bacteria and host cell walls: i t s relation to mechanisms of induced resistance. Physiol. Plant Pathol. 10, 43, 1977. SING, V.O. and SCHROTH, M.N. Bacteria-plant cell surface interactions. Active immobilization of saprophytic bacteria in plant leaves. Science. 197, 759, 1977. STONE, B.A. and CLARKE, A.E. Chemistry and Biology of 1,3-8--DGlucans. Macmillan, London. 1979. KUIJT, J. The Biology of Parasitic Flowering Plants. Univ. of C a l i f . Press, Berkeley. PUSTOVOIT,G.V., PLYTNIKOVA, T.G. and GUBIN, I.A. Breeding sunflower for resistance to broom rape. Sel'skokhozy a i s t vennaya Biol. 11, 240, 1976. (In Russian; in Plant Breeding Abstr. 46, 9322.) KUIJT, J. and TOTH, R. Ultrastructure of angiosperm haustoria - a review. Ann. Bot. 40, 1121, 1976.

588

PI2hNTS AND IM/41.rNITY

73.

DORR, I. and KOLLMANN, R. S t r u k t u r e l l e Grundlage des Paratismus Orobanche. I I . Die Differenzierung der Assimilat-Leitungsbahn Haustorialgewebe. Protoplasma. 83, 185, 1975. TAINTER, F.H. The u l t r a s t r u c t u r e of ArCeuthobium pusillum. Can. 49, 1615, 1971. DORR, I. and KOLLMANN, R. Strukturelle Grundlage des Paratismus Orobanche I. Wachstum der Haustorialzellen im Wirtsgewelse. 80, 245, 1974.

74. 75.

76. 77. 78.

79. 80. 81.

82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93.

Vol. 3, No. 4

bei in J. Bot. bei Protoplasma.

DORR, I. Zur Lokalisierung yon Zellkontakten zwischem Cuscuta odorata und verschiedenen hoheren Wirtspflanzen. Protoplasma. 65, 435, 1968. TSIVION, Y. Possible role of c y t o k i n i n s in non-specific recognition of a host and in early growth of haustoria in the p a r a s i t i c plant Cuscuta campestris. Bot. Gaz. 139, 27, 1978. ENDE, H. VAN DEN Sexual Interactions in Plants. Academic Press: London and New York. 1976. YEN, P.H. and BALLOU, C.E. Structure and immunochemistry of Hansenula winge.i.y-2340 mannan. Biochemistry. 13, 2420, 1974. KNOX, R.B. Pollen and A l l e r g y . Studies in Biology no. 107. Edward Arnold London. 1979. WIESE, L. and SHOEMAKER, D.W. On sexual a g g l u t i n a t i o n and mating type substances (gamones) in isogamous h e t e r o t h a l l i c chlamydomonads I I . The e f f e c t of Concanavalin A upon the mating reaction. Biological b u l l e t i n . 138, 88, 1970. CRANDALL, M.A. and BROCK, T.D. Molecular basis of mating in the yeast Hansenula wingei. Bacteriological Reviews. 32, 139, 1968. PETTITT, J.M. Detection in p r i m i t i v e gymnosperms of proteins and glycoproteins of possible s i g n i f i c a n c e in reproduciton. Nature. 266, 530, 1977. MCWILLIAH, J.R. I n t e r s p e c i f i c i n c o m p a t i b i l i t y in Pinus. Am. J. Bot. 46, 425, 1959. DE NETTANCOURT, D. I n c o m p a t i b i l i t y in Angiosperms. Springer-Verlag: Berlin and New York. 1977. p. 230. HOOD, L., CAMPBELL, J.H. and ELGIN, S.C.R. The o r g a n i z a t i o n , expression and evolution of antibody genes and other multi-gene f a m i l i e s . Annu. Rev. Genetics. 9, 305, 1975. HESLOP-HARRISON,J. I n c o m p a t i b i l i t y and the pollen-stigma i n t e r a c t i o n . Ann. Rev. Plant Physiol. 26, 403, 1975. CARLSON, R.W., SANDERS, R.E., NAPOL, C. and ALBERSHEIM, P. Host-symbiont interaction III. P u r i f i c a t i o n and p a r t i a l characterization of Rhizobium lipopolysaccharides. Plant Physiol. 62, 912, 1978. SMITH, V.A. and HINDLEY, J. Effect of agrocin 84 on attachment of Agrobacterium tumefaciens to cultured tobacco c e l l s . Nature 276, 498, 1978. CHESTER, K.S. The problem of acquired physiological immunity in plants. Q. Rev. Biol. 8, 275, 1933. MINTZ, B. & SILVERS, W.K. "Intrinsic" immunological tolerance in allophenic mice. Science 158, 1484, 1967. SEQUEI RA, L. Lectins and their role in host-pathogen specificity. Ann. Rev. Phytopathol. 16, 453-481, 1978. PEGG, G.F. The occurrence of 1,3-8 glucanase in healthy and Verticilliuminfected, resistant and susceptible tomato plants. J. Exp. Bot. 27, I093-1011, 1976.

Vol. 3, No. 4

94. 95.

96.

PLANTS AND II~UNITY

589

CARUSO, F.L. and KUC, J. Induced resistance of cucumber to anthracnose and angular leaf spot by Pseudomonas lachrymans and Colletotrichum iagenarium. Physiol. Plant Path. 14 191-201 (1979). CLARKE, A.E., GLEESON P., HARRISON, S.-and KNOX, R.B. Pollen-stigma i n t e r a c t i o n s ; i d e n t i f i c a t i o n and characterization of surface components with recognition p o t e n t i a l . Proc Natl. Acad. Sci. U.S.A. 76 3358-3362 (1979). STEAD, A.D., ROBERTS, I . N . , and DICKINSON, H.G. P o l l e n - p i s t i l i n t e r a c t i o n in Brassica oleracea: events p r i o r to pollen germination Planta (in press) 1979.