- Email: [email protected]
Induced resistance: from the basic to the applied
Research Update
TRENDS in Plant Science Vol.6 No.10 October 2001
transgenic studies. However, the experiments were performed in heterologous systems, and it is possible that other unidentified genes in tobacco might be contributing to the observed results. Moreover, the Pra2-DDWF1 interaction might not be universal in the plant kingdom because Arabidopsis plants expressing antisense Pra2 did not show a short hypocotyl phenotype. Kang et al.8 point out that this might simply be because of lack of sufficient sequence identity of the putative Arabidopsis Pra2 homolog for antisense suppression by pea Pra2, but it is also possible that in Arabidopsis, this mechanism of light-BR signal integration might not be functional. Pra2 regulation of DDWF1: a novel mechanism of integrating brassinosteroids and light signals
The work by Kang et al.8 increases our understanding of the molecular mechanisms involved in the interaction between light and BR signals in the regulation of cell expansion. Two different models (Fig. 2) were proposed to account for the increased activity of DDWF1 in vivo in response to Pra2: • Pra2 directly activates DDWF1. • It triggers the assembly of a functional enzyme complex on ER membranes by recruiting unknown, but essential cofactor(s) through vesicle trafficking. Several cytochrome P450s have been fully characterized that catalyze multiple steps in brassinolide biosynthesis in Arabidopsis6, and given that this species
contains a large multigene family of small G proteins, it would be a useful exercise to study the possible interactions of small G proteins with these enzymes. Thus, it might be possible to determine how general the Pra2–DDWF1 mechanism is, and if multiple cytochrome P450 steps in the biosynthetic pathway are regulated in the same manner. The molecular and biochemical data provided by Kang et al.8 shows that the Pra2–DDWF1 interaction might be a molecular switch regulating the etiolation–de-etiolation transition. High levels of Pra2 and DDWF1 in the dark lead to increased levels of BR and enhanced hypocotyl elongation. Upon exposure to light, phytochrome and bluelight photoreceptors signal the repression of Pra2 (and probably DDWF1) and the production of BR is reduced, leading to a slowing of hypocotyl growth in the light. Implicit in this argument, is a reduction in existing BR levels by metabolism. Actual measurements of endogenous BR levels in light and dark-grown pea epicotyls would be crucial to test this hypothesis. To what extent BRs affect other manifestations of de-etiolation in pea, such as leaf and chloroplast development, remains to be determined. References 1 Chory, J. et al. (1996) From seed germination to flowering, light controls plant development via the pigment phytochrome. Proc. Natl. Acad. Sci. U. S. A. 93, 12066–12071 2 Noguchi, T. et al. (2000) Biosynthetic pathways of brassinolide in Arabidopsis. Plant Physiol. 124, 201–219
445
3 Clouse, S.D. and Sasse, J.M. (1998) Brassinosteroids: essential regulators of plant growth and development. Annu. Rev. Plant Physiol. Plant Mol. Biol. 49, 427–451 4 Li, J. et al. (1996) A role for brassinosteroids in light-dependent development of Arabidopsis. Science 272, 398–401 5 Szekeres, M. et al. (1996) Brassinosteroids rescue the deficiency of CYP90, a cytochrome P450, controlling cell elongation and de-etiolation in Arabidopsis. Cell 85, 171–182 6 Clouse, S.D. and Feldmann, K.A. (1999) Molecular genetics of brassinosteroid action. In Brassinosteroids: Steroidal Plant Hormones (Sakurai, A. et al., eds), pp. 163–190, Springer 7 Neff, M.M. et al. (1999) BAS1: a gene regulating brassinosteroid levels and light responsiveness in Arabidopsis. Proc. Natl. Acad. Sci U. S. A. 96, 15316–15323 8 Kang, J.G. et al. (2001) Light and brassinosteroid signals are integrated via a dark-induced small G protein in etiolated seedling growth. Cell 105, 625–636 9 Ma, H. (1994) GTP-binding proteins in plants: new members of an old family. Plant Mol. Biol. 26, 1611–1636 10 Nagano, Y. et al. (1995) Location of lightrepressible, small GTP-binding protein of the YPT/rab family in the growing zone of etiolated pea stems. Proc. Natl. Acad. Sci U. S. A. 92, 6314–6318 11 Inaba, T. et al. (1999) Identification of a cisregulatory element involved in phytochrome down-regulated expression of the pea small GTPase gene pra2. Plant Physiol. 120, 491–500 12 Inaba, T. et al. (2000) DE1, a 12-base pair cisregulatory element sufficient to confer darkinducible and light down-regulated expression to a minimal promoter in pea. J. Biol. Chem. 275, 19723–19727
Steven D. Clouse Dept of Horticultural Science, North Carolina State University, Raleigh, NC 27695-7609, USA. e-mail: [email protected]
Meeting Report
Induced resistance: from the basic to the applied Linda L. Walling Induced Resistance in Plants Against Insects and Diseases, Wageningen, The Netherlands, 26–28 April 2001.
Induced resistance (IR) is a plastic response, which diverts carbon and nitrogen resources from plant growth and reproduction to provide a longlasting and systemic resistance to a broad spectrum of pathogens and pests. IR is activated by a variety of chemicals, pathogens, biocontrol bacteria and http://plants.trends.com
herbivores. The molecular underpinnings of IR, fitness costs and use in field applications were the foci of the Induced Resistance in Plants against Insects and Diseases conference organized by the International Organization of Biological Control. The diverse and exciting program fostered interactions between applied and theoretical scientists in the fields of plant–pathogen and plant–herbivore interactions. Joseph Kúc (University of
Kentucky, Lexington, KY, USA), a pioneer in the field of applied IR, provided a historical perspective and highlighted three themes that permeated the conference: (1) there is a diverse array of signals that stimulate IR; (2) IR is a sensitization process that primes the plant for more rapid deployment of defenses; and (3) when integrated into good agricultural practices, IR can both enhance plant productivity and resistance to disease.
1360-1385/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S1360-1385(01)02046-5
446
Research Update
Induced resistance: multiple pathways for an effective defense
Two types of IR are well characterized. Systemic acquired resistance (SAR) is dependent on salicylic acid-mediated signaling and is activated by benzo[1,2,3]thiadiazole-7-carbothioic acid S-methyl ester (BTH; BION®). Induced systemic resistance (ISR), which develops after colonization by some biocontrol rhizobacteria, is dependent on the sequential action of jasmonic acid and ethylene. SAR is correlated with an increase in salicylic acid and the expression of pathogenesis-related (PR) protein genes. Salicylic acid is essential for SAR, although is not the signal transported to distal sites to activate SAR. Chris Lamb (John Innes Centre, Norwich, UK) described the Arabidopsis dir1 mutant that does not produce the mobile SAR signal, but has a normal local defense response. DIR1 (a putative non-specific lipid-transfer protein) might be this signal or might be involved in production or transport of this elusive signal. Several laboratories discussed the importance of salicylic acid in priming cells for rapid expression of defense genes after elicitor or pathogen challenge. Uwe Conrath (University of Kaiserslautern, Germany) showed that salicylic acid accelerates increases in phenylalanine ammonia-lyase RNAs and MAP kinase activity after fungal elicitor treatments in parsley (Petroselinum crispum). Transgenic Arabidopsis that ectopically expressed a chimeric salicylate synthase (Brigitte Mauch-Mani, University of Fribourg, Switzerland) or NPR1/NIM1 (a regulator of the SAR and ISR signaling pathways) (Robert Dietrich, Syngenta, Research Triangle, NC, USA) have enhanced resistance to pathogens and accelerated PR expression after pathogen challenge. Application of other chemicals that stimulate IR, including BTH, β-aminobutryic acid (BABA) (Mauch-Mani), Messenger® (Zhongman Wei, Eden Biosciences, Bothwell, WA, USA), and OxycomTM (Anne Anderson, Utah State University, UT, USA) also accelerate defense gene expression. ISR is not correlated with major changes in RNA or protein profiles suggesting that ISR produces undiscovered defense products that contribute to resistance. Characterization of Arabidopsis isr1 (Juriaan Ton, Utrecht http://plants.trends.com
TRENDS in Plant Science Vol.6 No.10 October 2001
University, The Netherlands), tobacco etr1 (Bart Geraats, Utrecht University), and temporal accumulation of defense RNAs after ACC treatments in ISR-expressing plants (Corné Pieterse, Utrech University) suggest that enhanced ethylene sensitivity is crucial for ISR. Consistent with this theory, Karen Léon-Kloosterziel’s (Utrech University) poster described an Arabidopsis enhancer-trap line that responds to biocontrol rhizobacteria and the ethylene precursor ACC. Signal perception: elicitor-binding proteins and early signaling events in induced resistance
EDS1 mediates rapid perception of pathogens in several resistant plant–avirulent pathogen interactions. Jane Parker (John Innes Centre) described the necessity for the lipase-like proteins EDS1 and PAD4 in pathogen and insect resistance, ROS generation, and salicylic acid accumulation. Two-hybrid assays show that EDS1 and PAD4 physically interact suggesting that this affiliation might be essential for their action. Thomas Boller (Friedrich Miescher Institute, Basel, Switzerland) reported that the flagellin peptide (flg22) induces rapid alkalinization of culture media, callose deposition and an oxidative burst in tomato. However, flg22 does not affect monocots. FLG1 and FLG2 (leucine-rich repeat receptor kinase-like protein) influence flg22 perception or action in Arabidopsis. The harpin peptide (Messenger®) is another potent inducer of defense genes and IR (Wei). Putative harpin-binding protein (HrBP1) genes were identified in both monocots and dicots. Integrating multiple signaling pathways: salicylic acid and jasmonic acid cross-talk
An elegant and interactive computational model for cell signaling using Boolean gates was presented by Jean-Pierre Métraux (University of Fribourg). This model incorporates the complex network of signaling, including light signals (Métraux) and the reciprocal regulation of salicylic acid- and jasmonic acid-signaling pathways (Mauch-Mani, Pieterse, Thomma, and Métraux). However, these pathways are not always mutually exclusive and can provide a multi-layered defense. For example, both ISR and SAR can be induced simultaneously and their
effects are additive (Pieterse). Jasmonic acid- and salicylic acid -regulated genes are expressed together in response to many pathogens (Bart Thomma, Katholieke Universiteit Leuven, Belgium) and Messenger® (Wei), and salicylic acid and jasmonic acid can act synergistically (Luis Mur, University of Wales, Aberystwyth, UK). Richard Bostock (University of California, Davis, CA, USA) provided evidence that the timing of jasmonic acid and BTH applications, as well as their concentrations, influence the cross talk between these pathways, with lower concentrations of jasmonic acid and BTH allowing simultaneous expression of both pathways. ‘…both jasmonic acid perception in scions and biosynthesis in root stocks are crucial for the systemic accumulation of woundresponse RNAs.’
The characterization of the classes of mutants that influence light and defense cross talk (ltd, psi; Métraux), enhance susceptibility to pathogens (esa, Thomma; eds4-12, Ton), influence BABA perception or action (bai, Mauch-Mani), and the further characterization of known mutants that influence salicylic acid and jasmonic acid cross talk will substantially enhance our understanding of the intricacies of the defense network. Oxylipins: key regulators of defense responses
The octadecanoid pathway, which synthesizes jasmonic acid, is activated and amplified in response to pests or pathogens. Thierry Heitz (Insitut de Biologie Moléculaire des Plantes, Strasbourg, France) described the importance of a soluble phospholipase A2 in the release of linolenic acid from tobacco membranes, a crucial initial step in this pathway. Although jasmonic acid and the jasmonicbiosynthesis intermediates are known defense signals, the importance of other oxylipins is just being realized. Edward Farmer (University of Lausanne, Switzerland) showed that linolenic acid-derived 9- and 13-ketodienes accumulate after Pseudomonas syringae pv. tomato infection. These oxylipins and other molecules with the chemically reactive α,βunsaturated carbonyl group are potent signals for defense gene activation. Gregg Howe (Michigan State University, East Lansing, MI, USA)
Research Update
provided important new evidence for jasmonic acid’s involvement in systemic signaling in tomato. Reciprocal grafting experiments using control plants and mutants that disrupt jasmonic perception (jai1; a potential coi1 analogue) or jasmonic acid accumulation (spr2) showed that both jasmonic acid perception in scions and biosynthesis in root stocks are crucial for the systemic accumulation of wound-response RNAs. Systemin, a mobile peptide, activates the octadecanoid pathway in tomato. Andreas Schaller (ETH Zürich) identified a protease with a likely role in systemin catabolism. The tomato enzyme is similar to a mammalian insulin-degrading enzyme and removes three COOH-terminal residues from systemin to generate an inactive peptide. Herbivore-induced resistance: fitness costs and complexity of responses
Ian Baldwin (Max Planck Institute, Jena, Germany) discussed the fitness costs of jasmonic acid-induced defenses in Nicotiana attenuata. In the absence of herbivore pressure, jasmonic acid decreases seed production relative to the controls. However, in the presence of herbivores, jasmonic acid-treated plants have increased seed number relative to undamaged, jasmonic acid-treated plants. Erkki Haukoija (University of Turku, Finland) described a delayed IR, which is expressed in foliage a year after damage occurs in birch (Betula) and is influenced by light levels. This IR impedes larval growth and development, as well as influencing leaf size, branching and nitrogen and phytochemical content of plant organs. Martin Heil (Biozentrum, Würzburg, Germany) highlighted the need for continued evaluation of the fitness costs of IR. Herbivore-activated IR and its relationship to ISR and SAR is not completely understood. Jasmonic aciddependent defenses are important and influenced by the time and intensity of herbivore damage, as well as components of herbivore saliva. Genes activated by herbivores are dependent on the mode of feeding and components in saliva. Gary Felton (University of Pennsylvania, Philadelphia, PA, USA) showed that glucose oxidase (GOX) in Helicoverpa zea saliva suppresses many wound responses that are usually induced against tissuedamaging herbivores. GOX inhibits increases in polyphenol oxidase, http://plants.trends.com
TRENDS in Plant Science Vol.6 No.10 October 2001
peroxidase and lipoxygenase activities, nicotine and jasmonic acid. GOX also induces expression of PR genes, SAR and a tolerance to cold stress. Insects that do little damage to foliage activate different signaling pathways than most herbivores that chew or tear tissue. Phloem-feeding whiteflies (Linda Walling, University of California, Riverside, CA, USA) and aphids (Bostock) activate PR genes. In addition, closely related species of whiteflies evoke distinct differences in local and systemic gene expression using jasmonic acid-dependent or novel defense signaling pathways. Volatiles: potent signals for induced resistance
Over 100 different volatiles are released when damaged by herbivores, providing an indirect defense by attracting herbivore predators and parasites. Andre Keßler (Max Planck Institute) characterized the volatile blend released by N. attenuata when attacked in its natural environment. These volatiles are effective signals in the field, because treatment of field-grown plants with volatiles or jasmonic acid enhanced herbivore predation and deterred herbivore oviposition. Silvia Dorn (ETH Zürich, Switzerland) explained that, similar to foliar-feeding damage, volatiles released from herbivore-damaged apple fruit attract parasitoids and deter further herbivore infestation. Torsten Meiners (University of Berlin, Germany) also reported that egg deposition and oviduct secretions over small epidermal wounds on angiosperm and gymnosperm leaves can incite volatile production. Remo van Poeke (Wageningen University, The Netherlands) showed that larval feeding activates Arabidopsis genes that are important in the synthesis of several volatile classes (i.e. C6 volatiles, jasmonic acid, MeSA and terpenes). Marcel Dicke (Wageningen University) explained that the subset of volatiles induced by jasmonic acid are not as attractive to spider mite enemies, as the volatiles released after spider mite damage or when jasmonic acid-induced volatiles are supplemented with MeSA. Both the Marcel Dicke and the Ted Turlings laboratories (Maria Fritzche-Hoballah, University of Neuchâtel, Switzerland) reported that there were no apparent fitness costs to volatile production in two plant-herbivore-parasitoid interactions.
447
Marcel Dicke emphasized that given the ability of volatiles to have an impact on the entire community (herbivores, pathogens, carnivores, and neighboring competing plants), their usefulness in resistance strategies needs to be carefully evaluated. Usefulness of induced resistance in the field
The usefulness of IR in disease control and plant productivity has been tested in the field. IR stimulated by biocontrol bacteria (Kees van Loon, Wageningen University), NIM1-expressing plants (Dietrich), BION® (Duncan McKenzie, Syngenta, Basel, Switzerland), OxyxomTM (Anderson), Messenger® (Wei) or Reynoutria sachalinensis extracts (Milsana®; Annegret Schmitt, BBA, Darmstadt, Germany; Hans von Amsberg, KHH Bioscience Inc., Raleigh, NC, USA) is effective against pathogens, pests and even parasitic plants (Holger Buschmann, University of Hohenheim, Stuttgart, Germany). In most cases, the mode and timing of IR inducer application significantly influences its effectiveness. IR alone is often insufficient to completely control disease or pest damage. However, when used in conjunction with good agricultural practices, plant productivity, quality and disease resistance increase substantially. Use of low levels of fungicides with NIM1 over-expressing plants, BION®, Milsana® or biocontrol bacteria enhances disease resistance. In addition use of BION® with cultivars expressing partial pathogen resistance (Enrique Torres, Universidad Nacional de Columbia, Bogata, Columbia) or the application of avirulent bacteria with biocontrol bacteria (van Loon) are other alternatives that enhance the usefulness of IR horticultural and crop plants in the field. Rapid advances that are being made in our understanding of the mechanisms of pathogen, pest and elicitor perception, and the elucidation of defense signal transduction networks will ultimately enhance and refine the applied IR strategies. It is likely that IR provides an environmentally friendly and economically viable disease control strategy. Linda L. Walling Dept of Botany and Plant Sciences, University of California, Riverside, CA 92521, USA. e-mail: [email protected]
TRENDS in Plant Science Vol.6 No.10 October 2001
transgenic studies. However, the experiments were performed in heterologous systems, and it is possible that other unidentified genes in tobacco might be contributing to the observed results. Moreover, the Pra2-DDWF1 interaction might not be universal in the plant kingdom because Arabidopsis plants expressing antisense Pra2 did not show a short hypocotyl phenotype. Kang et al.8 point out that this might simply be because of lack of sufficient sequence identity of the putative Arabidopsis Pra2 homolog for antisense suppression by pea Pra2, but it is also possible that in Arabidopsis, this mechanism of light-BR signal integration might not be functional. Pra2 regulation of DDWF1: a novel mechanism of integrating brassinosteroids and light signals
The work by Kang et al.8 increases our understanding of the molecular mechanisms involved in the interaction between light and BR signals in the regulation of cell expansion. Two different models (Fig. 2) were proposed to account for the increased activity of DDWF1 in vivo in response to Pra2: • Pra2 directly activates DDWF1. • It triggers the assembly of a functional enzyme complex on ER membranes by recruiting unknown, but essential cofactor(s) through vesicle trafficking. Several cytochrome P450s have been fully characterized that catalyze multiple steps in brassinolide biosynthesis in Arabidopsis6, and given that this species
contains a large multigene family of small G proteins, it would be a useful exercise to study the possible interactions of small G proteins with these enzymes. Thus, it might be possible to determine how general the Pra2–DDWF1 mechanism is, and if multiple cytochrome P450 steps in the biosynthetic pathway are regulated in the same manner. The molecular and biochemical data provided by Kang et al.8 shows that the Pra2–DDWF1 interaction might be a molecular switch regulating the etiolation–de-etiolation transition. High levels of Pra2 and DDWF1 in the dark lead to increased levels of BR and enhanced hypocotyl elongation. Upon exposure to light, phytochrome and bluelight photoreceptors signal the repression of Pra2 (and probably DDWF1) and the production of BR is reduced, leading to a slowing of hypocotyl growth in the light. Implicit in this argument, is a reduction in existing BR levels by metabolism. Actual measurements of endogenous BR levels in light and dark-grown pea epicotyls would be crucial to test this hypothesis. To what extent BRs affect other manifestations of de-etiolation in pea, such as leaf and chloroplast development, remains to be determined. References 1 Chory, J. et al. (1996) From seed germination to flowering, light controls plant development via the pigment phytochrome. Proc. Natl. Acad. Sci. U. S. A. 93, 12066–12071 2 Noguchi, T. et al. (2000) Biosynthetic pathways of brassinolide in Arabidopsis. Plant Physiol. 124, 201–219
445
3 Clouse, S.D. and Sasse, J.M. (1998) Brassinosteroids: essential regulators of plant growth and development. Annu. Rev. Plant Physiol. Plant Mol. Biol. 49, 427–451 4 Li, J. et al. (1996) A role for brassinosteroids in light-dependent development of Arabidopsis. Science 272, 398–401 5 Szekeres, M. et al. (1996) Brassinosteroids rescue the deficiency of CYP90, a cytochrome P450, controlling cell elongation and de-etiolation in Arabidopsis. Cell 85, 171–182 6 Clouse, S.D. and Feldmann, K.A. (1999) Molecular genetics of brassinosteroid action. In Brassinosteroids: Steroidal Plant Hormones (Sakurai, A. et al., eds), pp. 163–190, Springer 7 Neff, M.M. et al. (1999) BAS1: a gene regulating brassinosteroid levels and light responsiveness in Arabidopsis. Proc. Natl. Acad. Sci U. S. A. 96, 15316–15323 8 Kang, J.G. et al. (2001) Light and brassinosteroid signals are integrated via a dark-induced small G protein in etiolated seedling growth. Cell 105, 625–636 9 Ma, H. (1994) GTP-binding proteins in plants: new members of an old family. Plant Mol. Biol. 26, 1611–1636 10 Nagano, Y. et al. (1995) Location of lightrepressible, small GTP-binding protein of the YPT/rab family in the growing zone of etiolated pea stems. Proc. Natl. Acad. Sci U. S. A. 92, 6314–6318 11 Inaba, T. et al. (1999) Identification of a cisregulatory element involved in phytochrome down-regulated expression of the pea small GTPase gene pra2. Plant Physiol. 120, 491–500 12 Inaba, T. et al. (2000) DE1, a 12-base pair cisregulatory element sufficient to confer darkinducible and light down-regulated expression to a minimal promoter in pea. J. Biol. Chem. 275, 19723–19727
Steven D. Clouse Dept of Horticultural Science, North Carolina State University, Raleigh, NC 27695-7609, USA. e-mail: [email protected]
Meeting Report
Induced resistance: from the basic to the applied Linda L. Walling Induced Resistance in Plants Against Insects and Diseases, Wageningen, The Netherlands, 26–28 April 2001.
Induced resistance (IR) is a plastic response, which diverts carbon and nitrogen resources from plant growth and reproduction to provide a longlasting and systemic resistance to a broad spectrum of pathogens and pests. IR is activated by a variety of chemicals, pathogens, biocontrol bacteria and http://plants.trends.com
herbivores. The molecular underpinnings of IR, fitness costs and use in field applications were the foci of the Induced Resistance in Plants against Insects and Diseases conference organized by the International Organization of Biological Control. The diverse and exciting program fostered interactions between applied and theoretical scientists in the fields of plant–pathogen and plant–herbivore interactions. Joseph Kúc (University of
Kentucky, Lexington, KY, USA), a pioneer in the field of applied IR, provided a historical perspective and highlighted three themes that permeated the conference: (1) there is a diverse array of signals that stimulate IR; (2) IR is a sensitization process that primes the plant for more rapid deployment of defenses; and (3) when integrated into good agricultural practices, IR can both enhance plant productivity and resistance to disease.
1360-1385/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S1360-1385(01)02046-5
446
Research Update
Induced resistance: multiple pathways for an effective defense
Two types of IR are well characterized. Systemic acquired resistance (SAR) is dependent on salicylic acid-mediated signaling and is activated by benzo[1,2,3]thiadiazole-7-carbothioic acid S-methyl ester (BTH; BION®). Induced systemic resistance (ISR), which develops after colonization by some biocontrol rhizobacteria, is dependent on the sequential action of jasmonic acid and ethylene. SAR is correlated with an increase in salicylic acid and the expression of pathogenesis-related (PR) protein genes. Salicylic acid is essential for SAR, although is not the signal transported to distal sites to activate SAR. Chris Lamb (John Innes Centre, Norwich, UK) described the Arabidopsis dir1 mutant that does not produce the mobile SAR signal, but has a normal local defense response. DIR1 (a putative non-specific lipid-transfer protein) might be this signal or might be involved in production or transport of this elusive signal. Several laboratories discussed the importance of salicylic acid in priming cells for rapid expression of defense genes after elicitor or pathogen challenge. Uwe Conrath (University of Kaiserslautern, Germany) showed that salicylic acid accelerates increases in phenylalanine ammonia-lyase RNAs and MAP kinase activity after fungal elicitor treatments in parsley (Petroselinum crispum). Transgenic Arabidopsis that ectopically expressed a chimeric salicylate synthase (Brigitte Mauch-Mani, University of Fribourg, Switzerland) or NPR1/NIM1 (a regulator of the SAR and ISR signaling pathways) (Robert Dietrich, Syngenta, Research Triangle, NC, USA) have enhanced resistance to pathogens and accelerated PR expression after pathogen challenge. Application of other chemicals that stimulate IR, including BTH, β-aminobutryic acid (BABA) (Mauch-Mani), Messenger® (Zhongman Wei, Eden Biosciences, Bothwell, WA, USA), and OxycomTM (Anne Anderson, Utah State University, UT, USA) also accelerate defense gene expression. ISR is not correlated with major changes in RNA or protein profiles suggesting that ISR produces undiscovered defense products that contribute to resistance. Characterization of Arabidopsis isr1 (Juriaan Ton, Utrecht http://plants.trends.com
TRENDS in Plant Science Vol.6 No.10 October 2001
University, The Netherlands), tobacco etr1 (Bart Geraats, Utrecht University), and temporal accumulation of defense RNAs after ACC treatments in ISR-expressing plants (Corné Pieterse, Utrech University) suggest that enhanced ethylene sensitivity is crucial for ISR. Consistent with this theory, Karen Léon-Kloosterziel’s (Utrech University) poster described an Arabidopsis enhancer-trap line that responds to biocontrol rhizobacteria and the ethylene precursor ACC. Signal perception: elicitor-binding proteins and early signaling events in induced resistance
EDS1 mediates rapid perception of pathogens in several resistant plant–avirulent pathogen interactions. Jane Parker (John Innes Centre) described the necessity for the lipase-like proteins EDS1 and PAD4 in pathogen and insect resistance, ROS generation, and salicylic acid accumulation. Two-hybrid assays show that EDS1 and PAD4 physically interact suggesting that this affiliation might be essential for their action. Thomas Boller (Friedrich Miescher Institute, Basel, Switzerland) reported that the flagellin peptide (flg22) induces rapid alkalinization of culture media, callose deposition and an oxidative burst in tomato. However, flg22 does not affect monocots. FLG1 and FLG2 (leucine-rich repeat receptor kinase-like protein) influence flg22 perception or action in Arabidopsis. The harpin peptide (Messenger®) is another potent inducer of defense genes and IR (Wei). Putative harpin-binding protein (HrBP1) genes were identified in both monocots and dicots. Integrating multiple signaling pathways: salicylic acid and jasmonic acid cross-talk
An elegant and interactive computational model for cell signaling using Boolean gates was presented by Jean-Pierre Métraux (University of Fribourg). This model incorporates the complex network of signaling, including light signals (Métraux) and the reciprocal regulation of salicylic acid- and jasmonic acid-signaling pathways (Mauch-Mani, Pieterse, Thomma, and Métraux). However, these pathways are not always mutually exclusive and can provide a multi-layered defense. For example, both ISR and SAR can be induced simultaneously and their
effects are additive (Pieterse). Jasmonic acid- and salicylic acid -regulated genes are expressed together in response to many pathogens (Bart Thomma, Katholieke Universiteit Leuven, Belgium) and Messenger® (Wei), and salicylic acid and jasmonic acid can act synergistically (Luis Mur, University of Wales, Aberystwyth, UK). Richard Bostock (University of California, Davis, CA, USA) provided evidence that the timing of jasmonic acid and BTH applications, as well as their concentrations, influence the cross talk between these pathways, with lower concentrations of jasmonic acid and BTH allowing simultaneous expression of both pathways. ‘…both jasmonic acid perception in scions and biosynthesis in root stocks are crucial for the systemic accumulation of woundresponse RNAs.’
The characterization of the classes of mutants that influence light and defense cross talk (ltd, psi; Métraux), enhance susceptibility to pathogens (esa, Thomma; eds4-12, Ton), influence BABA perception or action (bai, Mauch-Mani), and the further characterization of known mutants that influence salicylic acid and jasmonic acid cross talk will substantially enhance our understanding of the intricacies of the defense network. Oxylipins: key regulators of defense responses
The octadecanoid pathway, which synthesizes jasmonic acid, is activated and amplified in response to pests or pathogens. Thierry Heitz (Insitut de Biologie Moléculaire des Plantes, Strasbourg, France) described the importance of a soluble phospholipase A2 in the release of linolenic acid from tobacco membranes, a crucial initial step in this pathway. Although jasmonic acid and the jasmonicbiosynthesis intermediates are known defense signals, the importance of other oxylipins is just being realized. Edward Farmer (University of Lausanne, Switzerland) showed that linolenic acid-derived 9- and 13-ketodienes accumulate after Pseudomonas syringae pv. tomato infection. These oxylipins and other molecules with the chemically reactive α,βunsaturated carbonyl group are potent signals for defense gene activation. Gregg Howe (Michigan State University, East Lansing, MI, USA)
Research Update
provided important new evidence for jasmonic acid’s involvement in systemic signaling in tomato. Reciprocal grafting experiments using control plants and mutants that disrupt jasmonic perception (jai1; a potential coi1 analogue) or jasmonic acid accumulation (spr2) showed that both jasmonic acid perception in scions and biosynthesis in root stocks are crucial for the systemic accumulation of wound-response RNAs. Systemin, a mobile peptide, activates the octadecanoid pathway in tomato. Andreas Schaller (ETH Zürich) identified a protease with a likely role in systemin catabolism. The tomato enzyme is similar to a mammalian insulin-degrading enzyme and removes three COOH-terminal residues from systemin to generate an inactive peptide. Herbivore-induced resistance: fitness costs and complexity of responses
Ian Baldwin (Max Planck Institute, Jena, Germany) discussed the fitness costs of jasmonic acid-induced defenses in Nicotiana attenuata. In the absence of herbivore pressure, jasmonic acid decreases seed production relative to the controls. However, in the presence of herbivores, jasmonic acid-treated plants have increased seed number relative to undamaged, jasmonic acid-treated plants. Erkki Haukoija (University of Turku, Finland) described a delayed IR, which is expressed in foliage a year after damage occurs in birch (Betula) and is influenced by light levels. This IR impedes larval growth and development, as well as influencing leaf size, branching and nitrogen and phytochemical content of plant organs. Martin Heil (Biozentrum, Würzburg, Germany) highlighted the need for continued evaluation of the fitness costs of IR. Herbivore-activated IR and its relationship to ISR and SAR is not completely understood. Jasmonic aciddependent defenses are important and influenced by the time and intensity of herbivore damage, as well as components of herbivore saliva. Genes activated by herbivores are dependent on the mode of feeding and components in saliva. Gary Felton (University of Pennsylvania, Philadelphia, PA, USA) showed that glucose oxidase (GOX) in Helicoverpa zea saliva suppresses many wound responses that are usually induced against tissuedamaging herbivores. GOX inhibits increases in polyphenol oxidase, http://plants.trends.com
TRENDS in Plant Science Vol.6 No.10 October 2001
peroxidase and lipoxygenase activities, nicotine and jasmonic acid. GOX also induces expression of PR genes, SAR and a tolerance to cold stress. Insects that do little damage to foliage activate different signaling pathways than most herbivores that chew or tear tissue. Phloem-feeding whiteflies (Linda Walling, University of California, Riverside, CA, USA) and aphids (Bostock) activate PR genes. In addition, closely related species of whiteflies evoke distinct differences in local and systemic gene expression using jasmonic acid-dependent or novel defense signaling pathways. Volatiles: potent signals for induced resistance
Over 100 different volatiles are released when damaged by herbivores, providing an indirect defense by attracting herbivore predators and parasites. Andre Keßler (Max Planck Institute) characterized the volatile blend released by N. attenuata when attacked in its natural environment. These volatiles are effective signals in the field, because treatment of field-grown plants with volatiles or jasmonic acid enhanced herbivore predation and deterred herbivore oviposition. Silvia Dorn (ETH Zürich, Switzerland) explained that, similar to foliar-feeding damage, volatiles released from herbivore-damaged apple fruit attract parasitoids and deter further herbivore infestation. Torsten Meiners (University of Berlin, Germany) also reported that egg deposition and oviduct secretions over small epidermal wounds on angiosperm and gymnosperm leaves can incite volatile production. Remo van Poeke (Wageningen University, The Netherlands) showed that larval feeding activates Arabidopsis genes that are important in the synthesis of several volatile classes (i.e. C6 volatiles, jasmonic acid, MeSA and terpenes). Marcel Dicke (Wageningen University) explained that the subset of volatiles induced by jasmonic acid are not as attractive to spider mite enemies, as the volatiles released after spider mite damage or when jasmonic acid-induced volatiles are supplemented with MeSA. Both the Marcel Dicke and the Ted Turlings laboratories (Maria Fritzche-Hoballah, University of Neuchâtel, Switzerland) reported that there were no apparent fitness costs to volatile production in two plant-herbivore-parasitoid interactions.
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Marcel Dicke emphasized that given the ability of volatiles to have an impact on the entire community (herbivores, pathogens, carnivores, and neighboring competing plants), their usefulness in resistance strategies needs to be carefully evaluated. Usefulness of induced resistance in the field
The usefulness of IR in disease control and plant productivity has been tested in the field. IR stimulated by biocontrol bacteria (Kees van Loon, Wageningen University), NIM1-expressing plants (Dietrich), BION® (Duncan McKenzie, Syngenta, Basel, Switzerland), OxyxomTM (Anderson), Messenger® (Wei) or Reynoutria sachalinensis extracts (Milsana®; Annegret Schmitt, BBA, Darmstadt, Germany; Hans von Amsberg, KHH Bioscience Inc., Raleigh, NC, USA) is effective against pathogens, pests and even parasitic plants (Holger Buschmann, University of Hohenheim, Stuttgart, Germany). In most cases, the mode and timing of IR inducer application significantly influences its effectiveness. IR alone is often insufficient to completely control disease or pest damage. However, when used in conjunction with good agricultural practices, plant productivity, quality and disease resistance increase substantially. Use of low levels of fungicides with NIM1 over-expressing plants, BION®, Milsana® or biocontrol bacteria enhances disease resistance. In addition use of BION® with cultivars expressing partial pathogen resistance (Enrique Torres, Universidad Nacional de Columbia, Bogata, Columbia) or the application of avirulent bacteria with biocontrol bacteria (van Loon) are other alternatives that enhance the usefulness of IR horticultural and crop plants in the field. Rapid advances that are being made in our understanding of the mechanisms of pathogen, pest and elicitor perception, and the elucidation of defense signal transduction networks will ultimately enhance and refine the applied IR strategies. It is likely that IR provides an environmentally friendly and economically viable disease control strategy. Linda L. Walling Dept of Botany and Plant Sciences, University of California, Riverside, CA 92521, USA. e-mail: [email protected]