Plant Growth-Promoting Abilities in Cyanobacteria

Chapter 23

Plant Growth-Promoting Abilities in Cyanobacteria A.N. Rai, A.K. Singh, M.B. Syiem Biochemistry Department, North-Eastern Hill University, Shillong, India

1. INTRODUCTION The success of modern agriculture greatly depends on the availability of nutrients such as nitrogen, phosphorus, iron, zinc, etc. In spite of vast availability, their easily metabolizing form is scarce. The reason being that majority of organic phosphates cannot be utilized by many organisms and that there is a continuous unidirectional flow of soluble phosphates from terrestrial environments to aquatic environments and that maximum amount of phosphates occurs in immobilized state as salts of calcium and iron. These elements are supplied to crop plants in combined form as chemical fertilizers. In view of the known limited availability of fossil fuel, chemical fertilization-based agriculture technology will become unrealistic. It is against this background that scientist have intensified their efforts to find sustainable source of nutrients for enhancing the production of agricultural crops. Cyanobacteria have the ability of mobilizing insoluble organic phosphates with the help of phosphatase enzymes (Bose et al., 1971; Dorich et al., 1985). The other limiting nutrient nitrogen which is also abundantly present in the biosphere cannot be assimilated by most of the primary producers. This process is known to occur primarily in few groups of prokaryotes exhibiting chemotrophic and phototrophic mode of nutrition. In this regard, members of cyanobacteria become very important because of their ability to perform simultaneously the nitrogen-fixation activity and higher plant-type oxygenic photosynthesis. These attributes allow them to dwell and supply fixed nitrogen and organic carbon in a variety of habitats and provide nutrients to crop plants. Although cyanobacteria add organic biomass and improve the quality of soil, they have been researched extensively for their potential as source of nitrogen biofertilizer in free-living conditions as well in symbiosis. Ammonia liberation activity of wild and mutant types of cyanobacteria isolated from different habitats have been evaluated (Kannaiyan et al., 1994; Thomas et al., 1991). Cyanobacterial biofertilizer potential and colonization of crop plants have been reviewed extensively (Syiem et al., 2017). The association of these microorganism influences plant growth, development, and susceptibility to pathogens (Prasanna et al., 2013, 2015). Besides their significant role as biofertilizers, cyanobacteria are known to excrete a great number of substances that influence plant growth and development. They have been reported to produce growth-promoting regulators (resembling gibberellin, cytokinin, abscisic acid, and auxin), vitamins (particularly vitamin B), amino acids, polypeptides, and exopolysaccharides that act as antibacterial, antifungal and toxin-like substances (Ahmad and Winter, 1968; Singh and Trehan, 1973; Rodgers et al., 1979; Marsalek et al., 1992; Grieco and Desrochers, 1978; Vorontsova et al., 1988; Kulik, 1995). Growth-promoting effects of cyanobacterial inoculation have been reported for crops such as rice, wheat (Triticum aestivum L.), soybean (Glycine max L. Merr.), oat (Avena sativa L.), tomato (Solanum lycopersicum L.), radish (Raphanus sativus L.), cotton (Gossypium hirsutum L.), sugarcane (Saccharum sp.), maize (Zea mays L.), chili (Capsicum annuum L.), bean (Phaseolus vulgaris L.), muskmelon (Cucumis melo L.), and lettuce (Lactuca sativa L.) (Venkataraman, 1972; Rodgers et al., 1979; Singh, 1988; Arif et al., 1995; Thajuddin and Subramanian, 2005; Karthikeyan et al., 2007; Maqubela et al., 2008; Saadatnia and Riahi, 2009).

Cyanobacteria. https://doi.org/10.1016/B978-0-12-814667-5.00023-4 © 2019 Elsevier Inc. All rights reserved.

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2.  PROMOTION OF PLANT GROWTH THROUGH IMPROVEMENT OF SOILS 2.1  Reclamation of Usar Lands Usar soils are defined as saline or alkaline patch of land. The patches of saline soils show high concentration of any kind of salt, electrical conductivity (EC) of soil saturation extract more than 4 dS cm−1, and pH between 7.5 and 8.5. Alkaline soils are characterized by elevated concentrations of exchangeable sodium ion (>15%), free carbonate and bicarbonate, low calcium, high pH value (8.5–10), and the EC of saturation soil extract less than 4 dS cm−1. Soil structure, infiltration rate, and aeration are very poor for alkaline soils. Such soils are hard, deficient in nitrogen, and poor in water-holding capacity. They become sticky when wet and rigid in dry condition. Increase in pH of alkaline soil results from exchange of hydrogen ion on negatively charged clay particle for positively charged ions. The high alkalinity, osmotic pressure, and impermeability of Usar soil renders them inappropriate for agricultural applications. Crop yield in arid and semiarid areas suffers from limited water resources. Irrigation of agriculture fields in arid region brings additional amount of salts which convert such fields in to Usar land after evaporation. Elevated level of salinity poses enormous difficulty in reclamation of sodic soil. Many states of India, for example, Uttar Pradesh (12.95 lakh hectares) and Rajasthan (12.14 lakh hectares) are burdened with huge areas of unproductive Usar lands. Thus there has been realization for the reclamation of such unfertile lands to fulfill the increasing demand for food for increasing global population. Different methods of reclamation of Usar soil that reduce the salt content include irrigation with clean water, treatment with gypsum (Dhar and Mukherji, 1936), cultivation of salt tolerant crops, and application of cyanobacteria. Cyanobacteria in general are alkaliphilic, photosynthetic bacteria which grow profusely in waterlogged condition. Studies on the role of blue-green algae (cyanobacterial) in nitrogen fixation in rice field (De, 1939) arose as a result of observation made by Howards (1924) on cultivation of rice plants on same land for a long time and without addition of fertilizers. Their ability to grow in waterlogged condition and to exchange high percentage of salt make them good candidate for reclamation of Usar soil. Role of blue-green algae (cyanobacterial) in reclamation of Usar land was suggested and shown long ago (Singh, 1950, 1961). A wild-type Nostoc calcicola and its HCO3 − -resistant mutant strains both exhibited ability to grow in medium amended with 20% and 60% Usar soil extract, respectively, besides decreasing the pH significantly (Jaiswal et al., 2010). Ability of cyanobacteria to grow in waterlogged and nitrogen limiting conditions as well as to associate with other life forms makes them an attractive biological tool for treatment of Usar land. Singh (1950) has listed comprehensively a number of cyanobacteria to grow naturally in the alkaline soil. These include Nostoc (N. commune, N. muscorum, N. punctiforme), Scytonema (S. ocellatum, S. javanicum), Microcoleus (M. chthonoplastes, M. vaginatus), Porphyrosiphon (P. notarisii), Camptylonema (C. lahorense), and Cylindrospermum (C. licheniforme, C. muscicola). At later stages, when the soil became waterlogged, forms like Aulosira fertilissima, various species of Anabaena, Cylindrospermum gorakhporense, and Wollea bharadwajae appeared. These populations were found to be dominated by N. commune therefore assigned more credit in bioamelioration (Singh, 1950). Since then various studies have critically evaluated the potential of cyanobacterial as bioameliorant of Usar/saline soil and as biofertilizer (Whitton, 2000; Hedge et al., 1999). Inoculation of cyanobacterial in experimental pot and field conditions has been reported to enhance the organic and nitrogen content of soils (Venkataraman, 1993). A number of studies have revealed beneficial role of blue-green algae in qualitatively improving physical, chemical and nutritional properties of salt affected soils (Kaushik and Subhashini, 1985). For example, exopolysaccharides from cyanobacteria increase the soil aggregation of saline soils (De Caire et al., 1997), stimulate growth promotion in other microorganisms, and increase the soil enzymes activities resulting in the release of nutrients required by plants (De Caire et al., 2000). Whereas improvement of soil texture and water-holding capacity occurs through addition of their reduced carbon contents, the status of soil fertility is enhanced simultaneously by way of nitrogen-fixation process mostly by heterocystous cyanobacteria.

2.2 Biofertilizers Success of modern agriculture greatly depends on the availability of nitrogen nutrients and the present source of this has been fossil fuel-based nitrogen fertilizer factories. There has been a steep increase in the use of pesticides and chemical fertilizer after the advent of modern agriculture. Globally, India is the second largest consumer of chemical fertilizers next only to China. According to FAO, the total fertilizer demand in the world for 2018 is expected to be in excess of 200 million MT including nearly 120 million MT demand for nitrogen fertilizers alone. Due to limited installed capacity of fertilizers, many countries including India import a significant amount of their fertilizer requirements at great cost to foreign exchange reserves. Although use of chemical fertilizer has made a large number of developing countries self-sufficient in food grain production, their applications have caused enormous damage to the environment by polluting and affecting quality of soil, ground water, and health. The replacement of agrochemicals by biofertilizers is considered an environmentally

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healthy practice. Application of biofertilizers improves the soil quality and decreases our dependency on chemical fertilizer and pesticides. The diazotrophs (prokaryotes with nitrogen-fixation ability) capable of growth at the simple expense of light, air, and water have been suggested to be an ideal system as source of fixed nitrogen in agricultural and nonagricultural ecosystems. It is against this background that scientists have intensified their efforts to understand the biological process of nitrogen fixation at molecular level in order to manipulate it as a renewable source of nitrogen in modern agriculture. Oxygenic phototrophic mode of autotrophic nutrition evolved with appearance of prokaryotes like cyanobacteria and prochlorophytes. In comparison to the prochlorophyte Prochloron, majority of cyanobacteria are well-established fixers of molecular nitrogen. Studies on biological nitrogen fixation started in late 1880s, a time when diazotrophy was receiving considerable attention. Suggestions that cyanobacteria might fix nitrogen (Frank, 1889; Prantl, 1889) came at about the same time that Hellriegel and Wilfarth (1888) and Beijerinck (1888) demonstrated the involvement of root nodule bacteria in nitrogen fixation by legumes. This led to the practice of using legume crop as a biological mechanism to sustain nitrogen budget of the agricultural land. Similarly free-living cyanobacteria have been suggested to function as sources of biofertilizer in rice fields of tropical countries (De, 1939; Watanabe, 1956; Singh, 1961; Venkataraman, 1972; Stewart et al., 1979) and their potential in temperate agricultural soils has also been identified (Jenkinson, 1977). In addition, many cyanobacteria especially Nostoc and Anabaena form symbiotic association with many higher plants and act as a source of nitrogen. Biofertilizers, the essential component of organic farming, are defined as environment friendly formulation of living or latent microorganisms with capability to promote plants growth by increasing the accessibility of nutrients to plants. Biofertilizers are widely applied to improve soil fertility by accelerating microbial processes in order to enhance the availability of fixed nitrogen, mobilization of inorganic nutrients such as P, Zn, Fe, and mineralization of organic compounds.

2.2.1  Free-Living Cyanobacteria Cyanobacteria are a highly diverse group of Gram-negative organisms with eukaryotic-type oxygenic photosynthetic activity. The diverse morphologies of cyanobacteria are unicellular forms, nonheterocystous filamentous forms, and filamentous heterocystous forms (Desikachary, 1959). While majority of heterocystous forms produce filaments without branching, some produce branched filaments, for example, Mastigocladus, and false branching pattern, for example, Scytonema and Tolypothrix. Most of the cyanobacteria from various morphological domains are naturally gifted to fix atmospheric nitrogen. They have been described as various species of filamentous nonheterocystous forms and filamentous heterocystous forms (Vaishampayan et al., 2001). Nitrogen-fixing enzyme nitrogenase is an extremely oxygen labile protein (Rai et al., 1992a,b). Depending upon the level of protections, nonheterocystous forms have evolved to fix nitrogen under anaerobic, microaerobic, or aerobic conditions, whereas all heterocystous forms perform nitrogen fixation under aerobic conditions (Bergman et al., 1997). Early studies have revealed the role of heterocysts in nitrogen fixation. Fogg (1949) observed repression of heterocyst formation in combined nitrogen supplemented cultures. Heterocyst as the site of nitrogen fixation was reported by Fay et al. (1968). These fundamental studies were followed by comprehensive understanding of functioning of nitrogen fixation both in bacteria and cyanobacteria (Brill, 1983; Mulligan and Haselkorn, 1989; Kim and Rees, 1992a,b). Investigations have been performed to understand the regulation of heterocyst differentiation, nitrogenase activity, and ammonium transport activity in the cyanobacterium N. muscorum (Singh et al., 1989). Fixed nitrogen in cyanobacteria is assimilated via glutamine synthetase-glutamate synthase (GS-GOGAT) pathway (Stewart and Singh, 1975; Wolk et al., 1976). Cyanobacterial mutants resistant to chemical inhibitors of GS-GOGAT pathway have been shown to produce extracellular ammonia and partially defective biosynthetic activity of glutamine synthetase (Thomas et al., 1991; Singh et al., 1992; Mahasneh et al., 1994). Further even manipulation of growth conditions has been shown to induce the extracellular release of ammonia (Kannaiyan et al., 1994). Fixed nitrogen from the diazotrophs becomes available to neighboring plants either as free liberated ammonia or as organic nitrogen after death and decay of cells. A lot of released nitrogen is lost due to microbial attack. Such wastage can be prevented to a certain extent if fixed nitrogen is released directly on to the surface of host plants, for example, wheat and rice. In nature, phylogenetically diverse group of cyanobacteria occur but all are not equipped with colonization potential. Various laboratories therefore have conducted investigations to screen and evaluate the ability of cyanobacteria isolates to associate with rice plants (Nilsson et al., 2002; Syiem et al., 2007). In one such report, Singh et al. (2006) have analyzed colonization of roots and submerged segment of shoots of rice by a symbiotic strain of Nostoc, and associative N2-fixation activity. The colonization pattern was found to be biphasic and occurred on seedlings grown in both N2-media and combined nitrogen supplemented media. It was maximum in NO3 − media and under light. Most importantly, nitrogenase activity was always higher in associated condition than in free-living state. A study using 45 cyanobacterial isolates revealed colonization of roots of rice by about 50% of isolates only (Akoijam et al., 2012). A simple denaturing gradient electrophoresis (DGGE)-based method for screening the competent strains has also been suggested (Akoijam et al., 2012).

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These results suggest that a prior screening of the cyanobacterial strains should be performed for construction of efficient biofertilizer consortia. Cyanobacteria help in acceleration of seed germination and promote seedling growth (Gupta and Lata, 1964). Significant positive effects on seed germination and seedling growth have been observed in case of Senna notabilis and Acacia hilliana when seeds bio-primed with cyanobacteria genera Microcoleus and Nostoc were used (Muñoz-Rojas et al., 2018). This may be useful for restoring degraded ecosystems. Cyanobacteria grow luxuriantly under waterlogged conditions in paddy fields and sustain the N-fertility of such soils (Watanabe and Roger, 1984). Contribution of cyanobacteria toward total nitrogen budget of rice fields depends on their nitrogenfixation activity which is governed mainly by physical, chemical, and biotic factors. Estimates of the addition of fixed-N by cyanobacteria in rice fields are reported to be 18–45 kg N ha−1 (Watanabe and Cholitkul, 1979), 90 kg N ha−1 (Metting, 1981), and 20–30 kg N ha−1 (Issa et al., 2014). The environment-friendly nature of cyanobacterial biofertilizer application has drawn the attention of researchers worldwide. Biofertilization of degraded soils leads to improved organic matter content and quality (Pardo et al., 2010). The development and application of the cyanobacterial biofertilizers have been reviewed in recent past (Kaushik, 2009; Majeed et al., 2017; Syiem et al., 2017). Interaction between maize hybrids and cyanobacterial isolates has been carried out for identifying promising combination for improved yield (Prasanna et al., 2016a). The study revealed a considerable positive influence of cyanobacteria-based bioinoculants on soil nutrient availability, plant height, and yield with AnabaenaTrichoderma biofilm being most effective. Application of Anabaena torulosa-Trichoderma viride biofilm formulations led to increased yield and showed promise as plant growth promoting and disease-suppressing agent (Prasanna et al., 2016b). In a similar study involving interaction of cyanobacterial consortium and Anabaena-Trichoderma biofilm with Chrysanthemum showed promising positive effects in improving soil fertility (Prasanna et al., 2016c). Mohan and Kumar (2017) analyzed effect of 15 cyanobacterial strains individually and in combination showing improved growth performance and yields in cases of T. aestivum, Z. mays, and Hordeum vulgare. It was also observed that application of cyanobacterial consortia was more effective than the use of individual cyanobacterial strains. Two Nostoc isolates from a salt affected area have been used as biofertilizer for bioremediation of salt-affected soils and improved productivity under semiarid condition (Nisha et al., 2018). Several developing countries like India, China, and Brazil have been exploiting cyanobacterial biofertilizer potential in rice cultivation. In India, mass production of cyanobacterial biofertilizer was initiated at IARI, New Delhi (Venkataraman, 1972). In a laboratory-based study, ammonia-excreting strains of cyanobacteria have been tested in terms of their ability to support the growth of wheat plant (Spiller and Gunasekaran, 1990) which was found to be maximum when cyanobacteria were associated on the roots (Spiller et al., 1993). One of the disadvantages of ammonia-excreting biofertilizer strains is that they grow at slow rates in comparison with wild types. Considering their relatively slow growth rates, maintenance of such strains in batch cultures may lead to a gradual decrease of their biofertilizer potential. This might happen due to changes in environmental conditions leading to contamination with microbes with faster growth rate, or production of metabolic products by contaminants that might inhibit the growth of target biofertilizer strains. These problems can be resolved by mass cultivation of the biofertilizer strains in raceway open ponds and in raceway photo-bioreactors (Wang et al., 1991; Boussiba, 1993; Moreno et al., 2003; Silva and Silva, 2013) in strictly controlled conditions. During outdoor mass cultivation, care should be taken to use region specific strains. It is more appropriate to inoculate crop fields with consortia comprising at least five or six biofertilizer strains. The inocula raised in such ways can be preserved before field applications (Silva et al., 2007; Esteves-Ferreira et al., 2013). There are various methods reported and in use for preservation of biofertilizer strains/inocula. The accepted methods of preservation are to keep them on agar slant, immobilization in calcium alginate beads and agar flakes, lyophilization, and cryopreservation. Many studies in the past have shown viability of using immobilization matrix for long-term preservation of cyanobacterial inocula with no changes in native characters. In a similar study, preservation of Spirulina in agar flakes has been successfully demonstrated. Cyanobacterial spores (akinetes) which develop in response to adverse environmental conditions represent a highly resistant phase in cyanobacterial life. Methods have been developed for induction of sporulation in the cyanobacterium Nostoc ANTH (Kyndiah and Rai, 2007). They evaluated impact of pH, temperature, addition of various carbon sources, and limitations of phosphate and sulfate on akinete formation. Among these treatments, only phosphate and sulfate-induced akinete formation. However, under sulfate limitation induction of akinete differentiation was quicker resulting in profuse akinete differentiation. The akinetes due to their inherent capacity to endure harsh environmental conditions can be stored for long term. In addition, they are easy to transport from their place of storage to application site. Further, akinetes can be mixed with rice seeds and applied directly in the rice paddies or adsorbed on seedling roots and transplanted.

2.2.2 Cyanobacteria-Azolla Symbiotic Systems The heterosporous aquatic fern Azolla, a member of family Salviniaceae, is widely distributed in freshwater habitats in temperate and tropical climates. It has a fast growth rate and is very effective as biofertilizer. The biofertilizer property results from occurrence of nitrogen fixing cyanobacteria in leaf cavities which are present on the dorsal leaf lobe of Azolla. The

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extracellular cavity is approximately 0.3 mm long with a narrow opening to outside (Lechno-Yossef and Nierzwicki-Bauer, 2002). Cyanobacterial filaments are distributed in periphery region of the cavity and embedded in mucilage with center being gaseous. The diazotrophic partners (cyanobiont) in most of the associations are genera of heterocystous cyanobacteria particularly Nostoc and Anabaena. Under symbiotic association, cyanobacteria have enormous biological N-fixation ability (30–100 kg N ha−1 crop−1) and hence has been considered as a valuable source of nitrogen for rice crop (Ito and Watanabe, 1985; Singh and Singh, 1987; Roy et al., 2016). The beneficial effect of Azolla in enriching the rice paddies with fixed nitrogen and thus enhancing yield has been known for centuries (Fogg et al., 1973; Watanabe and Liu, 1992). Historically use of Azolla in China dates back to the record of Ming dynasty (AD 1368–1644) agricultural book (Lumpkin and Plucknett, 1982). It has been reported that cultivation of Azolla adds approximately 300 tons of green biomass per hectare in a year under normal subtropical climate which is equivalent to 800 kg N (1800 kg of urea) (Wagner, 1997). Besides its value in crop production, Azolla has also been suggested for bioremediation of industrial effluents and sewage wastewater (Sood et al., 2012), as animal feed, human food, and medicine (Wagner, 1997). The quantity of crop production in Azolla fertilized field depends on the manner of application. Prior cultivation of Azolla as monocrop has been shown to increase rice yield by 112% over control, and by 23% when applied as intercrop and by 216% when applied both as monocrop and intercrop (Peters, 1978). During growth of the fern in combined nitrogen-free medium, the cyanobiont releases fixed-N as ammonia to the host plant. But addition of exogenous ammonia interrupts the inorganic nitrogen metabolism and appears to affect the sustained maintenance of symbiosis (Newton and Selke, 1981). Usually in rice paddy, farmers allow Azolla to produce a dense mass. At later stages, growing rice plant eventually displaces the Azolla which decays in soil easily and subsequently releases fixed nitrogen which is utilized by rice plants. In addition, it provides phosphorus, potassium, zinc, iron, molybdenum, and other micronutrients (Mahanty et al., 2017). Under the present scenario of growing environmental concern associated with production and application of chemical fertilizer, Azolla offer promising environment friendly alternative for wetland rice paddy in India, Vietnam, and China (Singh and Singh, 1987). In India, Azolla pinnata is most commonly propagated (Mazid and Khan, 2015) and has been tested for increasing N content of rice paddies. In a comprehensive field experiment conducted during three successive seasons, average Azolla biomass of 38–63 and 43–64 tons ha−1 equivalent to 64–90 and 76–94 kg N ha−1 with significantly enhanced growth and yield of rice crop has been reported (Singh and Singh, 1987). Further study by the same group has shown that application of higher level of urea fertilizer reduced the growth of Azolla and cyanobacteria with more reduction in cyanobacteria than Azolla (Singh et al., 1988). A recent study has further emphasized importance of Azolla biofertilizer when partially substituted for chemical nitrogen fertilizer and showed that use of Azolla biofertilizer substantially increased nitrogen use efficiency due to enhanced nitrogen uptake and reduced loss of nitrogen (Yao et al., 2018). Therefore, Azolla biofertilizer provides a financially attractive and environmentally friendly option.

3.  PROMOTION OF PLANT-GROWTH THROUGH DIRECT TRANSFER OF FIXED NITROGEN 3.1  Cyanobacterial-Plant Symbioses Although suited for independent existence in nature due to their ability to fix both atmospheric CO2 and N2, some cyanobacteria enter into symbiotic association with a wide range of hosts. Such symbioses have evolved independently into phylogenetically diverse genera belonging to algae, fungi, bryophytes, pteridophytes, gymnosperms, and angiosperms among plants (Table 1). These N2-fixing symbioses contain heterocystous cyanobacteria, particularly Nostoc, as their cyanobacterial partners. The cyanobiont partner is hosted in a variety of host tissues under microaerobic environment. For a particular host species, the cyanobiont genus is specific although specificity does not extend to the strain level. Once the symbiotic association is established, the symbiosis is often stable over a wide range of environmental conditions. An intricate system of signaling and sensing is established between the partners to achieve optimization in synchronized growth, development and maintenance of the symbioses. Host controls the cyanobacterial population in relation to its biomass through controlled initiation of infection, nutrient supply and cell division. The main objective of forming a symbiosis is acquisition of fixed nitrogen for the host while it may be the advantage of a stable housing that is protected from environmental variations as well as regular supply of nutrients for the cyanobiont. Hosts carefully mediate structural-functional changes in the cyanobiont to suit their role as nitrogen provider to the far bigger host (Rai et al., 2000). Although over the billion years of evolution, symbiosis endowed all plants (except fungi) with chloroplast leading to autotrophy in carbon requirement, nature is yet to make provision of N-autotrophy to plants. However, many plants have

TABLE 1  Cyanobacterial-Plant Symbioses Host Plant Group

Plant Partner

Cyanobiont

Remarks

Autotrophs Algae (diatoms)

Freshwater: Rhopalodia, Epithemia, Denticulata

Coccoid cyanobacteria

Diatoms: Rhopalodia gibberula, R. gibba; Epithemia adnata, E. turgida, E. sorex, E. zebra; Denticulata vanheurcki. Bluish-green inclusions visible in the cytoplasm have been identified as cyanobacteria

Marine: Bacteriastrum Chaetoceros, Hemiaulus, Rhizosolenia Steptotheca Neostreptotheca Roperia

Richelia, Calothrixa

Bacteriastrum, Chaetoceros, Roperia tessellate, Steptotheca indica, Neostreptotheca subindica, Hemiaulus indica, H. hauckii, H. membranaceus, H. sinensis; Rhizosolenia clevei and other speciesb form symbiotic associations. The cyanobiont is in the periplasmic space

Hornworts: Anthoceros Notothylas Dendroceros Phaeoceros

Nostoca

Four of the six extant hornwort genera are symbiotically competent

Liverworts: Blasia Cavicularia

Nostoca

Only 2 of the 330 known liverwort genera form symbiosis

Mosses: Sphagnum

Nostoca

Cyanobacterium inhabits the hyaline cells of the moss.

Pteridophytes

Water fern: Azolla

Nostoc sp.c

Symbiosis is intercellular. All seven species of the genus Azolla undergo symbiosis with Nostoc sp.: Azolla mexicana, A. microphylla, A. caroliniana, A. rubra, A. filiculoides, (A. japonicum) A. pinnata (var. pinnata and imbricata), A. nilotica. Cyanobiont partner resides in the mucilage filled cavities on the ventral surface of the dorsal lobes of the leavesk

Gymnosperm

Cycadaceae: Cycas Stangeriaceae: Stangeria, Zamiaceae: Bowenia, Ceratozamia, Dioon, Encephalartos, Lepidozamia, Macrozamia, Microcycas, Zamia.d

Nostoc sp.a

Symbiosis is intercellular. All known cycads (150 spp. of 10 genera belonging to 3 families) are symbiotically competent. The cortical zone of the coralloid roots of cycads hosts the cyanobiont

Angiosperm

Haloragaceae: Gunnera

Nostoc sp.a

Symbiosis is intracellular. All 65 known species of Gunnerae form symbiosis. Cyanobiont is hosted in the stem nodules

Heterotrophs Fungi

Lichenized Other

Nostoc,a Scytonema,a Fishcerella,a, f, Calothrix,a unicellular cyanobacteriag Nostoc

The fungal symbioses are intercellular in nature. ~20% of all known fungi form symbioses known as lichens. 10% of known lichens are bipartite containing cyanobionts as both carbon and nitrogen provider. Tripartite lichens (3%–4%) contain both a cyanobiont and a green alga as phycobionth Only one coenocytic soil fungus closely related to arbuscular mycorrhiza-forming fungi of the genus Glomusi called Geosiphon pyriforme (phycomycete), is known till date. The cyanobiont is present in the bladders

Bryophytes

a

Symbiosis is intercellular. Cyanobiont is found in mucilage filled cavities on undersurface of gametophytic thallus of hornwort/liverwort j

Heterocystous form. Villareal (1992), Schenk (1992), and Carpenter et al. (1999). Usually called Anabaena azollae. d Stevenson (1990) suggested a new genus, Chigua. Phylogenetic analysis with chloroplast DNA suggest that it is a sister group of the genus Zamia (Caputo et al., 1991). e Bergman et al. (1992a,b). f Fischerella includes Hyphomorpha, Stigonema, and Mastigocladus. g Gloeocapsa, Chroococcus, Synechocystis, Aphanocapsa, Microcystis (Stewart et al., 1983; Galun and Bubrick, 1984; Bergman et al., 1997; Rai et al., 2000). h Honeggar (1991), Schenk (1992), and Hill (1994). Complications continue in the identification and nomenclature of mycobionts, requiring a combination of morphological, physiological, and molecular approaches (Jorgensen, 1991; Gargas and Taylor, 1992; Bridge and Hawksworth, 1998; Nimis, 1998; Rambold et al., 1998). i Schüßler et al. (1994) and Gehrig et al. (1996). j Two other hornworts, Megaceros and Folioceros, have no reported cyanobionts (Meeks, 1990). k Grouped into sections Rhizosperma (A. pinnata and A. nilotica) and Azolla (the other five spp.) (Braun-Howland and Nierzwicki-Bauer, 1990; Watanabe and Van Hove, 1996). b c

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acquired this illusive N-autotrophy by an indirect mode via symbiosis with nitrogen-fixing cyanobacteria particularly the genus Nostoc. The major role of cyanobacteria in symbiosis is N2 fixation and provision of fixed nitrogen to the hosts. Except in bipartite lichen symbiosis, the cyanobiont becomes functionally nonphotosynthetic in all other symbioses and fixed carbon is supplied to the cyanobiont by the host plants. In nonphotosynthetic fungal partners cyanobacteria have the added burden of providing fixed-C to the host as well as produce enough fixed-C to provide extra energy required for its enhanced N2 fixation. In all plant-cyanobacterial symbioses, fixed nitrogen is directly transferred from the cyanobacterial partners to host. Thus, responsibility of promoting wholesome plant growth lies on the N2 fixation by the cyanobiont. For this, the cyanobiont undergoes several alterations in their growth, morphology, photosynthesis and carbon metabolism as well as in the N2 fixation and assimilation of fixed nitrogen. As pointed out earlier, the most crucial aspect of plant-cyanobacterial symbiosis is N-metabolism. By bringing in changes such as increase in heterocyst frequency (cells where N2 is fixed), increasing the levels of nitrogenase enzyme, and by decreasing GS (the enzyme for N-assimilation) activity, the cyanobacteria in symbiosis carry out enhanced level of N2 fixation and transfer the fixed-N to the host. The cyanobacteria that are most commonly found in symbiosis belong to the genus Nostoc (Dodds et al., 1995; Friedl and Budel, 1996; Singh et al., 2016). Few others that occur in symbiosis are listed in Table 1. A few unicellular symbionts are restricted to diatoms and lichens. The filamentous types of cyanobacteria that are found in symbiosis are all heterocystous. In the absence of combined-N free-living cyanobacteria develop regularly spaced heterocysts with a frequency of 5%–10% of the total cells. Heterocysts are the site of N2 fixation and the enzyme nitrogenase is expressed in heterocysts as heterocysts’ morphology provides conducive environment for protection of nitrogenase against exposure to O2 which is otherwise irreversibly toxic to the enzyme (Gallon, 1992). When cyanobacteria enter into a symbiotic relationship, they undergo an altered spacing pattern increasing heterocyst percentage except in diatoms and bipartite lichens (Rai et al., 2000). In liverworts, cycads and Gunnera symbioses, the heterocyst frequency exceeds 40% and many heterocysts occur in double or multiple numbers in a row. Although heterocyst differentiation occurs in response to N-starvation in free-living cyanobacteria, in symbioses heterocyst differentiation occurs even when cyanobacteria do not show any sign of N-starvation (Rai, 1990a; Bergman et al., 1992a). Heterocyst (%) in cyanobionts is correlated with their carbon status. In bipartite lichen, where cyanobacteria provide both fixed carbon and nitrogen to the mycobiont, there is no increase in heterocyst frequency. In tripartite lichen where the cyanobacteria meet only their own carbon demand, the heterocyst frequency is moderately increased to 15%. However in other symbioses (Azolla, bryophytes, cycads, and Gunnera) where the hosts provide the fixed carbon, heterocyst frequency reaches a percentage as high as 30%–80%. Thus, differentiation of heterocysts is related to overall adjustments and modifications brought about during establishment of a functional and successful symbiosis. As pointed out earlier, cyanobionts in all plant-cyanobacteria symbioses fix atmospheric N2 (Stewart et al., 1983; Rowell and Kerby, 1991; Kluge et al., 1992). With increase in heterocyst frequency, the rate of N2 fixation is also significantly higher in the cyanobiont (Stewart and Rodgers, 1977). Except in diatoms and lichens, the total energy expense of N2 fixation is met by the host in the form of fixed carbon. GS is the primary ammonia-assimilating enzyme in diazotrophic cyanobacteria. In free-living cyanobacteria the intracellular activity and concentration of GS in heterocyst are double than in vegetative cells (Bergman et al., 1985), as enhanced expression of GS is required to assimilate N2-derived ammonia. There is an overall decrease in GS activities of cyanobionts in lichens (>90%), Azolla (>70%), bryophytes (>60%), and the angiosperm Gunnera (Rai, 1990a; Rai et al., 2000). Decrease in GS concentration and activity in the cyanobionts results in liberation of symbiosis-induced ammonia to the hosts. However, the intracellular concentration of GS and its distribution pattern in cycad cyanobiont is similar to those of free-living isolates (Lindblad and Bergman, 1990) and in these symbioses fixed-N from the cyanobiont is transferred to the host in the form of amino acids (see Rai et al., 2000). Having established these modifications in the lifestyle of the cyanobiont a biotrophic nutrient exchange is established between the partners as the symbiosis progresses. From young to mature symbiotic organs and tissues, there is increasing amount of fixed nitrogen released by the cyanobiont whereas increased carbon transfer occurs from the host to the cyanobiont (Table 2). However, in bipartite lichens both nitrogen and carbon are increasingly released by the cyanobiont to the mycobiont as one moves from young to mature parts of the lichen. In context of N2 fixation and transfer, it is a feature of all ­plant-­cyanobacteria symbioses where there is movement of fixed nitrogen from cyanobacteria to hosts (Rai, 1988, 1990a; Rowell and Kerby, 1991; Villareal, 1991, 1992). N2-derived ammonia transfer represents 50%–90% of N2 reduced by the cyanobacteria in lichens, 80%–90% in liverworts and hornworts, 40%–50% in Azolla, and 90% in Gunnera (Rai, 1990a;

466 Cyanobacteria

TABLE 2  Heterocyst Frequency, Status of Cyanobiont Photosynthetic Activity, and Nutrient Exchange in Cyanobacterial-Plant Symbioses Nutrient Exchange Host

Photosynthesis Activity (Cyanobiont)

Existence of Heterocysts

Fixed Carbon

Diatoms

Active

Unchanged

Indeterminate

Most likely as NH4 + to the host

Bryophytes

Inactive

40%–45%

Probably sucrose to cyanobiont from host

As NH4 + from cyanobiont to the host

Azolla

Inactive

25%–30%

As sucrose from the host to cyanobiont

As NH4 + from cyanobiont to the host

Cycads

Inactive

~45%

Fixed carbon to host from cyanobiont

Probably as glutamate/ citrulline from cyanobiont to the host

Gunnera

Inactive

60%–80%

Fixed carbon to host from cyanobiont

As NH4 + from cyanobiont to the host

Bipartite lichen

Active

Unchanged

As glucose by the cyanobiont to host

As NH4 + from cyanobiont to the host

Tripartite lichen

Active

15%–35%

No transfer of fixed-C from cyanobiont to mycobiont

As NH4 + from cyanobiont to the host

Fixed‑Nitrogen Transfer

Fungi:

Janson et al., 1995; Silvester et al., 1996). The ammonia received by the mycobiont in lichen symbiosis is assimilated via glutamate dehydrogenase (GDH) but in other hosts it is accomplished by GS-GOGAT pathway (Peters and Meeks, 1989; Meeks, 1990; Rai, 1990b; Silvester et al., 1996). Translocation of fixed nitrogen from the symbiotic tissues to other parts of the host occurs in the form of amino acids. In tripartite lichen it is alanine while in Azolla it is glutamine, glutamate, ammonia, and a derivative of glucose move from leaf cavities to stem apex. In cycads it is a mix of amino acids. In Gunnera, asparagine is the primary compound exported through the phloem to the rest of the plants (Stewart et al., 1983; Bergman et  al., 1992b; Stock and Silvester, 1994; Silvester et  al., 1996). The translocated amino acids further donate the amino groups in various biosynthetic pathways via transamination reactions in order to synthesize rest of the amino acids and nucleotides crucial of protein or nucleic acid biosynthesis required for cell division and overall growth of the plants (Fig. 1). The previous section relates to cyanobacteria’s contribution toward promotion of plant growth through direct transfer of fixed nitrogen. The exchange of nutrients in the forms of fixed nitrogen and fixed carbon from and to the cyanobiont during symbiosis occurs at cyanobiont-host interface where the cyanobiont is in close contact with the host (Rai et al., 2000) (Fig. 1). In mutualistic symbioses, such membranes at the interface are known to possess H+-ATPases indicating the presence of energy-dependent transport system (Smith and Smith, 1990; Quispel, 1992).

3.2  Other Cyanobacterial-Plant Associations In natural habitats soil-plant-microbe interactions are a complex phenomenon and influence of such interactions on plant growth, health and productivity are difficult to quantify, although it is well established that such close interactions do influence plant health (Prasanna et al., 2012). The increasing world population and subsequent demand on food has increased use of chemicals fertilizers leading to contamination and degradation of soil quality (Vaishampayan et al., 2001; Shariatmadari et al., 2013). This has led to various investigations into soil-microbe-plant interactions in quest to improve soil health, fertility, and nutrient status of commercially important plant such as rice, wheat, soybean, etc. Research in plant growth-promoting rhizobacteria (PGPR) and

Plant Growth-Promoting Abilities in Cyanobacteria Chapter | 23  467

Host-plant transfer cell

N2

Heterocyst

Nitrogenase

Transfer cell

Host cell

NH3 NH3 Glutamine synthetase

NH4+ Glutamine synthetase

GA T

Sugars

GO

Sugars

Glu

Gln Sugars

CO2-fixation (photosynthesis)

Amino acids

Glu

Gln

Vegetative cell

Proteins

Amino acids

Nutrient translocation

N compounds Sugars

FIG. 1  Nutrient exchange in plant-cyanobacterial symbioses.

in diazotrophic cyanobacteria is gaining momentum to find viable alternatives for sustainable agriculture (Morrissey et al., 2004; Frank et al., 2006; Piromyou et al., 2011). Despite the proven benefits of cyanobacterial coculture/association with crop plants especially rice, information about their role as plant growth-promoting inoculants/microbes are scarce (Gupta et al., 2013). However, research in this field is gathering momentum. Nilsson et al. (2002) have shown that many Nostoc sp. colonize rice roots on surfaces and intercellular spaces. Associative N2 fixation was shown to be higher than that by the free-living cyanobacteria. Positive influence on growth of vegetables such as cucumber (Cucumis sativus), tomato (S. lycopersicum), and squash (Cucurbita maxima) was shown in the presence of applied cyanobacterial inoculant during cultivation (Shariatmadari et al., 2013). Similar beneficial effects of various cyanobacterial strains and novel cyanobacterial-based biofilms on plant growth of rice, wheat, tomato have also been recorded (Prasanna et al., 2009, 2014; Triveni et al., 2012a,b). In recent times concept of “organic agriculture” with minimum use of chemical fertilizer is becoming popular with increasing application of biological inputs such as compost, farmyard manure, green manure, and biofertilizers. Among various biofertilizers, cyanobacteria constitute the major input in rice cultivation. They form an inexpensive farm grown input that improves soil fertility due to their carbon and nitrogen fixing abilities that aids to soil nutrients upon their turnover. Cyanobacteria are abundantly found in rice fields and many are seen to colonize submerged rice-shoots (Whitton et al., 1988). There are reports of cyanobacterial presence in the rhizosphere of several grasses (Rother et al., 1988; Gantar et al., 1991). Spiller and Gunasekaran (1990) reported an ammonia-excreting Anabaena variabilis mutant SA-1 that enhanced growth of wheat plants. Evidences are accumulating on production of signals in the cyanobionts that affects gene expression in host plants leading to qualitative and quantitative changes in soil microflora in the rhizosphere (Whitton et al., 1988; Karthikeyan et al., 2007). Further, there are evidences of presence of extra- and intracellularly associated cyanobacteria in wheat roots as in the case of rice roots (Nilsson et al., 2002) which show ability in producing growth-promoting substances (Karthikeyan et al., 2007). Three cyanobacteria (Calothrix sp., and two Anabaena sp.) were shown to influence nutritional quality of wheat grains in terms of protein content and important micronutrients (Fe, Cr, Zn, and Mn) indicating positive flow of fixed nitrogen from the cyanobacterial inoculants to the wheat plants for improved production of proteins (Rana et al., 2012). Researchers are now engaged in screening different bacterial and cyanobacterial strains as effective plant growthpromoting inoculants for various crops’ growth, development, and productivity (Prasanna et al., 2012; Gupta et al., 2013;

468 Cyanobacteria

Shariatmadari et al., 2013). They are also looking into making formulations of such inoculants using combination of bacteria and cyanobacteria in a consortium and evaluating their synergistic effects as biofertilizers for economically important crops such as rice, wheat, and vegetables. It has been shown that cereal plants inoculated with PGPR show a positive impact on root length, root weight, root volume, root area, shoot weight, and pinnacle weight (Adesemoye et al., 2009; Mader et al., 2011). Relatively higher impact was recorded when the wheat plants were inoculated with a combination of cyanobacterial and bacterial strains (Yanni and Dazzo, 2010; Prasanna et al., 2012) indicating inclusion of cyanobacteria produced superior results as compared to using PGPR alone. This can be directly attributed to availability of fixed-N from the cyanobacterial inoculant as other PGPR strains do not have the capability to fix atmospheric nitrogen. Soil enzyme activities are more sensitive to environment and therefore, reflect soil quality directly and quickly (Allison and Jastrow, 2006). A five to six fold increase in the soil alkaline phosphate and dehydrogenase activities in soil rhizosphere upon enrichment with microbial inoculants containing cyanobacteria indicated positive influence of these organisms on soil quality which in turn positively modulated plant growth (Mader et al., 2011; Prasanna et al., 2012). There are many reports of cyanobacteria’s favorable influence on crop plant growth besides enhancing carbon status of soil through their photosynthetic activities aiding to soil fertility (Karthikeyan et al., 2007, 2009; Prasanna et al., 2008, 2009, 2014). Other than that cyanobacteria can contribute to about 20–30 kg N ha−1 year−1 (Issa et al., 2014). Several cyanobacterial species such as N. muscorum, A. variabilis, Tolypothrix tenesis, and A. fertilissima have been established as effective biofertilizers and many Asian countries like India, China, and Vietnam have been using cyanobacteria in paddy cultivation for centuries as an alternative to chemical fertilizers (Venkataraman, 1972; Lumpkin and Plucknett, 1982). Applications of cyanobacteria as inoculant in wheat and rice cultivation have been reported to enhance plant root/ shoot length, dry weight, and yield (Stewart et al., 1968; Peters et al., 1977; Spiller and Gunasekaran, 1990; Obreht et al., 1993; Prasanna et al., 2014).

4.  PROMOTION OF PLANT GROWTH THROUGH DIRECT TRANSFER OF FIXED CARBON Cyanobionts present in bipartite lichen symbioses have unique role as providers of both fixed carbon and fixed nitrogen. Lichens are composite organisms made up of a fungus usually an ascomycete that grows symbiotically with an alga and/ or a cyanobacterium. Most common cyanobacterial genus in lichen symbioses is Nostoc, although several other genera, for example, Chroococcidiopsis, Gloeocapsa, Sertonema, and Stigonema are known to associate in forming different lichens. Lichens are classified as bipartite or tripartite depending on the number of partners involved in formation of the symbioses. In bipartite lichens where the photobiont is a cyanobacterium, the lichen is classified as cyanolichen. Since the mycobiont (fungal partner) is photosynthetically inactive, the burden of entire provision of both fixed carbon and nitrogen falls on the cyanobiont partner. In these symbioses, the cyanobiont is photosynthetically active and fix CO2 via C3 pathway. Significant level of CO2 fixation (15%–20% of that of light) also occurs in darkness via C4 pathway. In bipartite lichen, 70%–80% of total fixed CO2 is released by the cyanobiont to the mycobiont. The transfer of fixed carbon mostly occurs in light and in the form of glucose (Rai and Bergman, 2002). 14C labeling experiments in Peltigera polydactyla, Peltigera canina, and Cora paviona showed that phosphoglyceric acid is the most labeled initial product after 30S of exposure time (Rai, 1990a,b). The transferred glucose is converted to mannitol in lichenized fungi probably to safeguard the acquired glucose from being used up by other symbiotic partners. 15N-tracer studies have shown that the highest initial labeling was in NH 4 + followed by gradual increase of 15N labeled Gln, Asp, and Ala. NH 4 + assimilation in the cyanobacteria of P. canina appeared to be via GS-GOGAT pathway and via GDH in the fungus. The cyanobiont assimilated around 44% of the 15N2 fixed and the remainder was liberated almost exclusively as NH 4 + to the mycobiont. However, the unique feature of cyanobiont of lichen that sets it apart from its role in other symbioses is its role as provider of fixed-C, in addition to fixed-N provider, and transfer to the fungal partner for maintenance of the symbiotic association (Fig. 2).

5.  PROMOTION OF PLANT GROWTH THROUGH PRODUCTION OF GROWTH HORMONES, VITAMINS, AND OTHER SUBSTANCES Cyanobacteria plays important role in environment as nutrient supplements and soil compacting agents in agriculture. Diversity and abundance studies have generated information about the dominance of heterocystous cyanobacteria with Nostoc and Anabaena dominant (40%–90%) in many cropping lands in East and North India (Roger et al., 1993; Whitton, 2000). These isolates showed efficiency in enhancing the germination and growth of rice, wheat seeds and increased production indole acetic acid (IAA) and proteins. Once inoculated, a set of inoculated strains of cyanobacteria exhibited persistent presence on rice roots up to the harvesting stage, entry into root and enhancement in plant growth and yield in addition to bringing about substantial changes in soil microbial biomass carbon whenever present (Prasanna et al., 2009;

Plant Growth-Promoting Abilities in Cyanobacteria Chapter | 23  469

N2

Heterocyst

6%

NH3

55%–5

Nitrogenase

NH3

Glutamate dehydrogenase

NH4+

Glutamic acid

Glutamine synthetase

Gln Mycobiont

Transaminases

Sugars Nitrogen containing compounds Vegetative cell

Gln Sugars

ol

Mannit 70%–80%

Glucose

FIG. 2  Nutrient exchange between partners in lichen symbioses.

Gupta  et  al., 2013). Cyanobacteria develop network of their filaments on the soil that enmeshes soil particles at depth (Nisha et al., 2007). They additionally produce extracellular polysaccharides (EPS) that helps binding soil particles together leading to improvement of soil quality as they are hygroscopic in nature (Flaibani et al., 1989). In addition, cyanobacteria have been shown to produce various plant growth-promoting compounds. It is difficult to quantify individually the involvement of various aspects of cyanobacterial metabolism (N2 fixation, CO2 fixation, production of EPS, growth-promoting hormones and vitamins) on plant growth and development. However, there are quite a few studies relating to cyanobacteria’s plant growth-promoting activities especially relating to paddy crop. Misra and Kaushik (1989a,b) have reported that cyanobacterial inoculation enhances seed germination and root shoot growth in rice. That root and shoot weight increase on cyanobacterial inoculation was reported by Obreht et al. (1993). Gantar et al. (1995) and Nain et al. (2010) have reported the positive influence of cyanobacterial inoculation and colonization of wheat plant roots in terms of plant growth. In recent times, there are more and more reports on growth-promoting influence of cyanobacteria on crop plants. Shariatmadari et al. (2013) have reported a detailed account of cyanobacterial influence on various vegetable plants in terms of root length, plant height, leaf number, fresh root, dry root, fresh stem and leaf, and dry stem and leaf (in g). Their work was on cucumber, tomato, and squash. Kumar et al. (2013) have reported the potential of two cyanobacteria along with eight thermotolerant bacteria as plant growth-promoting (PGP) agents for spice crops such as coriander, cumin, and fennel. Prasanna et al. (2014) have reported preparation of cyanobacterial formulations and biofilmed inoculants for leguminous crops, especially mung bean and soybean where they have looked into various parameters of growth such as root and shoot length and their weight, increase in N2-fixing ability, microbial biomass and carbon of soil samples, and percent increase in available. These plant growth-promoting activities of cyanobacteria could be attributed to various substances such as hormones like auxin (Ahmad and Winter, 1968), gibberellin (Singh and Trehan, 1973), cytokinin (Rodgers et al., 1979), and abscisic acid (Marsalek et al., 1992) as well as to vitamins, particularly vitamin B (Grieco and Desrochers, 1978), and various amino acids (Vorontsova et al., 1988), antibiotic and toxins. Cyanobacterial extract as well as biomass has been used in plant

470 Cyanobacteria

TABLE 3  Important Phytohormones Produced by Cyanobacteria Type of Phytohormones

Cyanobacteria

General Functions

Auxin

Synechocystis, Chroococcidiopsis, Calothrix, Cylindrospermum, Glactothece, Plectonema, Anabaena, Anabaenopsis, Phormidium, Oscillatoria, Nostoc etc.a

Increases growth level, biomass production, stress tolerance, oil content

Cytokinin

Synechocystis, Chroococcidiopsis, Anabaena, Phormidium, Oscillatoria, Calothrix, Chlorogloeopsis, Cylindrospermum, Rhodospirillumb

Enhances growth rate, oil content, and stress tolerance

Gibberellins

Anabaena, Anabaenopsis, Cylindrospermum, Phormidiumc

Boost growth rate and biomass production

d

Abscisic acid

Nostoc, Anabaena, Synechococcus, Trichormus

Known to impart stress tolerance

Ethylene

Nostoc, Anabaena, Calothrix, Cylindrospermum, Scytonema, Synechococcuse

May be involved in programmed cell death, improved growth rate and biomass production

a

Sergeeva et al. (2002), Hussain et al. (2010), Mazhar et al. (2013), Singh et al. (2016). Hussain et al. (2010), Tsavkelova et al. (2006a,b), Singh et al. (2016). c Gupta and Agarwal (1973), Tsavkelova et al. (2006a,b), Singh et al. (2016). d Zahradnickova et al. (1991), Marsalek et al. (1992), Hartung (2010). e Tsavkelova et al. (2006a,b). b

growth experiment in vivo and in vitro. Many cyanobacteria that populate crop fields such as Anabaena, Anabaenopsis, Calothrix, Chlorogloeopsis, Cylindrospermum, Gloeothece, Nostoc, Plectonema, and Synechocystis have been reported to produce IAA (Tsavkelova et al., 2006a,b). A list of various phytohormones produced by cyanobacteria that show positive influence on plant growth is given in Table 3. Cyanobacteria have also been suggested to have the ability of solubilization and mobilization of insoluble organic phosphates with the help of phosphatase enzymes and thereby improving bioavailability of phosphorus to plants. They can solubilize insoluble forms of FePO4, AlPO4, hydroxyapatite [Ca5(PO4)3OH] as well as [(Ca)3(PO4)2] in soils and in sediments (Bose et al., 1971; Dorich et al., 1985) probably by using one of the following two mechanisms for solubilization of phosphates. (i) They produce a Ca+2 chelator which leads to dissolution of the phosphate compounds without changing the pH of the growth medium (Cameron and Julian, 1988; Roychoudhury and Kaushik, 1989): Ca10 ( OH )2 ( PO 4 )6 → 10Ca +2 + 2OH − + 6PO 4 3− (ii) Release organic acids that solubilize phosphate compounds (Bose et al., 1971) via the following reaction: Ca 3 ( PO 4 )2 + 2H 2 CO3 → 2CaHPO 4 + Ca ( HCO3 )2 (iii) Or possibly by a third option where once an organic phosphate is solubilized, the growing population of cyanobacteria uses the resulting PO 4 3− as their nutrition and after their death the PO 4 3− gets released into the soil and hence becomes available to other plants and microbes following mineralization (Arora, 1969; Saha and Mandal, 1979; Mandal et al., 1992, 1999). An observation by Fuller and Rodger (1952) that phosphorus uptake by plants from algal materials was higher than that from inorganic phosphates when provided in equal amounts over a long period of time led to a hypothesis that cyanobacteria concentrate available phosphorus by absorbing the excess amount from the sphere of chemical fixation and incorporating it into their cell constituents. They release the concentrated phosphorus gradually over a period of time through exudation, autolysis or microbial disintegration of dead cells thereby making them available to plant in the vicinity.

6. CONCLUSION It can be concluded that cyanobacteria most definitely influence plant growth by way of casual association, symbiotic relation and by simply being present in the vicinity of the plants. They contribute to such growth and development of plants either by direct or indirect fixed-N transfer or by improving soil quality and nutrient status and by solubilizing inorganic

Plant Growth-Promoting Abilities in Cyanobacteria Chapter | 23  471

phosphates as well as by production of growth-promoting hormones and vitamins. They further improve the overall soil environment by CO2-sequestration, N2 fixation, removing contaminants by biosorption as well as by aiding soil nutrient upon their turnover.

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FURTHER READING Akoijam, C., Langpoklakpam, J.S., Chettri, B., Singh, A.K., 2015. Cyanobacterial diversity in hydrocarbon-polluted sediments and their possible role in bioremediation. Int. Biodeterior. Biodegrad. 103, 97–104. Gupta, V., Natarajan, C., Kumar, K., Prasanna, R., 2011. Identification and characterization of endoglucanases for fungicidal activity in Anabaena laxa. J. Appl. Phycol. 23, 73–81. Lu, Y., Xu, J., 2015. Phytohormones in microalgae: a new opportunity for microalgal biotechnology? Trends Plant Sci. 20 (5), 273–282. Manjunath, M., Prasanna, R., Sharma, P., Nain, L., Singh, R., 2011. Developing PGPR consortia using novel genera Providencia and Alcaligenes along with cyanobacteria for wheat. Arch. Agron. Soil Sci. 57 (8), 873–887. Pandey, K.D., Shukla, P.N., Giri, D.D., Kashyap, A.K., 2005. Cyanobacteria in alkaline soil and the effect of cyanobacterial inoculation with pyrite amendments on their reclamation. Biol. Fertil. Soils 41, 451–457. Rai, A.N., Borthakur, M., Singh, S., Bergman, B., 1989. Anthoceros-Nostoc symbiosis: immunoelectronmicroscopic localization of nitrogenase, glutamine synthetase, phycoerythrin and ribulose-1,5-bisphosphate carboxylase/oxygenase in the cyanobiont and the cultured (free-living) isolate Nostoc 7801. J. Gen. Microbiol. 135, 385–395. Swarnalakshmi, K., Prasanna, R., Kumar, A., Pattnaik, S., Chakravarty, K., Shivay, Y.S., Singh, R., Saxena, A.K., 2013. Evaluating the influence of novel cyanobacterial biofilmed biofertilizers on soil fertility and plant nutrition in wheat. Eur. J. Soil Biol. 55, 107–116. Syiem, M.B., Singh, A.K., Rai, A.N., 2009. Nitrogen metabolism in cyanobacteria. In: Khattar, J.I.S., Singh, D.P., Kaur, G. (Eds.), Algal Biology and Biotechnology. IK International, New Delhi, pp. 81–96. Wolf, A.M., Baker, D.E., Pionke, H.B., Kunichi, H.M., 1985. Soil test for estimating labile, soluble and algal available phosphorus in agriculture soils. J. Environ. Qual. 14, 341–348.