By: Mohd Sadaq
Abiotic and biotic stresses are the major constraints that are affecting the productivity of the crops. Many tools of modern science have been extensively applied for crop improvement under stress, of which PGPRs role as bio protectants has become of paramount importance in this regard. Rhizobium trifolii inoculated with Trifolium alexandrinum showed higher biomass and increased number of nodulation under salinity stress condition. Pseudomonas aeruginosa has been shown to withstand biotic and abiotic stresses.
Paul and Nair (scientists) found that P. fluorescens MSP-393 produces osmolytes and salt-stress induced proteins that overcome the negative effects of salt. P. putida Rs-198 enhanced germination rate and several growth parameters viz., plant height, fresh weight and dry weight of cotton under condition of alkaline and high salt via increasing the rate of uptake of K+, Mg2+ and Ca2+, and by decreasing the absorption of Na+.Few strains of Pseudomonas conferred plant tolerance via 2,4-diacetylphloroglucinol (DAPG).
Interestingly, systemic response was found to be induced against P. syringae in Arabidopsis thaliana by P. fluorescens DAPG. Calcisol produced by PGPRs viz., P. alcaligenes PsA15, Bacillus polymyxa BcP26 and Mycobacterium phlei MbP18 provides tolerance to high temperatures and salinity stress. It has been demonstrated that inoculation of plant with AM fungi also improves plant growth under salt stress. Achromobacter piechaudii was also shown to increase the biomass of tomato and pepper plants under 172 mM NaCl and water stress.
A root endophytic fungus Piriformospora indica was found to defend host plant against salt stress. In one of the studies it was found that inoculation of PGPR alone or along with AM like Glomus intraradices or G. mosseae resulted in the better nutrient uptake and improvement in normal physiological processes in Lactuca sativa under stress conditions. The same plant treated with P. mendocina increased shoot biomass under salt stress.
Mechanisms involved in osmotic stress tolerance employing transcriptomic and microscopic strategies revealed a considerable change in the transcriptome of Stenotrophomonas rhizophila DSM14405T in response to salt stress . Combination of AM fungi and N2-fixing bacteria helped the legume plants in overcoming drought stress . Effect of A.brasilense along with AM can be seen in other crops such as tomato, maize and cassava.
1. brasilense and AM combination improved plant tolerance to various abiotic stresses. The additive effect of Pseudomonas putida or Bacillus megaterium and AM fungi was effective in alleviating drought stress . Application of Pseudomonades sp. under water stress improved the antioxidant and photosynthetic pigments in basil plants. Interestingly, combination of three bacterial species caused the highest CAT, GPX and APX activity and chlorophyll content in leaves under water stress. Pseudomonas spp. was found to cause positive affect on the seedling growth and seed germination of A. officinalis L. under water stress.
Photosynthetic efficiency and the antioxidative response of rice plants subjected to drought stress were found to increase after inoculation of arbuscular mycorrhiza. The beneficial effects of mycorrhizae have also been reported under both the drought and saline conditions . Heavy metals such as cadmium, lead, mercury from hospital and factory waste accumulate in the soil and enter plants through roots. Azospirillium spp, Phosphobacteria spp and Glucanacetobacter spp. isolated from rhizosphere of rice field and mangroves were found to be more tolerant to heavy metal specially iron. P. potida strain 11 [P.p.11], P. potida strain 4 [P.p.4] and P. fluorescens strain 169 [P.f.169] can protect canola and barley plants from the inhibitory effects of cadmium via IAA, siderophore and 1-aminocyclopropane-1-carboxylate deaminase [ACCD]. It was reported that rhizoremediation of petroleum contaminated soil can be expedited by adding microbes in the form of effective microbial agent [EMA] to the different plant species such as cotton, ryegrass, tall fescue, and alfalfa.
PGPRs as biological agents proved to be one of the alternatives of chemical agents to provide resistance to against various pathogen attacks. Apart from acting as growth-promoting agents they can provide resistance against pathogens by producing metabolites. Bacillus subtilis GBO3 can induce defense-related pathways viz., salicylic acid [SA] and jasmonic acid [JA]. Application of PGPR isolates viz., B. amyloliquefaciens 937b and B. pumilus SE-34 provide immunity against tomato mottle virus. B. megaterium IISRBP 17, characterized from stem of black pepper, acts against Phytophthor capsici. Bacillus subtilis N11 along with mature composts was found to control Fusarium infestation on banana roots.
Similarly, B. subtilis [UFLA285] was found to provide resistance against R. solani and also it induced foliar and root growth in cotton plants. In another interesting study Paenibacillus polymyxa SQR-21 was identified as a potential agent for the bio-control of Fusarium wilt in watermelon. Further, the exploitation of PGPRs was found to be effective to manage the spotted wilt viruses in tomato, cucumber mosaic virus of tomato and pepper, and banana bunchy top virus in banana.
In some cases it was shown that along with bacteria, mycorrhizae can also confer resistant against fungal pathogens and inhibit the growth of many root pathogens such as R. solani, Pythium spp., F. Oxysporum, A. obscura and H. annosum by improving plant nutrients profile and thereby productivity. For instance Glomus mosseae was effective against Fusarium oxysporum f. sp. Basilica which causes root-rot disease of basil plants. Medicago tranculata also showed induction of various defense-related genes with mycorrhizal colonization. It was shown that addition of arbuscular mycorrhizal fungi and Pseudomonas fluorescens to the soil can reduce the development of root-rot disease and enhance the yield of Phaseolus vulgaris L.
The writer is a student of GDC Bhaderwah-University of Jammu
