Sangeeta PaulNIVETHA N2021-07-172021-07-172020https://krishikosh.egranth.ac.in/handle/1/5810170669T-10450Drought is considered as major abiotic stress affecting seed germination, plant growth and crop productivity. Mustard an important oilseed crop is prone to be affected by drought. Rhizobacteria inoculation help crops to withstand stress. Plant root colonisation, an important factor for maximizing benefits from inoculation, is impacted by osmotic stress. So, deciding the optimum rhizobacterial density, mechanisms behind germination enhancement and colonization under osmotic stress is required for optimizing benefits from inoculation. In the present study, influence of different inoculum densities of Bacillus sp. strain MRD-17 and B. casamancensis strain MKS-6 on seed germination in vitro and growth of mustard in pot experiment, under water deficit stress were investigated. Rhizobacterial population densities ranging from 105 to 107 cfu/mL, were found to be stimulatory on seedling fresh weight in vitro; and on shoot and root fresh weight at 30 days after sowing in pot experiment under control and water deficit stress condition. However, higher inoculum densities (1011 cfu/mL) of both rhizobacteria had an inhibitory effect on seedling vigour and plant growth. Under sterile condition, there was beneficial effect of stimulatory and adverse effect of inhibitory population on plant growth while under unsterile conditions, adverse effects of inhibitory population was observed only on root fresh weight. Under sterile condition, rhizoplane population of inoculated bacteria more than 109 cfu/g was inhibitory and 106 to 108 cfu/g was beneficial for plant growth. Lower population densities of the inoculated rhizobacteria were observed on rhizoplane, under unsterile condition. Thus, population densities of rhizobacteria on roots had major impacts on seedling vigour and plant growth. Under no stress condition, germination of seedlings was not affected by IP inoculation, but post-germinative growth was affected. Inhibitory effect of IP and stimulatory effect of BP populations were observed on seedling fresh weight and length. Under osmotic stress condition, inoculation enhanced seedling germination, fresh weight and length. Rhizobacterial inoculation increased activities of enzymes involved in reserve hydrolysis in seedlings and highest was observed in IP under no stress condition and BP under osmotic stress condition. Higher gibberellic acid content was noted in inoculated plants under both conditions; while abscisic acid content was higher in inoculated seedlings onlyunder no stress condition. Inoculation with IP and BP reduced osmolyte accumulation in plants under osmotic stress condition. Expression of genes involved in the biosynthesis of reserve hydrolytic enzymes BjICL and BjMLS were significantly upregulated in IB and BP inoculated seedlings under no stress and osmotic stress conditions, respectively. BjABCD1 was upregulated under no stress and downregulated under osmotic stress upon rhizobacteria inoculation. Under no stress condition, inoculated seedlings showed higher expression of BjGA20Ox. There was upregulation of BjNCED3, BjABI5 and BjRGL2 genes involved in ABA biosynthesis, ABA and GA mediated repression of growth, respectively in IP inoculated seedlings. Under osmotic stress condition in inoculated seedlings, down-regulation of genes BjP5CS, BjBADH2 involved in osmolyte accumulation and BjDREB1-2, BjDREB2 involved in the regulation of ABA independent signalling pathway, indicated decrease in stress. Analysis of root exudates indicated that osmotic stress had significant impact on sugars, organic acids, fatty acids, phenolics and flavonoid composition. Osmotic stress also had a positive influence on swimming and twitching motility and chemotactic response B. casamancensis strain MKS-6. Osmotic stress had beneficial effect on biofilm production ability of rhizobacteria. Osmotic stress significantly enhanced cell wall hydrolytic enzymes production by Bacillus sp. strain MRD-17, in vitro and in vivo. There was induction of defence enzymes upon rhizobacterial inoculation. Osmotic stress and root exudates induced changes in the cell surface components of rhizobacteria and only root exudates influenced expression profile of rhizobacterial cell wall proteins. Rhizobacteria Bacillus sp. strain MRD-17 and B. casamancensis strain MKS-6 improved seed germination under osmotic stress condition with great colonization ability. The beneficial effect of these strains can be harnessed for drought stress alleviation in mustard by developing these into bio-formulations.EnglishThesis