Isolation, characterization and mechanisms of action of plant growth promoting rhizobacteria for growth promotion and drought tolerance in maize

Abstract

The global climate change has predicted drought as a major stress. Application of plant growth promoting rhizobacteria (PGPR) is an environment friendly alternative measure to impart tolerance in plants against stresses. Present investigation was based on the hypothesis that PGPR from stressed habitat can impart tolerance in plants in a better way. A series of experiments were conducted to screen PGPR from the rhizosphere of maize and rice grown at different moisture regimes and to evaluate their potential as bioinoculants under induced drought stressed conditions. Characterization of PGPR was made and their mechanisms of drought tolerance have been elucidated. Primarily, fifteen strains were screened from the rhizosphere of maize and rice grown in different moisture regimes, on the basis of their phosphate solublization and siderophore production abilities, and were identified using 16s rRNA gene sequence analyses. The identified strains isolated from maize rhizosphere grown in irrigated, semi-arid and arid regions were Pseudomonas sp. (1), Bacillus cereus, Bacillus pumilus, respectively, and from the rhizosphere of rice grown in irrigated fields and raised bed were Proteus sp. and Pseudomonas sp. (2) respectively. These 5 rhizobacteria were selected as bioinoculants on the basis of their growth promoting potential under normal and drought stressed conditions on maize plants grown in pots under axenic conditions. Subsequently, these isolates were characterized for C/N source utilization patterns, phytohormones, protease, hydrogen cyanide and ammonia production; antifungal, antibacterial and ACC deaminase activities. Emphasis was given on PGPR induced modulation of phytohormones content of leaves and the rhizosphere soil of maize using L-tryptophan as precursor of IAA, under induced drought stress under axenic conditions. The isolates were further evaluated under natural conditions of field. The cluster analyses based on QTS revealed that Bacillus and Pseudomonas strains exhibit similar C/N utilization patterns, irrespective of their habitat from where they were isolated. The Pseudomonas sp. (1) and Proteus sp. from irrigated field were phosphate solubilizers and produce higher amount of growth promoting hormones (IAA, GA). The Bacillus pumilus and Pseudomonas sp. (2) produced 14X and 17X higher ABA concentrations as compared to Pseudomonas sp. (1) and Proteus sp. respectively. Addition of L-tryptophan at the rate of 100 mg/L to the culture medium significantly enhanced the production of IAA, GA and ABA. The magnitude of ABA production by the PGPR isolates due to L-tryptophan increased linearly with decrease in moisture content of the rhizosphere soil from where isolation of PGPR was made. The PGPR isolated from water scarce environment showed higher ACC deaminase activity. The expression analyses of desiccation tolerance genes were carried out in Pseudomonas and Bacillus pumilus strains stored under desiccating (10% relative humidity) and hydrating (85% relative humidity) conditions. Up-regulation in the relative expression of trehalose synthase (treS), sigma factor (rpoS), alginate regulatory gene (mucA) and flagellar motor switch protein (fliM) gene were recorded in Pseudomonas sp. (4) and Pseudomonas sp. (2) strains in response to induced desiccation (10% RH). The relative water content, choline and glycine betaine production were higher in Arabidopsis plants inoculated with osmotically induced cells of Bacillus pumilus. It is inferred from the present study that the Bacillus pumilus and Pseudomonas sp. (2) isolated from the water deficit area, induced drought tolerance in maize because of the higher ABA production and enhanced activity of ACC deaminase. Up-regulation of treS, rpoS, mucA and fliM genes in Pseudomonas sp. (2) under desiccated conditions and greater accumulation of compatible solutes in response to osmotic stress in Bacillus pumilus, were adaptive mechanisms by these PGPR. The Bacillus pumilus can be used in the formulation of biofertilizer for arid and semi- arid regions.