Abstract
Although the physiological benefits of plant growth promoting rhizobacteria (PGPR) to plants are well known, the precise mechanisms that make these PGPR such wonderful friends of plants under adverse environmental conditions are still under investigation. We have previously reported that one such PGPR, Pseudomonas putida GAP-P45 ameliorates the adverse effects of water deficit in Arabidopsis thaliana by upregulating proline turnover and polyamine biosynthesis (Ghosh et al. Ann Microbiol 67:655–668, 2017; Sen et al. Plant Physiol Biochem 129:180–188, 2018). In this study, we investigated the impact of this phytohormone secreting strain on the regulation of endogenous phytohormone (abscisic acid, auxin, cytokinin and gibberellic acid) modulation in A. thaliana under normal and water stress conditions. We analyzed the content of all four phytohormones secreted by the bacteria in the nutrient medium as well as in the roots and shoots (separately) of the inoculated plants at three different days, post treatments. We observed that, while water stress increased the accumulation of abscisic acid and decreased the content of auxin and cytokinin in shoots and roots; the level of gibberellic acid decreased in shoots but increased in roots due to stress. Inoculation with GAP-P45 under water stress effectively reversed the trends of phytohormone accumulation, making their levels similar to the non-stressed, non-inoculated control plants. This happened despite there being no change in the water-potential of the medium due to GAP-P45 inoculation. We also observed that the pattern of phytohormones secreted by the PGPR varied depending on composition of nutrient media and culture conditions. We conclude that P. putida GAP-P45 alleviates water stress in A. thaliana by altering the endogenous hormone accumulation and re-distribution in both roots and shoots without causing any change to the water-potential of the medium.
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