Studies on Endurance of Azospirillum formosense Strains to Osmotic Stress and Modulation of Early Stage Pearl Millet Growth

In the scenario of water deficit stress, use of plant growth-promoting rhizobacteria (PGPR) adapted/tolerant to osmotic stress can be of great importance in designing suitable bioinoculants for the crops grown in stressed agro-ecosystems. The present investigation was conducted to study the in vitro response of selected Azospirillumformosense strains AIM3, AIM19, AIM38, AIM57 and AIM82 to PEG 6000-induced osmotic stress. The strains were first assessed for their growth and physiological responses under osmotic stress conditions (0, 10, 20, 30% PEG 6000). The growth of the cultures was not much affected under mild stress (10% PEG 6000), however, higher stress levels (30% PEG 6000) showed significant negative effect on growth with a sharp reduction in population of the cultures. Morphologically, the bacterial cells appeared as elongated rods and highly motile under 0 and 10% PEG 6000 whereas higher stress levels (20 and 30% PEG 6000) induced non-motile spheres with thick exopolymeric layer indicating stress adaptation. The intracellular accumulation of Pro and the biofilm-forming ability enhanced with increasing stress levels indicating osmoadaptation strategy of the strains. All the five strains could express PGPR traits (indole acetic acid and siderophore production and nitrogen fixation) under all levels of PEG 6000 indicating their PGP potential even under stress conditions. However, beyond 20% PEG 6000 there was drastic reduction in the expression of PGP traits. Seed germination bioassays indicated the positive effect of inoculation of bacterial strains on seed germination, seedling vigour index under 0, 10 and 20% PEG 6000 conditions. The present in vitro studies revealed responses and survival mechanisms of the A.formosense strains under osmotic stress conditions and their potential to alleviate moisture deficit effect in pearl millet plants.