Strain-specific salt tolerance and osmoregulatory mechanisms in Azospirillum brasilense

Salinity stress inhibits the growth and nitrogen fixation ability of the plant growth-promoting rhizobacterium Azospirillum brasilense. Five strains of A. brasilense were isolated from the rhizosphere of Indian cereals and grasses and identified on the basis of their phenotypic features and 16S rRNA gene sequence. The five Indian isolates and two standard strains of A. brasilense, Sp7 and Cd, showed notable differences in growth, acetylene-reducing activity under salt stress, and ability to take up and use glycine betaine for the restoration of growth and acetylene-reducing activity under salt stress. Salt stress also enhanced the production of exopolysaccharides and cell aggregates, the extent of which varied in different strains of A. brasilense at different carbon to nitrogen ratios in the culture medium. It can be concluded that the production of exopolysaccharides and cell aggregates is a more consistent physiological response of A. brasilense to salt stress than is the uptake and osmoprotection by glycine betaine.