Salinity stress responses in the plant growth promoting rhizobacteria, Azospirillum spp

In order to adapt to the fluctuations in soil salinity/osmolarity the bacteria of the genus Azospirillum accumulate compatible solutes such as glutamate, proline, glycine betaine, trehalose, etc. Proline seems to play a major role in osmoadaptation. With increase in osmotic stress the dominant osmolyte in A. brasilense shifts from glutamate to proline. Accumulation of proline in A. brasilense occurs by both uptake and synthesis. At higher osmolarity A. brasilense Sp7 accumulates high intracellular concentration of glycine betaine which is taken up via a high affinity glycine betaine transport system. A salinity stress induced, periplasmically located, glycine betaine binding protein (GBBP) of ca. 32 kDa size is involved in glycine betaine uptake in A. brasilense Sp7. Although a similar protein is also present in A, brasilense Cd it does not help in osmoprotection. It is not known if A. brasilense Cd can also accumulate glycine betaine under salinity stress and if the GBBP-like protein plays any role in glycine betaine uptake. This strain, under salt stress, seems to have inadequate levels of ATP to support growth and glycine betaine uptake simultaneously. Except A. halopraeferens, all other species of Azospirillum lack the ability to convert choline into glycine betaine. Mobilization of the bet ABT genes of E. coli into A. brasilense enables it to use choline for osmoprotection. Recently, a proU-like locus from A. lipoferum showing physical homology to the proU gene region of E. coli has been cloned. Replacement of this locus, after inactivation by the insertion of kanamycin resistance gene cassette, in A. lipoferum genome results in the recovery of mutants which fail to use glycine betaine as osmoprotectant.