Ak. Tripathi et al., SALINITY STRESS RESPONSES IN THE PLANT-GROWTH PROMOTING RHIZOBACTERIA, AZOSPIRILLUM SPP, Journal of Biosciences, 23(4), 1998, pp. 463-471
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 stres
s the dominant osmolyte in A. brasilense shifts from glutamate to prol
ine. Accumulation of proline in A. brasilense occurs by both uptake an
d synthesis. At higher osmolarity A. brasilense Sp7 accumulates high i
ntracellular concentration of glycine betaine which is taken up via a
high affinity glycine betaine transport system. A salinity stress indu
ced, periplasmically located, glycine betaine binding protein (GBBP) o
f ca. 32 kDa size is involved in glycine betaine uptake in A. brasilen
se 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 th
e GBBP-like protein plays any role in glycine betaine uptake. This str
ain, under salt stress, seems to have inadequate levels of ATP to supp
ort growth and glycine betaine uptake simultaneously. Except A. halopr
aeferens, all other species of Azospirillum lack the ability to conver
t 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 homol
ogy to the proU gene region of E. coli has been cloned. Replacement of
this locus, after inactivation by the insertion of kanamycin resistan
ce gene cassette, in A. lipoferum genome results in the recovery of mu
tants which fail to use glycine betaine as osmoprotectant.