Agriculture Reference
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Protein responses to NaCl stress were different even between isolates from the same
location. Salt induced increases and decreases of some polypeptides indicating an attempt of
cells to adjust to the new adverse conditions. According to Saxena et al. (1996) and Unni and
Rao (2001) the imposition of any stress to bacteria results in adaptive responses, which lead
to changes in the regular metabolic processes that are then reflected in protein profile
changes. In most isolates alterations corresponded to decreases in protein expression,
suggesting a deleterious effect on cell basic metabolism, which may have imposed an
inhibition of important metabolic pathways hence inhibiting growth and survival of
Rhizobium . However, in tolerant isolates increments of protein expression were detected,
mainly in polypeptides of high molecular weight (> 73.5 KDa), suggesting that these isolates
possess mechanisms to minimise stress. Synthesis of stress proteins in order to neutralize the
detrimental effects of environmental stresses has been reported (Dubey and Rani, 1987; Joshi,
1987; Saxena et al., 1996; Völker et al., 1992; Zahran et al., 1994).
4. C ONCLUSION
In conclusion, our results reveal that in environments with high water availability
throughout the year, Rhizobium populations displayed a high sensitivity to NaCl and are
expected to be highly vulnerable to increases in soil salinity. However, when naturally
exposed to large amplitudes of soil water availability and osmotic stress, populations reflect
these alterations by seasonal fluctuations in the population's global tolerance. Consequently,
the results seem to indicate that the salt tolerance of Rhizobium populations was strongly
influenced by their origin, which may represent evidence of the population's adaptation to the
environmental conditions experienced in the habitat they colonize. This emphasizes the
importance of more thorough evaluation of soil bacteria survival when facing to habitat
alterations, particularly the predicted climate alterations. Rhizobium assumes a double role: 1)
it can be used as a sensitive bioindicator of soil microflora when in free-living; 2) when is in
symbiosis with legumes it can reflect the influence that these alterations exert on the
symbiotic process of N 2 fixation, on which many natural and agricultural ecosystems depend.
Bringing into the mind that Rhizobium is a ubiquitous bacteria, the analysis of rhizobia
populations both as free-living and in symbiotic association can constitute an excellent way to
determine the influence of environmental changes, such as climate alterations on different
biota.
The establishment of any procedure that may help plant crops to cope with salt stress is of
extreme importance in world regions were salinity is a key issue in agriculture. The inhibitory
effect of salinity on N 2 fixation and the fact that legumes are generally considered sensitive or
moderately tolerant to salinity lead us to the assumption that legume cultivation under saline
conditions will only produce economic yields if species and cultivars with enhanced tolerance
to salinity are used. Inorganic N use can only be avoided if the host is able to establish an
effective symbiosis with a microssymbiont under salt conditions. The results presented in this
chapter bring to light new perspectives of agricultural sustainability in salt-constrained soils,
since there was a insignificant decrease of plant growth provided by inoculation with two salt
extremely tolerant strains when compared to NO 3 fed plants. This is indicative that efficient
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