Agriculture Reference
In-Depth Information
Three plasmids were only detected in tolerant and extremely tolerant strains and a
plasmid of 147 MDa appeared only in one sensitive strain and in 70% of strains from clusters
I and II. According to our results (Table 2 and Figure 3) salt tolerance may be related to the
presence of certain plasmids (828, 734, 147 and 82 MDa), however these plasmids
per se
are
not enough to bestow salt tolerance, since strains with no plasmids detected were able to
tolerate high levels of NaCl. These findings are in agreement with Baldani and Weaver
(1992) who observed that plasmid absence in
Rhizobium leguminosarum
bv.
trifolii
did not
impair their survival and development under osmotic stress.
The involvement of rhizobia plasmids in the nitrogen-fixing symbiosis is well
documented (Martínez et al., 1990; Moënne-Loccoz and Weaver, 1996). Rhizobia plasmids
are also related to catabolism of certain carbon compounds, metabolites found in root
exudates, chemotaxis and cell motility (Baldani et al., 1992; Boivin et al., 1991; Chen et al.,
1993; Hynes and McGregor, 1990). However, the influence that plasmid content of rhizobia
has on their ecological competence in the soil is still poorly understood. Moënne-Loccoz and
Weaver (1996) suggested that plasmid genes are important to the autecology of rhizobia
ex
planta
, and identified two plasmids that contributed to a higher salt tolerance in
Rhizobium
,
thus revealing the importance of plasmid genes for the activation of biochemical responses to
osmotic stresses. These responses may involve the regulation of intracellular accumulation of
certain metabolites (Graham, 1992).
3.2. Salinity Effects on Proteins
Protein synthesis is very sensitive to alterations in cell homeostasis (Yeo, 1998).
Therefore, the analysis of protein profile alterations in
Rhizobium
isolates is crucial to better
understand the degree of cell stress. The electrophoretic profiles of whole-cell soluble
proteins under control conditions and in the presence of salt are shown in Figure 4.
Figure 4. SDS-PAGE analysis of total soluble protein content of
Rhizobium leguminosarum
isolates and
strains growing in YEM supplemented with different NaCl concentrations. The molecular mass (KDa)
of polypeptides showing salt-induced alterations in band intensity are marked in the gel.
