Agriculture Reference
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of reiterated DNA (Fawzy and Kuykendall, 1994).
The correlation between the selection
pressure caused by stress and the existence of the same plasmids suggests that plasmids play a
major role in the adaptation of bacteria to environmental stresses (Lakzian et al., 2002). In
addition to symbiotic plasmids,
Rhizobium
strains may carry 1-10 other plasmids, which
range in size from about 30 to more than 1000 MDa (Fawzy and Kuykendall, 1994) and may
contribute to saprophytic competence of rhizobia (Chen et al., 1993).
Bacteria living under salt stress also experience a number of physiological alterations.
Salinity imposes both ionic and osmotic stress, which can be extremely detrimental for
microflora survival (Saxena et al., 1996). For example, according to Saxena et al. (1996) and
Unni and Rao (2001) the imposition of any stress to bacteria results in adaptative responses,
which lead to changes in the regular metabolic processes that are then reflected in protein
profiles. For this reason, one approach for understanding
Rhizobium
's ability to tolerate salt
has been to identify stress induced protein alterations. Several authors (Saxena et al., 1996;
Shamseldin et al., 2006; Soussi et al., 2001) reported that changes in protein profiles in
response to salt stress are sufficient to substantiate the presumption that proteins have a role
to play in salt tolerance. Völker et al. (1992) also suggested that stress protein induction is an
important cellular adaptations to growth-limiting conditions, such as heat and salt-stress.
This chapter aimed to study the halotolerance of rhizobial populations from different
agricultural ecosystems in order to evaluate
Rhizobium
's vulnerability to salinity, a growth
constraint that can affect many soils, even non saline, due to secondary salinization, which is
expected to increase due to climate alterations. To fix nitrogen in saline environments
leguminous plants require both free-living rhizobia and hosts tolerant to salt. Therefore, the
selection of tolerant phenotypes, which can withstand the negative impact of saline soils, can
be of great use to improve nitrogen fixation and productivity in salt-affected soils. For these
reasons,
Rhizobium
isolates were also screened for their efficiency to fix N
2
under salt
conditions in symbiosis with a legume.
Pisum sativum
plants were the host chosen, since they
are reported as one of the most halotolerant cultivated legumes (Subbarao and Johansen,
1994). Since plasmids code for genes that are important for bacterial adaptation and survival
to environmental constraints, plasmid profile analysis can offer a basic genetic tool to
elucidate if different populations have the same or different genetic features to cope with salt
stress. The comparative study of protein pool alterations between halotolerant strains under
salt stress shad some light the mechanisms underlying salt tolerance.
2.
S
ALT
T
OLERANCE OF
R
HIZOBIUM
I
SOLATED FROM
D
IFFERENT
A
GRICULTURAL
E
COSYSTEMS
Six soils were obtained from arable fields in different locations of Portugal subjected to
different climates. S. Bernardo (SB) soil is a silt loam soil with low salinity (0.29 g Na
+
Kg
-1
dry soil) and water availability throughout the year (69.3% field capacity in June); Vagos (V)
soil is a sandy loam soil with low salinity (0.23 g Na
+
Kg
-1
dry soil) and high water
availability throughout the year (86.6% field capacity in June); Costa Nova (CN) soil is a
sandy soil with sea influence (1.12 g Na
+
Kg
-1
dry soil) and a low water content during part of
the year (53.6% field capacity in June); Alentejo (A) soils are a clay loam soils with low
water availability and affected by high temperatures during part of the year (84.4% field
