Agriculture Reference
In-Depth Information
1. Surprisingly, shoot dry weights of plants inoculated with extremely tolerant strains AWA
1-2 and AWA 1-3 were not significantly different from plants supplied with nitrate.
The results indicated some success for the establishment of an effective symbiosis under
moderate salinity conditions. The significant increase in plant growth provided by the
inoculation of the isolates AWA 1-2 and AWA 1-3 may reflect their ability for effective N
fixation even under salt conditions. Most tolerant and extremely tolerant strains were able to
establish a moderately effective symbiosis under 90 mM NaCl, providing survival in saline
soils and increasing the probability of an effective symbiosis under the highest salt
concentration tolerated by the legume.
3.
M
OLECULAR
B
ASIS OF
S
ALT
T
OLERANCE
3.1. Involvement of Plasmids in NaCl Tolerance
Although chromosomal genes are important to bacterial adaptation and survival to
environmental constraints, resistance genes are mainly located in plasmids. Their analysis can
be a genetic tool to understand the way that bacteria tolerate stress. Furthermore, plasmid-
mediated tolerance may be ecologically important because tolerance can be rapidly
transferred from tolerant to sensitive bacteria (Gadd and Griffith, 1978). Weaver and Holt
(1990) showed that some plasmids influence survival under stress conditions, which open
new perspectives for the improvement of the
Rhizobium
-legume symbiosis under salinity.
Pereira et al. (2006) also reported the involvement of some plasmids with Cd tolerance in
Rhizobium leguminosarum
.
Table 2 represents the frequency of different plasmids in salt sensitive, tolerant and
extremely tolerant isolates. Our results demonstrate striking differences on plasmid profiles of
Rhizobium
isolates. Fifteen plasmids with sizes ranging from 82 to 828 MDa were separated
and each strain had between two to six plasmids, except for isolates AWB 1-1, AWA 2-2,
AWB 5-1, AWA 1-2 and AWA 1-3, where no plasmids were detected.
When the plasmid profiles were submitted to classification analysis (Bray-Curtis
coefficient) four distinct cluster groups were identified (Figure 3). Cluster I included five
isolates from Alentejo and one from USDA, all showing moderate (100-500 mM NaCl) or
high (600-1800 mM NaCl) tolerance to salinity. Plasmids 828 and 82 were only detected in
this group. Plasmid 734 was present in three tolerant strains and additionally a 147 MDa
plasmid was present in all Alentejo isolates. Cluster II contained only two strains, one with
moderate tolerance (USDA2343) and one tolerant (USDA2433), which had in common the
734 MDa plasmid. Cluster III was the most heterogeneous group, formed by seven strains:
five sensitive, one tolerant and one extremely tolerant. Most genotypes of this group shared
the 242 MDa, 201 MDa and 170 MDa plasmids. Cluster IV was formed only by salt sensitive
strains. Clustering of these strains was due to the presence of the 343, 275 and 220 MDa
plasmids. Clusters III and IV exhibited a higher number of plasmids than clusters I and II.
