Agriculture Reference
In-Depth Information
Rm8530,
sinRI
genes are required for the synthesis of EPSs needed in the nodule infection
steps [Marketon
et al.
, 2003].
S. meliloti
strain Rm41 carries a third type of QS system, homologous to the
traRI
systems controlling plasmid transfer
in
Agrobacterium
and
Rhizobium leguminosarum
bv.
viciae
. This system seems exclusive of strain Rm41 and is localized to a non-symbiotic
plasmid [González and Marketon, 2002].
Most wild type strains of
S. meliloti
carry an
expR
gene, also encoding a LuxR-type
regulator.
expR
defective mutants lack any nodulation phenotype, but interruption of
expR
influences EPS production and alters gene expression and protein levels in laboratory cultures
[Marketon
et al
., 2003, Sánchez-Contreras
et al
., 2007].
8.2. QS in Bradyrhizobium japonicum
In contrast to
Rhizobium
and
Sinorhizobium
, no AHLs made by
Bradyrhizobium
have
been purified nor characterized so far, although 22% of 142
Bradyrhizobium
spp. isolates
displayed AHLs-like activity when they were challenged in bioassays carried out with a QS-
reporter strain [Pongsilp
et al.
, 2005].
Bradyrhizobium japonicum
are the slow-growing
bacterial symbionts of soy bean [Loh and Stacey, 2003]. In
B. japonicum
strain USDA110, it
was demonstrated that the expression of the nodulation
genes is repressed at high cell
densities, evidencing the involvement of a QS mechanism in such regulation. A CDF named
bradioxetin (Figure 1) has been characterized, which keeps structural similarities with the
antibiotic oxetin and a siderophore (mugeneic acid) [Loh and Stacey, 2003]. Bradyoxetin
synthesis is actually regulated by the available Fe
+3
concentration, being maximally expressed
under iron starvation conditions [Loh
et al.
, 2002].
In
B. japonicum
, expression of the nodulation genes in response to plant-made flavonoids
is complexly regulated by several transcriptional activators [Loh and Stacey, 2003]. One of
these proteins, NswB, is also involved in the QS regulated repression of the nodulation genes.
NswB is the response regulator of a two-component system, which responds to cell density
increases by detecting the rising of bradyoxetin concentrations. NswB response is the
induction of
nolA
, which acts as a transcriptional activator of the
nodD2
gene [Loh and
Stacey, 2002]. NodD2 is a regulatory protein required for the repression of nodulation genes.
Thus, NwsB seems to work as a switch that results in either activation or repression of
nod
gene expression, depending on cell density [Loh and Stacey, 2003].
It is well established that the nodulation genes are not expressed in nodules, by a
mechanism still unclear. Loh
et al.
(2002) hypothesized that the concentration of bacteroids
inside the nodules reproduces a high cell density situation, thus leading to a QS-based
repression of the
nod
genes. Conclusive evidence for the role of QS in bacteroids requests
further studies using a QS-knockout mutant. To date, no mutants defective in bradyoxetin
production are available, and the genes regulating its synthesis are yet unidentified. Many
questions remain unanswered regarding the extent of QS implication in establishment and
functioning of the
B. japonicum
-soy bean symbiosis.
