Agriculture Reference
In-Depth Information
N
OD
F
ACTORS
-
B
ACTERIAL
I
NDUCERS OF
L
EGUME
N
ODULE
O
RGANOGENESIS
One key event in nodule formation is the synthesis and release of rhizobia
Nod factors, which trigger nodule formation. Nod factors are synthesized by
proteins encoded by rhizobia nodulation genes (
Nod
,
Nol
and
Noe
genes).
These genes are induced following the bacteria's perception of highly specific
flavonoid molecules secreted by the plant roots, which activate the bacterial
transcriptional regulator NodD. This in turn induces the transcription of the
other bacterial nodulation genes (Redmond
et al.
1996; Zuanazzi
et al
. 1998).
Nod factors are lipo-chitooligosaccharides consisting of a β-1,4-linked
N
-
acetyl-D-glucosamine backbone having four or five attached residues. The
non-reducing terminal residue is substituted at the C-2 position with an acyl
chain (Cullimore
et al
. 2001). Nod factors of different rhizobia species contain
different chemical substitutions on the sugar residues and/or variations in the
structure of the acyl chain. The variation in the amount and the structure of the
Nod factor produced by the different rhizobia species appears to be a key
factor in determining host specificity (Perret
et al
. 2000). The Nod factors
produced by
B. japonicum
are predominantly pentameric molecules with C
18:1
,
C
16:0
, and C
16:1
fatty acid chains at the non-reducing end and 2-
O
-methylfucose
at the reducing end of the chitin backbone (Carlson
et al.
1993).
The detection of Nod factors by a legume host induces major
developmental changes that are required for rhizobia infection. Indeed, Nod
factors initiate a variety of responses in epidermal, cortical and pericycle cells.
This includes inducing the expression of critical nodulation genes (Ferguson
et
al
. 2010). Morphological changes are also induced, including the swelling,
branching and curling of epidermal root hairs (
e.g.
, Esseling
et al
. 2003).
N
OD
F
ACTOR
R
ECEPTORS
-
F
ACILITATORS OF
H
OST
-M
ICROBE
S
PECIFICITY
Two genes predicted to function in Nod factor perception have been
isolated and cloned in soybean (Indrasumunar 2007; Indrasumunar
et al
.
2010). These genes called
Glycine max
Nod Factor Receptor 1
(
GmNFR1
) and
G. max
Nod Factor Receptor 5
(
GmNFR5
) encode receptor-like kinases with
LysM domain in the predicted extracellular domain. They are orthologs of
LjNFR1
and
LJNFR5
in
Lotus japonicus
,
PsSYM2a
and
PsSYM10
in
Pisum
