Agriculture Reference
In-Depth Information
A
UTOREGULATION OF
N
ODULATION
-
T
HE
L
EGUME
H
OST
C
ONTROLS ITS
R
OOT
N
ODULE
N
UMBERS
Typically, less than 10% of rhizobia infection events actually lead to the
formation of a fully functional nodule. This is due to a number of internal and
environmental factors that can inhibit the progression of nodule formation.
Legumes internally regulate the number of nodules they form via a process
called the Autoregulation Of Nodulation (AON). The AON pathway involves
long-distance root-shoot signalling initiated during nodule development by the
synthesis of a root-derived signal called ‗Q'. Recent work has indicated that Q
is likely a CLE peptide(s) (Okamoto
et al
. 2009; Mortier
et al
. 2010; Reid
et
al
. 2011). Grafting experiments have shown that Q travels to the shoot (Delves
et al
. 1986) where it, or a product of its action, is perceived by a LRR receptor
kinase called
GmNARK
/
LjHAR1/MtSUNN
(
e.g.
Searle
et al
. 2003). This
perception results in the production of a novel shoot-derived inhibitor, named
‗SDI'. SDI subsequently travels from the shoot back down to the roots where
it acts to inhibit further nodulation events (Ferguson
et al.
2010). It has
recently been established in soybean that SDI is small (
<
1 kDa), heat stable,
Nod factor-dependent, requires GmNARK-activity for its biosynthesis, and is
unlikely to be an RNA or a protein molecule (Lin
et al
. 2010, 2011b).
N
ITROGEN
F
IXATION
-
A
B
IOLOGICAL
A
LTERNATIVE TO
N
ITROGEN
-B
ASED
F
ERTILIZERS
Within a mature nodule, the bacteroids synthesize nitrogenase and other
supporting enzymes required for nitrogen fixation. These enzymes act to
reduce or ‗fix' atmospheric di-nitrogen gas (N
2
) into products such as
ammonia. This reduced nitrogen is subsequently exported into the host plant
after its conversion to glutamine by glutamine synthase and then to glutamate
by glutamate synthase.
In just one growing season, soybean can produce an estimated 200 kg N
ha
-1
in its above ground biomass (Peoples
et al
. 2009). This nitrogen is sourced
either directly from the soil, from symbiotic N
2
fixation, and/or from nitrogen-
based fertilizer uptake (Salvagiotti
et al
. 2008). It is estimated that 58-68% of
the nitrogen content of soybean is derived from symbiotic nitrogen fixation
(Salvagiotti
et al
. 2008; Peoples
et al
. 2009). After harvesting, the nitrogen
remaining in the nodules and roots, which represents between 30-60% of the
