Agriculture Reference
In-Depth Information
It is possible that SR160/BRI1 initially functions in the defense response but was
co-opted as a component in brassinolide signaling during the course of evolution,
although the reverse may be the case (Ryan & Pearce, 2003). Nevertheless, it raises
an interesting question as to how perception of systemin and brassinolide by the
same receptor activates two distinctive intracellular biochemical pathways.
It is yet to be determined if SB160 is also the receptor for the other systemins.
However, even if different receptors are involved, it is clear that the signaling path-
ways mediated by different systemins converge at an early step and, therefore, they
activate a similar physiological response. As in many signaling pathways mediated
by peptide hormones in animals, a kinase cascade may be involved in activation of
the plant wound response. Both the tomato systemin and the TobHypSys systemins
activate a 48-kDa MAPK (Schaller & Ryan, 1996; Stratmann
et al.
, 2000; Ryan &
Pearce, 2003). Other intracellular events involved in the systemin signaling path-
way include the release of Ca
2
+
from vacuoles to the cytosol as well as activation
of phospholipase A2. These signaling events lead to generation of jasmonate from
linolenic acid, which, together with ethylene, activates defense genes.
2.2.2 RALF regulates plant growth and development
During the purification of the tobacco systemins, another peptide from tobacco
leaves was found to cause rapid alkalinization of the medium of the tobacco sus-
pension cells. The peptide was named rapid alkalinization factor (RALF) (Pearce
et al.
, 2001b). RALF is a 49-amino acid peptide derived from a 115-amino acid pre-
proprotein. RALF induces a stronger and more rapid alkalinization response than
the two tobacco systemins. However, unlike the systemins, it does not induce the
tobacco PIN genes and, therefore, is not a signal of the wound response.
To determine its possible biological function, RALF was applied to excised plants
and germinating seeds. It was found that RALF caused an arrest of seedlings' root
growth by affecting the elongation zone and the root meristem (Pearce
et al.
, 2001b).
It also blocked root hair formation. RALF appears to be expressed in a variety of
tissues and its homologues are present in plants species across the plant kingdom.
The mechanism by which RALF regulates root growth remains elusive. Neither is
it clear whether RALF affects other growth and developmental process.
2.2.3
ENOD40 regulates nodulation and cell proliferation
Some nitrogen fixation bacteria (rhizobia) in soil establish an endosymbiotic asso-
ciation with legumes. Biological nitrogen fixation is carried out by rhizobia in a
specific type of organs called root nodules, which form following rhizobia infec-
tion. The nodule formation involves communication between the bacteria and the
host as well as between different types of root cells. Flavonoids secreted by legume
roots activate the rhizobial genes involved in production of Nod factors. Nod factors
share the similar basic structure comprising lipo-chitin oligomers with a four or five
b -1,4-linked
N
-acetyl glucosamine backbone in which the
N
-acetyl group of the
