Agriculture Reference
In-Depth Information
serine carboxypeptidase (BRS1) whose overexpresstion suppresses the
bri1
mutant
phenotype (Li
et al.
, 2001). Serine carboxypeptidases belong to the serine protease
family. BRS1 shares a sequence similarity to Kex1, which is involved in matura-
tion of a -mating factor in yeast. The precursor of a -mating factor is processed by
cleavage at a specific pair of basic amino acids by Kex2 (another serine protease)
and followed by removal of dibasic amino acids at the C-terminus by Kex1. The
authors proposed that BRS1 might be involved in processing a peptide signal that
is an unidentified component of the brassinolide signaling pathway.
The disease resistance response in plants has been very well studied. A quick
recognition of an invading pathogen is often the key for the plant to mount an effec-
tive defense response. This recognition triggers a battery of physiological responses
including the hypersensitive response (HR), which is often accompanied by rapid
cell death at the site of infection. HR, together with other defense mechanisms,
contains and eventually eliminates the pathogen. In addition to such localized re-
sistance, the plant develops mechanisms of systemic immunity following the initial
pathogen attack. This systemic acquired resistance (SAR) occurs when a mobile
signal is generated at the infected site and translocated throughout the rest of the
plant. SAR is an enhanced resistance against a broad spectrum of pathogens.
To date, the identity of the mobile signal that activates SAR remains obscure.
Possible candidates for long-range and/or short-range mobile SAR signals have
included several small molecules: salicylate, H
2
O
2
, nitric oxide, and unidentified
lipid molecules. Recently, the activation of an extracellular aspartic protease (CDR1)
in
Arabidopsis
was found to generate an endogenous mobile signal that activates
the SAR response (Xia
et al.
, 2004). The endogenous elicitor is associated with a
protein fraction whose molecular size ranges from 3 to 10 kDa. The elicitor activity
is inactivated by heating and after treatment with a digestive protease. The results
suggest that the elicitor activity comes from a peptide signal. The study raises the
possibility that SAR signaling is also mediated by a peptide signal in a manner
analogous to the systemin-mediated systemic wound response.
In addition to proteolytic maturation, some proteases may be involved in inac-
tivation of peptide signals. Since peptide signals generally act on cell surface, its
degradation is not likely mediated through the widely used ubiquitin-mediated pro-
tein degradation pathway. Instead, extracellular proteases play an important role
in the inactivation process. A tomato serine protease was initially identified as a
systemin-binding protein (SBP50) (Schaller & Ryan, 1994). SBP50 is a membrane-
associated protein with the ability to cleave systemin
in vitro
, which suggests that
it could be involved in inactivating the systemin signaling pathway.
2.4
Technologies for discovering new peptide signals
As indicated from the above examples, peptide signals regulate a variety of important
biological processes in plants. However, compared to the hundreds of peptide signals
in animals, the number of known plant peptide signals is low. Growing evidence
