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of systemin. The search for additional wound signaling molecules in tomato has
led to the identification of three new peptide signals, tomato hydroxyproline-rich
systemin (TomHypSys) I, II, and III (Pearce & Ryan, 2003). These three peptides
are 20, 18, and 15 amino acids in length respectively (Fig. 2.1). Like the TobHypSys
systemins, they are hydroxyproline-rich and glycosylated. All three TomHypSys
systemins are derived from a single wound-inducible 146-residues precursor that
shares sequence similarity to the precursor of the tobacco systemins, which suggests
they evolved from a common ancestor gene.
The systemin precursor, prosystemin, is synthesized in phloem tissues and lo-
calized in the cytosol (Narvaez-Vasquez & Ryan, 2004). In contrast, the precursors
of the TomHypSys and TobHypSys signals are likely synthesized on the rough ER,
processed and modified in ER and the Golgi apparatus, and then enter the secretory
pathway. Therefore, it is plausible that the TomHypSys and TobHypSys systemins
move from cell to cell through intercellular fluid and act as short-range signals to
induce the local wound response, whereas the localization of systemin provides the
advantage of rapid transportation through phloem to activate the systemic wound
response.
Jasmonate is another mobile signal molecule that interplays with systemin to
amplify wound signaling (Ryan & Pearce, 2003). Wounding causes the generation
and release of systemin from damaged cells. It moves through the apoplast and is
perceived by neighboring cells, resulting in the induction of jasmonate synthesis. As
jasmonate moves through the plant, it induces the production of more prosystemin.
This systemin-jasmonate cycle amplifies the wound signaling process to achieve a
strong systemic wound response and effectively defend the plant from its predator.
However, the question as to how prosystemin is released from the cytosol of un-
wounded distal cells to the apoplast remains unanswered. Since polypeptides usually
do not diffuse across the plasma membrane, a specific transporter would be required
to transport prosystemin to the intercellular space where it is activated by proteolytic
processing. In species such as tobacco that lacks systemin, jasmonate is mainly re-
sponsible for long-range wound signaling. The peptide signals such as TobHypSys
I and II help to amplify wound signaling and activate the local wound response.
The receptor of systemin was recently identified by its binding to systemin. The
receptor is called SR160, a 160-kDa transmembrane protein with an extracellular
leucine-rich repeat (LRR) domain and an intracellular serine/threonine kinase do-
main (Scheer & Ryan, 2002). It turns out that SR160 shares a very high sequence
similarity to BRI1 in Arabidopsis . BRI1 was previously identified as the cell sur-
face receptor of brassinolide, a steroid hormone that regulates plant growth and
development (Li & Chory, 1997). Further evidence that SR160 is indeed the tomato
brassinolide receptor came from another independent study aimed to isolate the
tomato gene CU-3 that functions like BRI1 (Montoya et al. , 2002). The study found
that the cloned CU-3 gene, mutation of which results in insensitivity to brassinolide,
encodes SR160. It was further revealed that the cu- 3 mutant is not only defective
in brassinolide signaling but also compromised in systemin signaling (Scheer et al. ,
2003). The above studies demonstrate that BRI1/SR160 has a dual function.
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