Agriculture Reference
In-Depth Information
Figure 2.1
The sequences of systemins and the structures of their precursors. Prolines (P) and
hydroxyprolines (O) are in boldface. In the precursor diagram, the systemin peptides are shown as
black boxes. The gray boxes represent the leader sequences. After Ryan and Pearce (2003).
As a result, the transgenic plant is compromised in defense against
Manduca sexta
larvae.
Systemin homologues have been identified in many other species of the Solaneae
subtribe in the Solanaceae family but not in other subtribes of this family, such as
tobacco. In addition, the tomato systemin does not induce the wound response in
tobacco, which suggests that it is highly diverged from the tobacco wound signal
and not perceived by tobacco cells. Tobacco appears to be missing a strong leaf-to-
leaf long-distance wound signaling system, although strong leaf-to-root and local
wound signaling systems are present (Pearce
et al.
, 1993; Zhang & Baldwin, 1997;
Constabel
et al.
, 1998).
Exogenous application of systemin to tomato suspension cultures causes a rapid
increase in the pH of the medium. This alkalinization is a convenient assay for the
biological activity of systemin. Although the tomato systemin does not generate
an alkalinization response when applied to tobacco suspension cells, extracts from
tobacco leaves can do so, suggesting that a systemin-like activity exists in tobacco.
Facilitated by the alkalinization assay, the Ryan group isolated two peptides
from tobacco, Tobacco Systemin I and Tobacco Systemin II (which were later
renamed TobHypSys I and II), that function to induce the localized wound response
(Pearce
et al.
, 2001a). Both tobacco systemins are hydroxyproline-rich 18-amino
acid peptides generated from proteolytic processing of a single 165-amino acid
precursor (Fig. 2.1). TobHypSys I and II do not share significant sequence similarity
to each other or to the tomato systemin. In addition, unlike the tomato systemins,
the tobacco systemins are glycosylated. However, they are all proline-rich (or
hydroxyproline-rich in the case of tobacco systemins) and have a -PPS- (or -OOS-)
motif. Prolines and hydroxyprolines may allow some limited conformations in
their secondary structures, which could play important roles in interaction with
their receptors (Pearce & Ryan, 2003; Narvaez-Vasquez & Ryan, 2004).
Although the suppression of the tomato systemin gene blocks systemic wounding
signaling, it does not have a significant affect on local wound signaling (McGurl
et al.
, 1992), which suggests that local wound signaling in tomato is independent
