Peptides as signals - Intercellular Communication in Plants - page 28

The advantages of using peptides as signals include their enormous structural di-
versity and the existence of various cellular mechanisms for controlling their activi-
ties (Alberts et al. , 1994; Bisseling, 1999). However, it was once thought that plants
have not evolved signaling systems that use a peptide as a signal. Instead, plant hor-
mone signaling was believed to be mediated by small molecules including the five
classical phytohormones: auxin, cytokinin, gibberellin, ethylene, and abscisic acid.
The question of how a small number of chemically simple molecules could account
for the observed diversity of cellular responses remains a paradox (Miklashevichs
et al. , 1996; Lindsey et al. , 2002). Ryan and colleagues at Washington State Univer-
sity set a milestone by discovering the first plant peptide hormone, called systemin,
that induces the systemic wounding response in tomato (Pearce et al. , 1991). Since
the discovery of systemin, several additional peptide signals have been found to
function as signals in cell-to-cell communication in plant developmental and physi-
ological processes. Growing evidence indicates that many more peptide signals are
likely involved in a diverse range of biological pathways.

2.2 Peptide signals in plants and their biological functions

2.2.1 Systemins mediate systemic and local wound responses
Plants, being sessile organisms, cannot move to escape attack by pathogens and
predators. Thus, plants have evolved complex defense mechanisms to protect them-
selves. In addition to preformed mechanisms such as physical barriers, plant cells
develop different inducible mechanisms in response to an attack. Wounding of plant
tissues by herbivory or mechanical damage activates a battery of defense responses
including production of protease inhibitors (pin) I and II which block protein degra-
dation in insects' digestive systems and affect larval growth (Johnson et al. , 1989).
The defense response is activated not only at the wound site but also in distal tissues,
suggesting that the local wound response leads to generation of a mobile signal that
activates the systemic response.
Application of an extract from wounded tomato leaves to an excised tomato plant
through a cut stem induces pin production in the leaves, indicating the existence of a
mobile wound signal in the extract. This biological assay was used to purify the sys-
temic signal, systemin (Pearce et al. , 1991). Systemin was found to be a proline-rich
18-amino acid peptide derived from its 200-amino acid precursor through limited
proteolysis (Fig. 2.1). When isotope-labeled systemin was applied to leaves through
fresh wounds, it was loaded into the phloem and transported into distal leaves of the
plant within 1-2 h. Systemin is biologically active at a femtomole concentration.
This strong inducing activity, together with its high mobility, makes it a powerful
systemic wound signal.
Transgenic tomato in which the systemin gene is suppressed through the
antisense-mediated gene suppression technology is defective in the systemic wound
response, which demonstrates its key role in distal signaling (McGurl et al. , 1992).