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 themselves. 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 degradation in insects' digestive systems and affect larval growth (Johnson etal., 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 systemic 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).

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