The systemin signaling pathway: differential activation of plant defensivegenes

Systemin, an 18-amino-acid polypeptide released from wound sites on tomato leaves caused by insects or other mechanical damage, systemically regulates the activation of over 20 defensive genes in tomato plants in response to herbivore and pathogen attacks. Systemin is processed from a larger prohorm one protein, called prosystemin, by proteolytic cleavages. However, prosyst emin lacks a signal sequence and glycosylation sites and is apparently not synthesized through the secretory pathway, but in the cytoplasm. The polype ptide activates a Lipid-based signal transduction pathway in which the 18:3 fatty acid, linolenic acid, is released from plant membranes and converted to the oxylipin signaling molecule jasmonic acid. A wound-inducible system in cell surface receptor with an M-r of 160 000 has recently been identifie d, The receptor regulates an intracellular cascade including, depolarizatio n of the plasma membrane, the opening of ion channels, an increase in intra cellular Ca2+. activation of a MAP kinase activity and a phospholipase A? a ctivity. These rapid changes appear to play important roles leading to the intracellular release of linolenic acid from membranes and its subsequent c onversion to jasmonic acid, a potent activator of defense gene transcriptio n. Although the mechanisms for systemin processing, release, and transport are still unclear, studies of the timing of the synthesis and of the intrac ellular localization of wound- and systemin-inducible mRNAs and proteins in dicates that differential syntheses of signal pathway genes and defensive g enes are occurring in different cell types. This signaling cascade in plant s exhibits extraordinary analogies with the signaling cascade for the infla mmatory response in animals. (C) 2000 Elsevier Science B.V. All rights rese rved.