The role of a wound-inducible polygalacturonase and hydrogen peroxide in the activation of plant defensive genes

A novel wound-inducible polygalacturonase catalytic subunit (PGcat) and its β subunit were identified in leaves of young tomato plants. The PGcat belongs to a new subfamily of PG enzymes in plants having about 40% identity at the amino acid level with the tomato fruit and absicion zone PGcat subunits. The leaf wound-inducible β subunit has 100% identity to a β subunit gene previously identified in the tomato genome, but which has never been known to be expressed in leaf tissue. The mRNAs encoding the PGcat and β subunit were expressed in leaves in response to wounding, systemin and oligosaccharide elicitors. The synthesis of the two mRNAs was accompanied by an increase in PG activity, in both the wounded leaf (localized response) and in the unwounded leaf (systemic response) of young tomato plants.; Since oligouronides (OGA) released by PG are known to cause a strong oxidative burst and the activation of defense gene expression, the accumulation of the reactive oxygen species H₂O₂ in response to wounding was also investigated. H₂O₂ accumulated locally and systemically in the leaves of wounded tomato plants with a kinetics of induction similar to that of PG activity. The production of H₂O₂ was activated by systemin, OGA, chitosan and methyl jasmonate (MJ), all inducers of PG, and was mediated through the octadecanoid pathway. PG and H₂O ₂ were also wound-inducible in leaves of species from several plant families.; To investigate the role of hydrogen peroxide (H₂O₂) in the wound response of tomato plants, several chemical inhibitors which block the accumulation of H₂O₂ were used. DPI, an NADPH oxidase inhibitor, inhibited H₂O₂ production and the accumulation of proteinase inhibitors induced by systemin, oligosaccharides and MJ. However, the expression of genes encoding the signal-related protein prosystemin, or the early wound-inducible enzymes lipoxygenase and allene oxide synthase of the octadecanoid signaling pathway, were not inhibited by DPI. Treatment of plants with the H₂O₂-generating enzyme glucose oxidase along with glucose resulted in the induction of genes involved in plant defense but did not activate the early signaling-related genes in tomato leaves. H₂O₂ was cytochemically detected in the cell walls of vascular parenchyma cells following wounding of wild-type tomato leaves as well as in the cell walls of mesophyll cells of transgenic tomato plants overexpressing prosystemin. The cummulative results indicate that H₂O₂ produced in the vascular bundles acts as a second messenger for the wound induction of defense genes in the mesophyll cells. The similarities in the systemic induction of both PG activity and H₂O₂ in several plant species suggests that herbivory may activate signaling for defense against both herbivores and pathogens, and that this defense response is widespread in the plant kingdom.