RESEARCH SUMMARY

Our research focuses primarily on wound signalling in plants, with Arabidopsis thaliana, tomato and potato as model systems. Upon wounding, defence-related gene expression is induced at both damaged and distal tissues well apart from the wound site. The plant hormone jasmonic acid (JA) plays a pivotal role in the complex pathway that triggers activation of wound-responsive genes.

Our laboratory studies the role of JA and related compounds (oxylipins) in plant defence responses. We determined the localisation of various biosynthetic enzymes and studied possible interactions among them that could regulate oxylipin synthesis. Using transgenic overexpression and co-suppression approaches, we modified JA and oxylipin biosynthesis in transgenic potato and tomato plants, and studied the consequences on defence gene activation. Allene oxide synthase, which catalyses the first committed step in JA synthesis, is an important target of our research to elucidate a role for the isoforms found in potato and tomato. Overexpression of w-3 fatty acid desaturases in tomato results in production of higher levels of 3-hexenal, a potent antimicrobial oxylipin and a major component of the aroma of tomato fruit.

Reversible protein phosphorylation is a common molecular mechanism in the regulation of signal transduction pathways. We established that a regulatory phosphorylation switch controls the spatial and temporal pattern of wound-induced gene expression. In this switch, dephosphorylation of target protein(s) is essential for JA-dependent gene activation. Pharmacological studies indicate that PP2A and/or PP4 are the most likely candidates for dephosphorylating target effector proteins.

We have undertaken a reverse genetic approach to identify loss-of-function mutants in PP2A catalytic subunits (PP2Ac). These mutants will help to elucidate the role of specific PP2A in the regulation of signal transduction networks in Arabidopsis. The Arabidopsis genome encodes five PP2Ac genes. Homozygous knockout mutant lines have been generated with T-DNA insertions in all PP2Ac genes. T-DNA-mediated disruption of the PP2Ac-2 gene leads to ABA hypersensitivity. PP2Ac-2 thus appears to be a specific negative regulator of ABA signal transduction in Arabidopsis. However, all other single PP2Ac mutants show no obvious phenotypes, suggesting that the encoded proteins might play largely redundant roles, consistent with their high degree of similarity. Double mutants have also been generated for each gene pair. Thorough characterisation of these mutants is revealing PP2A involvement in the regulation of developmental pathways and in responses to environmental stress. Identification and characterisation of target proteins that are specifically dephosphorylated by PP2A in these signalling pathways is one main goal in our future work.