Soluble phospholipase A(2) activity is induced before oxylipin accumulation in tobacco mosaic virus-infected tobacco leaves and is contributed by patatin-like enzymes

Recent evidence suggests that oxidized lipid-derived molecules play signifi cant roles in inducible plant defence responses against microbial pathogens , either by directly deterring parasite multiplication, or as signals invol ved in the induction of sets of defence genes. The synthesis of these oxyli pins was hypothesized to be initiated by the phospholipase A(2)-mediated re lease of unsaturated fatty acids from membrane lipids. Here, we demonstrate that, in tobacco leaves reacting hypersensitively to tobacco mosaic virus, a strong increase in soluble phospholipass A(2) (PLA(2)) activity occurs a t the onset of necrotic lesion appearance. This rapid PLA(2) activation occ urred before the accumulation of 12-oxophytodienoic and jasmonic acids, two fatty acid-derived defence signals. Three PLA(2) isoforms were separated a nd the most active enzyme was partially purified, its N-terminal sequence d isplaying similarity with patatin, the major storage protein in potato tube rs. Three related tobacco patatin-like cDNAs, called NtPat1, NtPat2 and NtP at3, were cloned, with NtPat2 encoding the PLA(2) isolated from infected le aves. RT-PCR experiments showed a rapid transcriptional activation of the t hree NtPat genes in virus infected leaves, preceding the increase in PLA(2) activity. Recombinant NtPat1 and NtPat3 enzymes were active in an assay us ing labelled bacterial membranes, and also displayed high bona fide PLA(2) activity on phosphatidylcholine substrate. These results point to a possibl e new role of patatin-like phospholipases in inducible plant defence respon ses. The induction kinetics together with the enzymatic activity data indic ate that the NtPat proteins may provide precursors for oxylipin synthesis d uring the hypersensitive response to pathogens.