The Gα protein controls a pH-dependent signal path to the induction of phytoalexin biosynthesis in Eschscholzia californica

The function of a G alpha protein in the elicitation of phytoalexin (benzophenanthridine) biosynthesis was characterized in cultured cells of California poppy (Eschscholzia californica). Both the decrease of Ga content via antisense transformation and the expression of recombinant anti-G alpha single-chain antibodies strongly impaired the induction of alkaloid biosynthesis by low elicitor concentrations. All transgenic cell types were deficient in two elicitor-triggered early signal events: activation of phospholipase A2 (PLA(2)) and efflux of vacuolar protons. The lacking H+ efflux could be restored ( 1) by adding lysophosphatidylcholine (LPC), a product of PLA(2) activity, to vacuoles in situ and ( 2) by exposing intact cells to isotonic, near-neutral HEPES buffers. The latter treatment induced alkaloid biosynthesis in the absence of elicitor and in G alpha-deficient cells. We conclude that Ga mediates the stimulation of PLA(2) by low elicitor concentrations and that the resulting peak of LPC initiates a transient efflux of vacuolar protons. In this way, an acidic peak of the cytoplasmic pH is generated that causes the expression of enzymes of phytoalexin production independent of the hypersensitive response.