The Gα Protein Controls a pH-Dependent Signal Path to the Induction of Phytoalexin Biosynthesis in Eschscholzia california

The function of a Gα protein in the elicitation of phytoalexin (benzophenanthridine) biosynthesis was characterized in cultured cells of California poppy (Eschscholzia californica). Both the decrease of Gα content via antisense transformation and the expression of recombinant anti-Gα 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 (PLA₂) and efflux of vacuolar protons. The lacking H⁺ efflux could be restored (1) by adding lysophosphatidylcholine (LPC), a product of PLA₂ 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 Gα-deficient cells. We conclude that Gα mediates the stimulation of PLA₂ 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.