We show that differentiation of zoospores of the late blight pathogen Phytophthora infestans into
cysts, a process called encystment, was triggered by both
phosphatidic acid (PA) and the
G-protein activator
mastoparan.
Mastoparan induced the accumulation of PA, indicating that encystment by
mastoparan most likely acts through PA. Likewise, mechanical agitation of zoospores, which often is used to induce synchronized encystment, resulted in increased levels of PA. The levels of diacylglycerolpyrophosphate (
DGPP), the phosphorylation product of PA, increased simultaneously. Also in
cysts, sporangiospores, and mycelium,
mastoparan induced increases in the levels of PA and
DGPP. Using an in vivo assay for
phospholipase D (
PLD) activity, it was shown that the
mastoparan-induced increase in PA was due to a stimulation of the activity of this
enzyme.
Phospholipase C in combination with
diacylglycerol (
DAG) kinase activity also can generate PA, but activation of these
enzymes by
mastoparan was not detected under conditions selected to highlight 32P-PA production via
DAG kinase. Primary and secondary
butanol, which, like
mastoparan, have been reported to activate
G-proteins, also stimulated
PLD activity, whereas the inactive tertiary isomer did not. Similarly, encystment was induced by n- and sec-
butanol but not by
tert-butanol. Together, these results show that Phytophthora infestans contains a
mastoparan- and
butanol-inducible
PLD pathway and strongly indicate that
PLD is involved in zoospore encystment. The role of
G-proteins in this process is discussed.