1s Regulated by a lnositol

A signaling role for cytosolic free Caz+ ([Ca2+Ii) in regulating Papaver rhoeas pollen tube growth during the self-incompati-bility response has been demonstrated previously. In this article, we investigate the involvement of the phosphoinositide signal transduction pathway in Ca2+-mediated pollen tube inhibition. We demonstrate that R rhoeas pollen tubes have a Caz+-dependent polyphosphoinositide-specific phospholipase C activity that is inhibited by neomycin. [Ca2+Ii imag-ing after photolysis of caged inositol (1,4,5)-trisphosphate (lns[1,4,5]P3) in pollen tubes demonstrated that lns(1,4,5)P3 could induce CaZ+ release, which was inhibited by heparin and neomycin. Mastoparan, which stimulated lns(1,4,5)P3 production, also induced a rapid increase in Ca2+, which was inhibited by neomycin. These data provide direct evidence for the involvement of a functional phosphoinositide signal-transducing system in the regulation of pollen tube growth. We suggest that the observed Ca2+ increases are mediated, at least in part, by Ins(l,4,5)P3-induced CaZ+ release. Fur-thermore, we provide data suggesting that Caz+ waves, which have not previously been reported in plant cells, can be induced in pollen tubes