A signaling role for cytosolic free Ca2+ ([Ca2+]i) in regulating Papaver rhoeas pollen tube growth during the self-incompatibility 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 P. rhoeas pollen tubes have a Ca2+-dependent polyphosphoinositide-specific phospholipase C activity that is inhibited by neomycin. [Ca2+]i imaging after photolysis of caged inositol (1,4,5)-trisphosphate (Ins[1,4,5]P3) in pollen tubes demonstrated that Ins(1,4,5)P3 could induce Ca2+ release, which was inhibited by heparin and neomycin. Mastoparan, which stimulated Ins(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(1,4,5)P3-induced Ca2+ release. Furthermore, we provide data suggesting that Ca2+ waves, which have not previously been reported in plant cells, can be induced in pollen tubes.
A key event in signal transduction in many eukaryotes is activation of polyphosphoinositide‐specific phospholipase C (PIC). This enzyme hydrolyses the plasma membrane‐associated lipid, phosphatidylinositol(4,5)bisphosphate (Ptdlns(4,5)P2) which leads to the production of the two second messenger molecules: inositol(1,4,5)trisphosphate (Ins(1,4,5)P3) and 1,2‐diacylglycerol (DG). In plants, an enzyme which functionally resembles mammalian PIC is known to exist in the plasma membrane, but little is understood about how its activity is regulated. The recent discovery of several plant proteins with 30–40% homology to the mammalian actin‐ and phosphoinositide‐binding protein, profilin, has prompted an investigation as to whether these proteins (plant profilins) are able to interact with polyphosphoinositides and, if so, whether such interactions have physiological relevance for signal transduction via the plant phosphoinositide system. In this study it is demonstrated that a direct and highly specific interaction does exist between plant profilin and polyphosphoinositides and that these interactions dramatically affect the ability of plant plasma membrane phosphoinositide phospholipase C to utilize phosphoinositides for second messenger production. These data are the first to demonstrate a functional role of plant profilin in controlling polyphosphoinositide turnover and also provide the first evidence for a direct effect of an actin‐binding protein on a membrane‐associated signalling enzyme. These findings indicate a novel mechanism for control of plant phosphoinositide turnover, and suggest a possible link between plant cell activation, second messenger production and modulation of cytoskeletal dynamics.
A signaling role for cytosolic free Ca2+ ([Ca2+]i) in regulating Papaver rhoeas pollen tube growth during the self-incompatibility 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 P. rhoeas pollen tubes have a Ca2+-dependent polyphosphoinositide-specific phospholipase C activity that is inhibited by neomycin. [Ca2+]i imaging after photolysis of caged inositol (1,4,5)-trisphosphate (Ins[1,4,5]P3) in pollen tubes demonstrated that Ins(1,4,5)P3 could induce Ca2+ release, which was inhibited by heparin and neomycin. Mastoparan, which stimulated Ins(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(1,4,5)P3-induced Ca2+ release. Furthermore, we provide data suggesting that Ca2+ waves, which have not previously been reported in plant cells, can be induced in pollen tubes.
Salinity and hyperosmotic stress are environmental factors that severely affect the growth and development of plants. Adaptation to these stresses is known to be a complex multistep process, but a rise in cytoplasmic Ca 2+ and increased polyphosphoinositide turnover have now been identified as being amongst the early events leading to the development of tolerance. To determine whether a causal link exists between these two events we have investigated the effects of several salts and osmotic agents on levels of inositol(1,4,5)trisphosphate (Ins(1,4,5)P 3 ) in plant cells. Our data show that salts as well as osmotic agents induce a rapid and up to 15-fold increase in cellular Ins(1,4,5)P 3 levels. The increase in Ins(1,4,5)P 3 occurs in a dose-dependent manner and levels remain elevated for at least 10 min. These data indicate that increased Ins(1,4,5)P 3 production is a common response to salt and hyperosmotic stresses in plants and that it may play an important role in the processes leading to stress tolerance. ß
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