Second-messenger-induced signalling events in pollen tubes of Papaver rhoeas

Abstract

A role for cytosolic free Ca2+ (Ca i 2+ ) in the regulation of growth of Papaver rhoeas pollen tubes during the self-incompatibility response has recently been demonstrated [Franklin-Tong et al. Plant J. 4:163–177 (1993); Franklin-Tong et al. Plant J. 8:299–307 (1995); Franklin-Tong et al. submitted to Plant J.]. We have investigated the possibility that Ca i 2+ is more generally involved in the regulation of pollen tube growth using confocal laser scanning microscopy (CLSM). Data obtained using Ca2+ imaging, in conjunction with photolytic release of caged inositol 1,4,5-trisphosphate [Ins(1,4,5)P 3], point to a central role of the phosphoinositide signal transduction pathway in the control of Ca“ fluxes and control of pollen tube growth. These experiments further revealed that increases in cytosolic levels of Ins(1,4,5)P 3 resulted in the formation of distinct Ca2+ waves. Experiments using the pharmacological agents heparin, neomycin and mastoparan further indicated that Ca2+ waves are propagated, at least in part, by Ins(1,4,5)P 3-induced Ca2+ release rather than by simple diffusion or by “classic” Ca2+-induced Ca2+ release mechanisms. We also have data which suggest that Ca2+ waves and oscillations may be induced by photolytic release of caged Ca2+. Ratio-imaging has enabled us to identify an apical oscillating Ca2+ gradient in growing pollen tubes, which may regulate normal pollen tube growth. We also present evidence for the involvement of Ca2+ waves in mediating the self-incompatibility response. Our data suggest that changes in Ca i 2+ and alterations in growth rate/patterns are likely to be closely correlated and may be causally linked to events such as Ca2+-induced, or Ins(1,4,5)P 3-induced wave formation and apical Ca2+ oscillations.