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

Abstract

A role for cytosolic free Ca2+ (Ca2+i) 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 Ca2+i 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)P3], point to a central role of the phosphoinositide signal transduction pathway in the control of Ca2+ fluxes and control of pollen tube growth. These experiments further revealed that increases in cytosolic levels of Ins(1,4,5)P3 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)P3-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 Ca2+i 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)P3-induced wave formation and apical Ca2+ oscillations.