F. C. H. Franklin

Cell and Molecular Biology of Self-Incompatibility in Flowering Plants

The potential to exploit self-incompatibility (SI) in plant breeding and its attraction as a system in which to investigate the molecular basis of cell-cell recognition and signaling has resulted in it becoming one of the most intensively studied and exciting topics in plant biology. Much research has encompassed the molecular cloning of genes involved in SI; consequently many reviews of this subject have focused on this area. In this article we have attempted to strike a somewhat different balance, with the objective of giving as broad a picture of the topic of SI as is possible within the confines of a single article. We provide a comprehensive summary of the genetics and population genetics of SI and, in describing progress toward the elucidation of the molecular basis of SI, have surveyed a much wider range of species than has previously been reviewed.

The intracellular events triggered by the self-incompatibility response inPapaver rhoeas

SummaryIn recent years self-incompatibility (SI) has come to be recognised as an important model system for studying cell-cell interactions and signalling in flowering plants. In this article we discuss the intracellular events associated with the SI response in the field poppy,Papaver rhoeas. The SI response inP. rhoeas is known to involve a Ca2+-based signalling pathway, activated following molecular interactions on the surface of incompatible pollen tubes. Evidence demonstrates that, following a transient increase in the concentration of cytosolic free Ca2+ ([Ca2+];) initiated by the SI response, phosphorylation of certain cytosolic proteins occurs, followed by activation of pollen gene expression. The magnitude of this transient Ca2+ wave and the localisation of cytosolic [Ca2+]i following the SI response are discussed. We also describe the character of the proteins specifically phosphorylated in the SI response and the nature of the protein kinases involved in their phosphorylation. Finally, we consider the possibility that the end result of the SI response inP. rhoeas may be analogous to programmed-cell-death mechanisms such as those seen in developmental processes and defence responses in various plant cells.

Molecular basis of the incompatibility mechanism in Papaver rhoeas L.

In this study we have begun to dissect the molecular mechanism of the self-incompatibility reaction of Papaver rhoeas. In order to gain an insight into the cellular activities, which lead to the inhibition of pollen tube growth following a self-incompatible response, we have been studying the effects of various metabolic inhibitors on pollen-stigmatic extract interactions in vitro. The results indicate that both transcription and glycosylation are required for the full inhibition of pollen tube growth during an incompatible response in P. rhoeas. The ability of actinomycin D to alleviate an incompatible reaction suggests that during the response pollen gene expression is induced; we have evidence that this is indeed the case and have identified novel proteins produced in the pollen which are associated with the incompatibility response. These findings give a clear indication that de novo transcription of pollen genes which are specific to this response, play an important role in the inhibition of pollen tube growth in this species. This provides a significant step towards the elucidation of the mechanism whereby pollen tube growth is arrested following an incompatible reaction in this species. Ribonuclease assays have revealed that, in contrast to the S-linked glycoprotein of Nicotiana alata, there is no detectable ribonuclease activity that correlates with the presence of the functional stigmatic S-gene product in P. rhoeas.

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

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.