Diverse cell signalling pathways regulate pollen‐stigma interactions: the search for consensus

Summary Siphonogamy, the delivery of nonmotile sperm to the egg via a pollen tube, was a key innovation that allowed flowering plants (angiosperms) to carry out sexual reproduction on land without the need for water. This process begins with a pollen grain (male gametophyte) alighting on and adhering to the stigma of a flower. If conditions are right, the pollen grain germinates to produce a pollen tube. The pollen tube invades the stigma and grows through the style towards the ovary, where it enters an ovule, penetrates the embryo sac (female gametophyte) and releases two sperm cells, one of which fertilizes the egg, while the other fuses with the two polar nuclei of the central cell to form the triploid endosperm. The events before fertilization (pollen–pistil interactions) comprise a series of complex cellular interactions involving a continuous exchange of signals between the haploid pollen and the diploid maternal tissue of the pistil (sporophyte). In recent years, significant progress has been made in elucidating the molecular identity of these signals and the cellular interactions that they regulate. Here we review our current understanding of the cellular and molecular interactions that mediate the earliest of these interactions between the pollen and the pistil that occur on or within the stigma – the ‘pollen–stigma interaction’. Contents Summary 286 I. Introduction 287 II. The pollen–stigma interaction 289 III. Pollen–stigma interactions in species with wet stigmas 290 IV. Pollen–stigma interactions in species with dry stigmas 295 V. Is there any consensus among cell signalling pathways regulating pollen–stigma interactions? 299 VI. Incompatibility and the pollen–stigma interaction 300 VII. New directions in pollen–stigma interaction research 302 VIII. Future prospects 310 Acknowledgements 312 References 312