ABSTRACT
Angiosperms have evolved a system of sexual reproduction where water is not required at the point of fertilization, immobile sperm being delivered directly to the egg by a pollen tube. This key innovation, termed ‘siphonogamy’, allowed them to reproduce sexually in most terrestrial environments. Before a pollen tube (male gametophyte) can access an ovule to release its two sperms into the embryo sac (female gametophyte) it must fi rst penetrate the gynoecium and navigate its way through this material sporophytic tissue to fi nd an ovule. The events and interactions that occur during this prezygotic cellular and molecular ‘courtship’ between haploid pollen and diploid gynoecium have been termed the pollen-pistil interaction (Heslop-Harrison 1975) and consist of six key stages: pollen capture and adhesion; 2) pollen hydration; 3) germination of the pollen to produce a pollen tube; 4) penetration of the stigma by the pollen tube; 5) growth of the pollen tube through the stigma and style; 6) entry of the pollen tube into the ovule and discharge of the sperm cells. Understanding the physiological and molecular basis of the cellular interactions that occur during the pollenpistil interactions involving a continuous exchange of signals between the haploid pollen and the diploid maternal tissue of the pistil (sporophyte) is a major goal for plant biologists, because ultimately we all depend on this
fundamental biological process for our food. Despite the recent exponential increase in the number of molecules implicated in pollen-pistil interactions in different model plant species and the signifi cant progress that has been made in elucidating the molecular identity of these signals and the cellular interactions that they regulate, no general consensus has yet emerged of a universal set of pollen-pistil mediation molecules that regulate a common programme of cellular interactions (Hiscock and Allen 2008).
