Surfaces and secretions in the pollen-stigma interaction: a brief review

Interactions between pollen and pistil are key events in angiosperm reproduction, and for this reason they have been the target of an increasing volume of research in recent years. The regulation of fertilization has proved to be remarkably complex, involving controls of various kinds imposed at several different levels. Pollen hydration, governance of germination and stigma penetration, guidance and nutrition of the pollen tube in the style-as well as various, still enigmatic, direct interactions between male and female gametophytes - may all be concerned, individually or in combination, in determining whether fertilization will or will not be effected. The controls differ in degree of efficiency and specificity. Depending largely on the taxonomic remoteness of the parents, interspecific incompatibility may result simply from lack of physiological co-adaptation between the partners, or from rejection mechanisms of a more specific kind. Intraspecific incompatibility, although generally having a deceptively simple genetic basis, has proved to be surprisingly diverse in its physiological manifestations. The paper will present a brief conspectus of the present status of research in the field, and review some recent interpretations of the specific recognition events that may be involved in some of the interactions.