A model for self-recognition and regulation of the incompatibility response of pollen.

Recent biochemical studies with Brassica indicate that the pollen grain has a primary role in the control of self incompatibility. Combining this new evidence with that from prior genetic, biochemical, and ultrastructural studies, a working model is hypothesized for the molecular events which occur during self recognition and the subsequent control of pollen germination. Self recognition is postulated to involve the interaction of a presynthesized, genotype-specific recognition molecule (effector) produced by the stigma with a presynthesized receptor molecule produced by and located in or on the pollen grain. The consequence of self recognition is a selective inhibition of pollen protein synthesis within about 2-4 minutes after imbibition. We deduced that protein synthesis is programmed to occur in pollen - unless interrupted as a consequence of self-recognition - and leads to the sequential production of opposing regulators: first a germination inhibitor (G-Inh), then a germination activator (G-Act). These regulators in turn control the activities of presynthesized, and probably sequestered enzymes required for germ tube formation. Sequential appearances of the G-Inh and G-Act occur unless synthesis of the G-Act is blocked as a result of self recognition. Thus, following a self pollination, recognition occurs in sufficient time to block production of the G-Act but not of the G-Inh, and inhibition of germination (incompatibility) results. For a cross pollination, there is no self recognition and production of the G-Act is unimpeded; it then nullifies the effect of the G-Inh and pollen germination (compatibility) results. The model and evidence for its support are discussed in detail.