Molecular recognition and response in pollen and pistil interactions

Many bisexual flowering plants possess a reproductive strategy called self- incompatibility (SI) that enables the female tissue (the pistil) to reject self but accept non-self pollen for fertilization. Three different SI mecha nisms are discussed, each controlled by two separate, highly polymorphic ge nes at the S-locus. For the Solanaceae and Papaveraceae types, the genes co ntrolling female function in SI, the S-RNase gene and the S-gene, respectiv ely, have been identified. For the Brassicaceae type, the gene controlling male function, SCR/SP11, and the gene controlling female function, SRK, hav e been identified. The S-RNase based mechanism involves degradation of RNA of self-pollen tubes; the S-protein based mechanism involves a signal trans duction cascade in pollen, including a transient rise in [Ca2+](i) and subs equent protein phosphorylation/dephosphorylation; and the SRK (a receptor k inase) based mechanism involves interaction of a pollen ligand, SCR/SP11, w ith SRK, followed by a signal transduction cascade in the stigmatic surface cell.