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 mechanisms are discussed, each controlled by two separate, highly polymorphic genes at the S-locus. For the Solanaceae and Papaveraceae types, the genes controlling female function in SI, the S-RNase gene and the S-gene, respectively, have been identified. For the Brassicaceae type, the gene controlling male function, SCR/SP11, and the gene controlling female function, SRK, have been identified. The S-RNase based mechanism involves degradation of RNA of self-pollen tubes; the S-protein based mechanism involves a signal transduction cascade in pollen, including a transient rise in [Ca²⁺]_i and subsequent protein phosphorylation/dephosphorylation; and the SRK (a receptor kinase) based mechanism involves interaction of a pollen ligand, SCR/SP11, with SRK, followed by a signal transduction cascade in the stigmatic surface cell.