Self-incompatibility patterns and signal transduction

Abstract

Self-incompatibility (SI), an important mechanism, enables plant species to increase their genetic variability and to avoid inbreeding depression. The recognition and rejection reactions for incompatibility involve a single polymorphic and multi-allelic S-locus. The pistil is enabled by the mechanism of SI to discriminate between self and non-self- pollen. The self-pollen when recognised by the pistil is either rejected just on the stigma or its pollen tube growth is halted in the style. The non-self-pollen is allowed to germinate and grow by the pistil. Plants showing variation with respect to S-locus are termed as haplotypes and are designated as S1, S2, S3, etc. In this review, focus is laid on the discussion about the recent developments in the understanding of three different mechanisms of SI. These mechanisms are controlled by two distinct determinant genes present at the S -locus. In Brassicaceae , the S-locus encodes two proteins: SRK which is a receptor kinase and a small protein SCR which acts as a ligand. SRK and SCR interact with each other and lead to the incompatible signalling in the papilla cells of the stigma. In case of Solanaceae , the rejection response is determined by a female determinant, a ribonuclease, and male determinant, an F-box protein. The mechanism of SI in Solanaceae is thought to be accomplished by the degradation of RNA and protein. In case of Papaveraceae , only the female determinant is known and the SI mechanism is supposed to involve a Ca2+ -dependent signalling cascade which in the end causes the death of non-self-pollen.