Brassica self-incompatibility: a glimpse below the surface.

Self-incompatibility (SI) has emerged as an evolutionary strategy to enhance the genetic variability of plant species. In Brassica, it is controlled by a single multiallelic locus, the S-locus, encoding a receptor kinase (SRK) expressed in the stigma papilla cells and its ligand, a small protein (SCR) located in the pollen coat. Pollen rejection is achieved only when the receptor recognizes SCR coming from the same S-allele. If a single papilla cell is simultaneously pollinated by a self- and a cross-pollen grain, it is capable of distinguishing between the two and responding accordingly, rejecting self while accepting cross pollen. This phenomenon reveals that SI response is strictly localized and does not involve the whole papilla cell. It also suggests that the distribution of SRK inside the cell may play an important role in regulating this dual response. We have recently demonstrated that SRK is mostly intracellular, only small amounts being present in distinct domains of the plasma membrane (PM), where interaction with SCR occurs. Following ligand recognition, the receptor-ligand complex is endocytosed and degraded. Based on this, we propose a model of the significance of SRK intracellular trafficking for the functioning and specificity of SI response.