The recognition and rejection of self-incompatible pollen in the Brassicaceae

Sporophytic self-incompatibility in the Brassicaceae is a reproductive strategy that prevents self-fertilization, and as a result, allows genetic diversity to be maintained. The conserved 'self' pollen recognition system is necessary due to the close proximity of male and female reproductive organs in the flower. This elaborate system is regulated by two tightly-linked polymorphic S genes encoding the pollen-coat protein S Cysteine-Rich/S Protein-11 (SCR/SP11) and the stigma-specific S Receptor Kinase (SRK). When the pollen is recognized as 'self', SCR/SP11 binds and activates SRK, and a signaling cascade is triggered in the stigma. While it is known that this signaling pathway leads to rejection of 'self' pollen, the cellular events downstream of SRK are less well understood. More recent research has identified new signaling proteins functioning downstream of SRK, Such as the M Locus Protein Kinase (MLPK) and the ARM-Repeat Containing-1 (ARC1) an E3 ubiquitin ligase. As well, changes in the actin cytoskeleton have been documented. Finally, with the premise that pollen rejection occurs by blocking cellular responses in the stigma required for the acceptance of compatible pollen, Exo70A1 has been identified as a 'compatibility' factor that is negatively regulated by the self-incompatibility pathway.