Figure 2.

Model for the mechanism of SSI in Brassica species. Inhibition of incompatible pollen occurs at the stigma surface, usually before germination of the pollen, and appears to involve a deregulation of water flow to the pollen from the stigma during hydration of the pollen grain [29]. Here, pollen from an S₁S₂ individual is inhibited on an S₁S₃ stigma as a consequence of a haplotype-specific interaction between male (pollen) and female (stigma) products of the S₁ haplotype (see Figure 1b). The S proteins are color coded: S₁, green; S₂, blue; and S₃, pink. SCR₁, a pollen S protein, is recognized by, and binds to, the extracellular receptor domain of SRK₁, a stigma S protein, thereby inducing dimerization of SRK₁ and autophosphorylation (shown by P) on serine and threonine residues in the kinase domain (shown as a zig-zag tail). 'Activation' of the SRK protein then initiates an intracellular signaling cascade within the stigma that leads to localized rejection of the pollen. SLG acts as an accessory protein in the formation of the receptor complex (see text for further details). PCP-A1, a small cysteine-rich pollen-coat protein similar to SCR, but not genetically linked to S, binds nonspecifically to SLGs from all haplotypes and may function as an additional accessory protein alongside SLG during formation of the receptor complex. Signaling downstream of SRK has not yet been characterized, but essential for the SI response is ARC1, an arm-repeat protein, that binds to the kinase domain of SRK in a phosphorylation-dependent manner, but whose function is as yet unknown. Exine is the outer region of the pollen cell wall.