Recent Progresses on Self-incompatibility Research in Brassica Species

Many hermaphrodite plant species have evolved mechanisms to prevent self-fertilization. One such mechanism is self-incompatibility (SI), which is defined as the inability of a fertile hermaphrodite plant to produce zygotes after self-pollination. SI prevents self-fertilization by rejecting pollen from plants with the same genotype. The SI system in Brassica is controlled sporophytically by multiple alleles at a single locus, designated as S, and involves cell-cell communication between male and female. When the S phenotype of the pollen is the same as that of the stigma, pollen germination and/or pollen tube penetration are disturbed on the papilla cells. On the female side, two genes (SLG and SRK) located at the S locus, are involved in the SI reaction. SLG is a secreted glycoprotein expressed abundantly in the papilla cell, and SRK is a membrane-spanning receptor-like serine/ threonine kinase whose extracellular domain is highly similar to SLG. Gain-of-function experiments have demonstrated that SRK solely determines S haplotypespecificity of the stigma, while SLG enhances the recognition reaction of SI. The sequence analysis of the S locus genomic region of Brassica campestris (syn. rapa) has led to the identification of an anther-specific gene, designated as SP11, which encodes a small cysteine-rich basic protein. Pollination bioassay and gain-of-function experiments have indicated that SP11 is the male S determinant. When the sequence of SP11 was aligned, six cysteine residues were found to be completely conserved among alleles. These conserved cysteine residues could be important for the tertiary structure of SP11. Recent biochemical analysis has suggested that SP11 operates as a sole ligand to activate its cognate SRK specifically. Because the activity of the S allele is controlled sporophytically, dominance relationships influence the ultimate phenotype of both the stigma and pollen. Molecular analysis has demonstrated that the dominance relationships between S alleles in the stigma were determined by SRK itself, but not by the relative expression level. In contrast, in the pollen, the expression of SP11 from the recessive S allele was specifically suppressed in the S heterozygote, suggesting that the dominance relationships in pollen were determined by the expression level of SP11.