The S-LOCUS CYSTEINE-RICH PROTEIN (SCR): A Small Peptide with A High Impact on the Evolution of Flowering Plants

Self-incompatibility (SI) is an archetypal cell-to-cell communication system in which self-pollen is rejected to prevent inbreeding. In crucifers (or Brassicaceae family), the pollen SI determinant is a small peptide, the S-LOCUS CYSTEINE-RICH PROTEIN (SCR, also known as SP11). During self-pollination, SCR binds to the extracellular domain of its cognate stigmatic receptor, the S-LOCUS RECEPTOR KINASE (SRK). This initiates a signaling cascade leading to self-pollen inhibition. The genes encoding both SI determinants are tightly linked in a multiallelic genomic region defined as the S-locus. Among S alleles, SCR shows extreme variability, and few residues in the protein have been reported to be critical for its specificity. In a heterozygous situation, SCR alleles display complex dominance relationships based on the silencing of certain SCR genes. This chapter provides a summary of the role of SCR in SI, the relationships between SCR alleles, and the role SCR loss played in the model plant Arabidopsis thaliana.