Members of the S-receptor kinase multigene family in Senecio squalidus L. (Asteraceae), a species with sporophytic self-incompatibility

While the molecular basis of sporophytic self-incompatibility (SSI) has been investigated extensively in the Brassicaceae, almost nothing is known about the molecular regulation of SSI in other families, such as the Asteraceae. In species of Brassica and in Arabidopsis lyrata, a stigma-specific serine-threonine receptor kinase (SRK) and its cognate ligand, a pollen coating-borne cysteine-rich protein (SCR/SPII), determine the female and male sides of the SSI response, respectively. Here we have used RT-PCR with degenerate primers to conserved regions of SRK to amplify three SRK-like gene fragments expressed in stigmas of Senecio squalidus (Asteraceae). The Senecio S-receptor-like kinase (SSRLK) sequences share similar to43% amino acid sequence identity with Brassica SRK3 but higher amino acid sequence identity (similar to50%) with two Solanum bulbocastanum receptor-like kinase genes of unknown function. Despite expression in stigmas, all three SSRLKs were expressed at varying levels in floral and vegetative tissues. No allelic polymorphism was detected for the three SSRLKs in two S homozygous lines of S. squalidus or three other lines of S. squalidus carrying different S alleles. A full-length cDNA clone was obtained for SSRLK1 and its predicted amino acid sequence revealed significant structural differences to Brassica SRKs, most notably a major N-terminal truncation of 169 amino acids and the presence of just 8 conserved cysteine residues within the putative receptor domain instead of 12. Together, the sequence characteristics and expression characteristics of SSRLKs suggest that they are unlikely to be directly involved in the SSI response of S. squalidus. These findings are discussed in terms of the evolution of the SRK multigene family and the molecular basis of SSI in S. squalidus and the Asteraceae.