Identification of a novel self-compatible phenotype and its mutant SRK allele harbourin an LTR-retrotransposon in radish (Raphanus sativus L.)

Self-incompatibility (SI) responses of radish (Raphanus sativus L.) are determined by two tightly linked genes encoding an S receptor kinase (SRK) and an S-locus cysteine-rich protein/S locus protein 11 (SCR/SP11), respectively. A radish showing an almost self-compatible (SC) phenotype was identified in this study. Inheritance patterns showed that this SC phenotype was dominant over an SI phenotype. In addition, the SC phenotype co-segregated with an S haplotype in an F2 population. This SC radish contained an RsS-26 haplotype in which duplicate SRK-like genes were previously identified. Full-length sequences of two SRK-like genes of 18,133-bp and 6,200-bp in length were obtained, and they were designated as RsSRK-26-1 and RsSRK-26-2, respectively. No critical mutation was found in the coding region of SRK-like genes. However, a 4,146-bp intact LTR-retrotransposon was identified in the third intron of RsSRK-26-1 of the SC radish. Interestingly, this LTR-retrotransposon was not detected in three other breeding lines containing the same RsS-26 haplotype. While normal transcription of RsSRK-26-1 and SI phenotypes were observed in these three breeding lines, no transcript of RsSRK-26-1 was detected in the SC radish, suggesting that recent transposition of an LTR-retrotransposon in the RsSRK-26-1 gene might be responsible for the SC phenotype of radish.