The most important Brassica species, B. rapa, is naturally self-incompatible. Self-compatible mutants would be useful for dissecting the molecular mechanism of self-incompatibility (SI), a process that promotes outcrossing by recognizing and refusing self-pollens. The S haplotype in a new self-compatible B. rapa cultivar, Dahuangyoucai, was characterized for the first time in this study. Sequence analysis of the S-locus genes, SLG (S-locus glycoprotein), SRK (S-locus receptor kinase) and SCR (S-locus cysteine-rich protein) revealed that Dahuangyoucai contained S haplotype highly similar to S-f2, a non-functional class I S haplotype identified in another self-compatible B. rapa cultivar, Yellow Sarson. Mutations of MLPK (M-locus protein kinase) and non-transcription of the male determinant, SCR, were observed in this cultivar, which is similar to the situation reported in Yellow Sarson. With respect to the female determinant, SRK, no transcript was detected in Yellow Sarson but two fragments were detected in Dahuangyoucai. One fragment was highly similar to SRK-f2, but the other fragment was different from the signal factors previously identified in the SI reaction. The results suggest that Dahuangyoucai and Yellow Sarson have the same origin and a similar mechanism of self-compatibility, but diverge after mutations in SRK, SCR and MLPK. Further studying the self-compatibility of Dahuangyoucai might identify novel factors involved in the SI signalling cascade and provide new insights into the mechanisms of SI in Brassicaceae.
The S locus glycoprotein and the S receptor kinase are sufficient for self-pollen rejection in Brassica