The AtPP gene of the Brassica napus S locus region is specifically expressed in the stigma and encodes a protein similar to a methyltransferase involved in plant defense

Self-incompatibility (SI) in Brassica is controlled by the S locus. The specificity of the Sl response is controlled on the stigma side by the S receptor kinase (SRK) and on the pollen side by the SCR (S locus cysteine-rich) protein, but other proteins might be involved in the process of self-pollen rejection. In this study, we show that the AtPP gene linked to the S locus of Brassica napus is expressed in the stigmas of Sl lines. AtPP has a developmental pattern of expression similar to the SRK gene. The AtPP protein has similarity with members of an Arabidopsis protein family and with an S-adenosyl-L-methionine:salicylic acid carboxyl methyltransferase, which is a plant defense-related protein of Clarkia breweri representing a new class of methyltransferases. A member of the AtPP gene family is present in the homeolog region of the S locus in Arabidopsis. Therefore, this gene might have co-evolved with S genes from an ancestral S locus of Brassicaceae. Possible functions of the AtPP protein in the self-recognition process are discussed.