A pollen coat protein, SP11/SCR, determines the pollen S-specificity in the self-incompatibility of Brassica species

Many flowering plants have evolved self-incompatibility (SI) systems to pre vent inbreeding. In the Brassicaceae, SI is genetically controlled by a sin gle polymorphic locus, termed the S-locus. Pollen rejection occurs when sti gma and pollen share the same S-haplotype. Recognition of S-haplotype speci ficity has recently been shown to involve at least two S-locus genes, S-rec eptor kinase (SRK) and S-locus protein 11 or S-locus Cys-rich (SP11/SCR). S RK encodes a polymorphic membrane-spanning protein kinase, which is the sol e female determinant of the S-haplotype specificity. SP11/SCR encodes a hig hly polymorphic Cys-rich small basic protein specifically expressed in the anther tapetum and in pollen. In cauliflower (B. oleracea), the gain-of-fun ction approach has demonstrated that an allele of SP11/SCR encodes the male determinant of S-specificity. Here we examined the function of two alleles of SP11/SCR of B. rapa by the same approach and further established that S P11/SCR is the sole male determinant of SI in the genus Brassica sp. Our re sults also suggested that the 522-bp 5'-upstream region of the S-9-SP11 gen e used to drive the transgene contained all the regulatory elements require d for the unique sporophytic/gametophytic expression observed for the nativ e SP11 gene. Promoter deletion analyses suggested that the highly conserved 192-bp upstream region was sufficient for driving this unique expression. Furthermore, immunohistochemical analyses revealed that the protein product of the SP11 transgene was present in the tapetum and pollen, and that in p ollen of late developmental stages, the SP11 protein was mainly localized i n the pollen coat, a finding consistent with its expected biological role.