A Pollen Coat Protein, SP11/SCR, Determines the PollenS-Specificity in the Self-Incompatibility ofBrassica Species

Abstract

Abstract Many flowering plants have evolved self-incompatibility (SI) systems to prevent inbreeding. In the Brassicaceae, SI is genetically controlled by a single polymorphic locus, termed theS-locus. Pollen rejection occurs when stigma and pollen share the same S-haplotype. Recognition ofS-haplotype specificity has recently been shown to involve at least two S-locus genes,S-receptor kinase (SRK) andS-locus protein 11 or S-locus Cys-rich (SP11/SCR). SRK encodes a polymorphic membrane-spanning protein kinase, which is the sole female determinant of the S-haplotype specificity. SP11/SCRencodes a highly polymorphic Cys-rich small basic protein specifically expressed in the anther tapetum and in pollen. In cauliflower (B. oleracea), the gain-of-function approach has demonstrated that an allele of SP11/SCRencodes the male determinant of S-specificity. Here we examined the function of two alleles of SP11/SCR ofB. rapa by the same approach and further established that SP11/SCR is the sole male determinant of SI in the genusBrassica sp. Our results also suggested that the 522-bp 5′-upstream region of theS  9  -SP11 gene used to drive the transgene contained all the regulatory elements required for the unique sporophytic/gametophytic expression observed for the nativeSP11 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 pollen of late developmental stages, the SP11 protein was mainly localized in the pollen coat, a finding consistent with its expected biological role.