The S locus glycoprotein and the S receptor kinase are sufficient for self-pollen rejection in Brassica

Self-incompatibility (SI) is one of several mechanisms that have evolved to prevent inbreeding in plants. SI in Brassica is controlled by the polymorp hic S locus complex. Two S locus-encoded proteins are coordinately expresse d in the stigma epidermis: the cell wall-localized S locus glycoprotein (SL C) and the plasma membrane-anchored S receptor kinase (SRK). These proteins are thought to recognize a pollen factor that leads to the rejection of se lf-pollen. Evidence has accumulated that indicates that both proteins are n ecessary for the ability of the stigma to inhibit self-pollen. However, it has not been possible to prove this necessity definitively or to demonstrat e that these genes are sufficient for this phenotype, because previous atte mpts to transfer this phenotype via transformation have not been successful . In this study, two overlapping S locus genomic clones, which cover approx imate to 55 kilobases of DNA and contain the SLG, SRK, and an anther-expres sed gene in the region common to the two, were introduced into a self-compa tible Brassica napus line. The resulting transgenic plants were shown to ca rry the female part of the SI phenotype, rejecting pollen in a haplotype-sp ecific manner. However, the pollen St phenotype was not found in any of the transgenic plants. These data show that the SLC and SRK are sufficient for the female side but not the male side of the SI phenotype in Brassica and that there must be an independent pollen S factor encoded outside the clone d region.