SRK, the stigma-specific S locus receptor kinase of Brassica, is targeted to the plasma membrane in transgenic tobacco

The S locus receptor kinase (SRK) gene is one of two S locus genes required for the self-incompatibility response in Brassica. We have identified the product of the SRK(6) gene in B. oleracea stigmas and have shown that it has characteristics of an integral membrane protein. When expressed in transgenic tobacco, SRK(6) is glycosylated and targeted to the plasma membrane. These results provide definitive biochemical evidence for the existence in plants of a plasma membrane-localized transmembrane protein kinase with a known cell-cell recognition function. The timing of SRK expression in stigmas follows a time course similar to that previously described for another S locus-linked gene, the S locus glycoprotein (SLG) gene, and correlates with the ability of stigmas to mount a self-incompatibility response. Based on SRK(6) promoter studies, the site of gene expression overlaps with that of SLG and exhibits predominant expression in the stigmatic papillar cells. Although reporter gene studies indicated that the SRK promoter was active in pollen, SRK protein was not detected in pollen, suggesting that SRK functions as a cell surface receptor exclusively in the papillar cells of the stigma.