A superfamily of S locus-related sequences in Arabidopsis: diverse structures and expression patterns

Six sequences that are closely related to the S gene family of the largely self-incompatible Brassica species have been identified in self-fertilizing Arabidopsis. The sequences define four genomic regions that map to chromosomes 1 and 3. Of the four functional genes identified, only the previously reported Arabidopsis AtS1 gene was expressed specifically in papillar cells and may function in pollination. The remaining three genes, including two novel genes designated ARK2 and ARK3, encode putative receptor-like serine/threonine protein kinases that are expressed predominantly in vegetative tissues. ARK2 promoter activity was detected exclusively in above-ground tissues, specifically in cotyledons, leaves, and sepals, in correlation with the maturation of these structures. ARK3 promoter activity was detected in roots as well as above-ground tissues but was limited to small groups of cells in the root-hypocotyl transition zone and at the base of lateral roots, axillary buds, and pedicels. The nonoverlapping patterns of expression of the ARK genes and the divergence of their sequences, particularly in their predicted extracellular domains, suggest that these genes perform nonredundant functions in specific aspects of development or growth of the plant body.