Self-recognition between pollen and stigma during pollination in Brassica olracea is genetically controlled by the multiallelic self-incompatibility locus (S). We describe the S receptor kinase (SRK) gene, a previously uncaracterize gene that resides at the S locus. The nucleotide sequences of genomic DNA and of cDNAs corresponding to SRK predict a putative transmembrane receptor having serine/threoninespecific protein kinase activity. Its extracellular domain exhibits king homology to the secreted product of the S-locus glycoprotein (SLG) gene and is connected via a single pass trausmembrane domain to a protein kina catalytic center.SRK alleles derived from different S-locus genotypes are highly polymorphic and have apparently evolved in unison with genetically linked alleles of SLW. SRK directs the synthesis of several alternative transcripts, which potentially encode different protein products, and these transcripts were detected exclusively in reproductive organs. The identification of SRK may provide new perspectives into the signal transduction mechanism underlying pollen recognition.Pollination and the subsequent invasive growth of pollen tubes into the female stigmatic and pistil tissues prior to fertilization provide an opportunity to study cell-cell interactions in flowering plants. In crucifers such as Brassica oleracea, self-recognition between pollen and stigma is controlled by the multiallelic self-incompatibility, or S, locus (1). In general, pollen germination and/or tube growth are arrested at the stigma surface if the pollen and stigma are borne by plants having identical S-locus genotypes. This arrest prevents self-fertilization and is termed the self-incompatibility (SI) response. Two related genes have been identified at the S locus by molecular methods (2-4). Of these, only one gene, the S-locus glycoprotein (SLG) gene has been characterized extensively. SLG encodes a secreted glycoprotein that is highly polymorphic in different S-locus genotypes (2) and may therefore be involved in determining the recognition specificity displayed in SI. Furthermore, SLG is expressed in stigmatic papillae (3) and anthers (5, 6), consistent with models for SI in which both pollen and stigma bear recognition determinants derived from the S locus.In this study, we show that the second S-locus-linked gene (4) encodes a putative receptor protein kinase, and we have therefore designated it SRK, for S receptor kinase.t Its structure is similar to that predicted in a recently described maize root cDNA clone, ZmPKJ (7), and is analogous to the growth factor receptor tyrosine kinases in animals. The putative ligand-binding domain is homologous to SLG and displays genotype-specific sequence polymorphisms that parallel those of SLG. SRK transcripts were detected only in the male and female reproductive organs, thus showing a pattern of expression similar to that of SLG. These findings offer foundation to the hypothesis that SI is mediated by receptor-ligand interactions between pollen and pistil and provide a ...

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Primary signal transduction plays a vital role in the way plants react to environmental and developmental signals. We report the sequence and expression of a putative receptor kinase gene, ARK1, in Arabidopsis thaliana that may be important in this regard. This Arabidopsis gene encodes a transmembrane protein with a cytoplasmic kinase catalytic domain, a transmembrane region, and an extracellular domain with sequence similarity to the secreted S-locus glycoprotein (SLG) gene of Brassica oleracea. This structure is similar to the S-locus receptor kinase (SRK) gene of Brassica and to the receptor kinase ZmPK1 gene of maize. RNA blots indicate that transcripts accumulate predominantly in leaf tissue, with limited amounts in stem and floral bud tissue and no detectable transcripts accumulating in root tissue. A smaller transcript that could be an alternative transcript of ARK1 also accumulates in leaf tissue. This transcript possibly encodes a secreted SLG-like glycoprotein that lacks transmembrane and kinase domains. The predominantly vegetative expression of ARK1 indicates that this gene is not primarily involved in pollen/pistil interactions in Arabidopsis.

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