| Self-incompatibility in plants is a process which involves cell-cell recognition events between pollen and stigma that prevents self-fertilization. Two members of the Brassica oleracea S-locus lined glycoprotein (SLSG) family, the S-locus glycoprotein (SLG) and the S-receptor kinase (SRK), are implicated by virtue of their close linkage to the genetically defined S-locus in the self-incompatibility process. The genome of Arabidopsis thaliana contains at least seven sequences which have similarity to members of the SLSG gene family. Like the Brassica SLSG gene family, the Arabidopsis sigma gene family appears to encode two types of products; putative single-pass receptor protein kinases and soluble glycoproteins. The putative receptor kinase members are encoded by the {dollar}Sigma{dollar}1, {dollar}Sigma{dollar}3 and {dollar}Sigma{dollar}4 genes, whereas the soluble product is encoded by the {dollar}Sigma{dollar}5 gene. At least one member of this family, {dollar}Sigma{dollar}2, appears to be a pseudogene. The {dollar}Sigma{dollar}7 sequence may encode a receptor kinase message that is not processed; this family member may also be a pseudogene. The {dollar}Sigma{dollar}3 and {dollar}Sigma{dollar}4 genes, and the {dollar}Sigma{dollar}1 and {dollar}Sigma{dollar}7 genes are present as small tandem duplications in the Arabidopsis genome. Except for these duplications, there does not appear to be any clustering of sigma genes within the genome. The sigma genes may be divided into classes based on their sequence similarity and expression pattern. The {dollar}Sigma{dollar}5 gene is most closely related to the {dollar}Sigma{dollar}2 pseudogene and the {dollar}Sigma{dollar}5 gene is expressed exclusively in floral tissues. The {dollar}Sigma{dollar}1, {dollar}Sigma{dollar}3 and {dollar}Sigma{dollar}4 genes are also closely related and are expressed in vegetative tissues. Experiments with reporter gene fusions indicate that neither the {dollar}Sigma{dollar}1 nor the {dollar}Sigma{dollar}3 gene is constitutively expressed. The two genes show distinct developmentally regulated patterns of tissue and organ expression that are virtually nonoverlapping. The relationship of these genes to the Brassica SLG and SRK genes, taken together with their homology to receptor protein kinases, makes it likely that these gene products are involved in some type of cell-cell signalling event. The nature of the signal transduction pathway and its relevance to the plant remain unknown. |
