Self-incompatibility (SI), a mechanism that prevents self- fertilization in plants with perfect flowers, is based on the ability of the pollinated flower to inhibit the development of self-related but not of genetically unrelated pollen. In Brassica, SI is genetically controlled by the highly polymorphic S locus, and work in the Nasrallahs' laboratory has demonstrated that two genes are tightly linked to this locus and located within 200 kb of each other: one gene encodes a transmembrane receptor protein kinase, and the other encodes a secreted glycoprotein highly homologous to the extracellular domain of the receptor. To understand the role of S-locus genes in SI, Dr. Nasrallah has begun an analysis of self- compatible variants in two predominantly self-incompatible species, B. oleracea and B. campestris. They identified a strain carrying a mutation in a trans-acting locus that regulates in the stigma S- locus function and the expression of the secreted glycoprotein gene at the RNA level. Two other strains were identified that carry S- locus linked mutations, or"self-fertility" alleles, and lack transcripts encoded by the receptor kinase gene They propose to continue the molecular genetic characterization of these spontaneous variants with the aim of deciphering the role of the receptor kinase gene in SI. In addition, other self-compatible variants will be isolated from among the available genetic base in an effort to obtain additional genetic markers at the complex S- locus region, and to ascertain the number of genes that are necessary and sufficient to impart an SI response. The genetic studies will be complemented by an analysis of engineered receptor kinase mutations expected to be dominant based on studies of receptor kinase mediated signalling pathways in other systems. The proposed analyses should not only yield insight into the mechanisms of self-recognition but also define the unique and novel aspects of cell-cell signalling in plants.
