9631921 Nasrallah Genetic self-incompatibility, whereby the stigma of the flower is able to inhibit pollen germination and pollen tube development after self-pollination, represents one of the more promising systems for analyzing cell-cell communication in higher plants. In Brassica, Dr. Nasrallah has demonstrated that the self-incompatibility S locus is a multifunctional gene complex that controls signal perception through genes that are expressed in cells of the stigma epidermis and that encode plasma membrane- and cell wall-localized receptors. Thus, elucidation of the mechanism of self-incompatibility in this system is likely to provide insight into receptor-mediated cell-cell signaling in plants. They will continue their analysis of mutations that lead to the breakdown of self-incompatibility with the goal of identifying genes, either at the S locus or at unlinked loci, that encode the various components of the recognition and signal transduction phases of the self-incompatibility response. They will focus on a series of deletions that they have recently generated in strains carrying defined S haplotypes. Two classes of lesions that perturb the self-incompatibility response in stigma and/or pollen have been identified. In one class, the lesions map to the S locus and will be exploited to isolate novel genes required for the perception phase of the response. A second class of lesions results in the stigma-specific breakdown of self-incompatibility; these lesions affect a locus unlinked to the S locus which is a condidate for a gene controlling a step in the self-incompatibility signaling cascade. The study promises to influence our thinking on how plant cells transduce signals across their outer cell wall, how they utilize a precise molecular recognition system, and how a multi-component signal perception system has evolved and is maintained in a coadaped gene complex.
