We are trying to understand the molecular basis of specificity and of receptor-based signaling in the self-incompatibility response of crucifers.

The goal of the proposed research is to understand how cell-cell recognition signals the arrest of a critical developmental pathway in the reproductive phase of the plant’s life cycle. Our experimental system is the self-incompatibility response of crucifers, which is an intra-specific mating barrier that prevents self-fertilization by arresting the development of genetically related “self” pollen grains. The recognition of self in this system is based on the activity of highly diverged alleles of a transmembrane receptor protein kinase and its defensin-like peptide ligand, which are the female and male determinants of specificity, respectively.

We have demonstrated that specificity in the self-incompatibility response is based on allele-specific receptor-ligand interactions. We are performing structure-activity relationship studies of receptor and ligand with the goal of mapping domains that determine specificity in their interaction and in activation of the pollen-inhibitory signal transduction pathway. We are also investigating the self-incompatibility signal transduction pathway using molecular genetic and proteomics approaches.

The intra-specific divergence of the receptor-ligand pairs that determine specificity in the self-incompatibility response exceeds the inter-specific divergence typical of many reproductive proteins, such as mammalian proteins that function in species-specific sperm-egg interaction. Also like other self/nonself recognition molecules, these proteins are subject to diversifying selection and must co-evolve to maintain their specific interaction. Because the two interacting partners in self-incompatibility recognition are known, and a large number of natural receptor-ligand variants are available for study, this system is ideally suited for exploring the structural basis of specific receptor-ligand interactions and for gaining insight into the co-evolution of gene functions that typically underlies recognition phenomena.

Mikhail Nasrallah