Cell-cell interactions are crucial for the development of multicellular organisms. In flowering plants, the cellular interactions that take place between the pollen and pollen tube (the male gametophyte) on the one hand and the pistil (the female reproductive structure) on the other hand are required for successful fertilization and the completion of the plant life cycle. These highly specific interactions occur at every step along the path of pollen tube development into the pistil, and ensure that only appropriate sperm cells are delivered to the ovules. Pollen-pistil interactions provide a unique opportunity in plants for elucidating the molecular basis of cell-cell communication as well as for understanding the operation and evolution of intra-specific and inter-specific barriers to fertilization. One goal of our research on the pollination biology of crucifers is to investigate the molecular mechanisms that underlie pollen-pistil interactions in compatible (productive) and incompatible (unproductive) pollinations through the use of molecular genetic, biochemical, and cell biological approaches. A second goal is to elucidate the genetic bases of transitions between outbreeding and selfing mating systems through the use of a phylogenetic/comparative genomics approach.
Like many other plant species with perfect flowers, approximately half of the species in the crucifer family possess a genetic self-incompatibility system. Self-incompatibility is a barrier to self-fertilization that is thought to have evolved as a mechanism for reducing inbreeding in natural populations. In the crucifer Brassica, self-incompatibility is controlled genetically by a Mendelian S locus. Our molecular analysis has shown that this locus is a complex of highly polymorphic and apparently coadapted genes. These genes encode cell surface receptors required for the stigma to distinguish self-related from self-unrelated pollen, as well as a small cysteine-rich protein borne by pollen that identifies the pollen grain as being self or non self and that likely represents the ligand for the stigma-localized receptor protein kinase. Based on these findings, the Brassica self-incompatibility system is now recognized as a model system for the study of receptor-ligand interaction in plants. Our current work relates to the interaction of the receptor protein kinase with its putative pollen ligand, activation of the receptor, and the signaling cascade that leads to the inhibition of pollen tube development. In parallel to these studies, we have initiated expressed sequence tag (EST) and proteomics projects aimed at uncovering genes and proteins involved in the adhesion of pollen to the pistil epidermis and in the guidance of pollen tubes to the ovules.