Self-incompatibility in the Brassicaceae: receptor-ligand signaling and cell-to-cell communication.

Cell–cell interactions in plants are expected to occur between adjoining cells that share a common developmental history. One exception is the interaction of the pollen or pollen tube with cells of the pistil. As a result, pollen–pistil interactions have emerged as models for the study of cell-to-cell signaling, particularly in the context of genetic self-incompatibility (SI). SI is an intraspecific mating barrier found in a large number of species distributed among many plant families that allows cells of the pistil to recognize and reject selfrelated pollen (De Nettancourt, 2001). Many SI systems are controlled genetically by a single highly polymorphic locus, the S locus, and pollen inhibition occurs when pollen and pistil are derived from plants that express the same S locus variant(s). Molecular studies performed during the last two decades have demonstrated that the term “self-incompatibility” does not represent one mechanism of self-recognition and that the S loci of different families are not homologous. Rather, the term encompasses a collection of disparate systems that have distinct evolutionary histories and are based on mechanistically different strategies for the inhibition of selfrelated pollen. One strategy used by members of the Solanaceae (McClure et al., 1989), Rosaceae (Sassa et al., 1993), Scrophulariaceae (Xue et al., 1996), and Campanulaceae (Stephenson et al., 2000) is directed at inhibiting pollen tubes after they have grown into the style and is based on the cytotoxic activity of stylar-secreted S-RNases, which act inside the pollen tube to inhibit its growth. Other strategies, such as those used by the Brassicaceae and Papavaraceae, are directed at preventing pollen germination or pollen tube ingress into the pistil; in this strategy, self pollen is inhibited at the stigma surface within minutes of pollen–stigma contact. It is in these two families that SI relies on the perception and transduction of specific signals, albeit using distinct molecular determinants and fundamentally different mechanisms, for the inhibition of self pollen. In the Brassicaceae, a signal is carried by the pollen grain, which is perceived and transduced into a cellular response within the stigma epidermis. In Papaver , the signal is produced by stigmatic cells and perceived by pollen, resulting in a transduction cascade within the pollen tube (Rudd and Franklin-Tong, 2001). Here, we review our understanding of cell-to-cell signaling in the SI system of the Brassicaceae, which is currently the only system for which the stigma and pollen recognition components are known.