Self-incompatibility. Prospects for a novel putative peptide-signaling molecule.

The recent identification of the long elusive pollen determinant of self-incompatibility (SI) in the Brassicaceae family (Schopfer et al., 1999) marks a major advance in SI research. Not only is it now unambiguously proven that in the genus Brassica SI specificity of pollen and stigma is governed by separate genes, but in addition, the availability of the pollen specificity gene opens new avenues for the study of the genetic, biochemical, and physiological aspects of an intriguing cell-cell communication system in plants. The Brassicaceae comprise many predominantly outbreeding species. These species favor crosspollination over self-pollination by means of a sophisticated SI system, whereby stigma epidermal cells (the papillar cells) recognize and reject selfrelated pollen. The SI recognition reaction is controlled genetically by a single locus, the complex and rearranged S locus (Nasrallah, 2000, and refs. therein) for which over 50 variants, designated S haplotypes, are known. Two sequence-related S-locus-encoded proteins, the S-locus glycoprotein (SLG) and the S-locus receptor kinase (SRK), are expressed specifically in stigmatic papillar cells. The SLG and SRK genes display extensive sequence polymorphism, which has been well documented for over 30 alleles (Stein et al., 1991; Kusaba et al., 1997). Genetic and biochemical evidence indicates that both genes are involved in the SI response of stigmas: SRK determines SI specificity of the papillar cells, and SLG seems to enhance the strength of the SI reaction (Takasaki et al., 2000), possibly by contributing to the stabilization of SRK (Dixit et al., 2000). Because SRK encodes a plasma membrane-localized receptor-like protein kinase, its biochemical function in SI recognition has been proposed to be the perception of a pollen-borne signal molecule and subsequent activation of a cytoplasmic signal transduction cascade that ultimately causes rejection of self-related pollen. Although the kinase activity of SRK has been confirmed experimentally, its receptor function remains circumstantial in the absence of any evidence that binding of a ligand to its ectodomain is translated into a cytoplasmic signal. Unlike the molecules responsible for stigma SI specificity, the molecules encoding SI specificity in pollen remained hypothetical until last year, when the S-locus Cys-rich (SCR) gene, represented by alleles from three S haplotypes, was identified and shown in a transgenic approach to confer SI recognition specificity to pollen (Schopfer et al., 1999). Alleles from several other S haplotypes subsequently have been identified (Fig. 1A). In agreement with the three prototypic sequences (SCR6, SCR8, and SCR13), all SCR alleles encode small polypeptides with a bipartite structure consisting of a conserved, mostly hydrophobic N-terminal part and a variable, mostly hydrophilic C-terminal part (Fig. 1A). The larger portion of the conserved N terminus has the characteristics of a secretion signal, which in the absence of other known localization signals suggests that the SCR polypeptides are processed via the secretory pathway with the apoplast being a likely destination. Unless further processing occurs, the mature gene products of SCR can therefore be assumed to be small (,8 kD) and hydrophilic polypeptides. Genetic control of pollen SI specificity has long been used as a primary criterion in the classification of SI systems. In sporophytically controlled SI systems such as those of the Brassicaceae, the phenotype of a pollen grain reflects the genotype of its diploid parent plant and not its own haploid genotype. Three hypotheses have been put forth to explain sporophytic control in SI. One hypothesis is based on premeiotic expression of the S-locus pollen gene within the meiocytes (Pandey, 1970). Another more widely accepted hypothesis proposes that the pollen S gene is expressed in the sporophytically derived cells of the tapetum (Heslop-Harrison, 1975), followed by transfer of its product to the pollen coat either by active secretion from the tapetal cells or by release from degenerating tapetal cells. More recently, the observation that a pollen coat protein is expressed gametophytically prompted a hypothesis of pseudosporophytic control of pollen coat proteins, and by inference of the pollen SI determinant (Doughty et al., 1998). This study counters the longheld belief that pollen coat components must be derived from the tapetum and that the thick exine layer presents a barrier to secretion from the pollen grain. In contrast, this new hypothesis assumes the free diffusion and mixing of gametophytically encoded coat proteins between pollen grains. It was hoped that the expression pattern of the anther-specific SCR gene would provide unambiguous support for one of the above hypotheses. However, the SCR gene exhibits a complex expression * Corresponding author; e-mail [email protected]; fax 607– 255–5407.