Self-recognition between pollen and stigma during pollination in Brassica olracea is genetically controlled by the multiallelic self-incompatibility locus (S). We describe the S receptor kinase (SRK) gene, a previously uncaracterize gene that resides at the S locus. The nucleotide sequences of genomic DNA and of cDNAs corresponding to SRK predict a putative transmembrane receptor having serine/threoninespecific protein kinase activity. Its extracellular domain exhibits king homology to the secreted product of the S-locus glycoprotein (SLG) gene and is connected via a single pass trausmembrane domain to a protein kina catalytic center.SRK alleles derived from different S-locus genotypes are highly polymorphic and have apparently evolved in unison with genetically linked alleles of SLW. SRK directs the synthesis of several alternative transcripts, which potentially encode different protein products, and these transcripts were detected exclusively in reproductive organs. The identification of SRK may provide new perspectives into the signal transduction mechanism underlying pollen recognition.Pollination and the subsequent invasive growth of pollen tubes into the female stigmatic and pistil tissues prior to fertilization provide an opportunity to study cell-cell interactions in flowering plants. In crucifers such as Brassica oleracea, self-recognition between pollen and stigma is controlled by the multiallelic self-incompatibility, or S, locus (1). In general, pollen germination and/or tube growth are arrested at the stigma surface if the pollen and stigma are borne by plants having identical S-locus genotypes. This arrest prevents self-fertilization and is termed the self-incompatibility (SI) response. Two related genes have been identified at the S locus by molecular methods (2-4). Of these, only one gene, the S-locus glycoprotein (SLG) gene has been characterized extensively. SLG encodes a secreted glycoprotein that is highly polymorphic in different S-locus genotypes (2) and may therefore be involved in determining the recognition specificity displayed in SI. Furthermore, SLG is expressed in stigmatic papillae (3) and anthers (5, 6), consistent with models for SI in which both pollen and stigma bear recognition determinants derived from the S locus.In this study, we show that the second S-locus-linked gene (4) encodes a putative receptor protein kinase, and we have therefore designated it SRK, for S receptor kinase.t Its structure is similar to that predicted in a recently described maize root cDNA clone, ZmPKJ (7), and is analogous to the growth factor receptor tyrosine kinases in animals. The putative ligand-binding domain is homologous to SLG and displays genotype-specific sequence polymorphisms that parallel those of SLG. SRK transcripts were detected only in the male and female reproductive organs, thus showing a pattern of expression similar to that of SLG. These findings offer foundation to the hypothesis that SI is mediated by receptor-ligand interactions between pollen and pistil and provide a ...

In the S locus-controlled self-incompatibility system of Brassica, recognition of self-related pollen at the surface of stigma epidermal cells leads to inhibition of pollen tube development. The female (stigmatic) determinant of this recognition reaction is a polymorphic transmembrane receptor protein kinase encoded at the S locus. Another highly polymorphic, anther-expressed gene, SCR, also encoded at the S locus, fulfills the requirements for the hypothesized pollen determinant. Loss-of-function and gain-of-function studies prove that the SCR gene product is necessary and sufficient for determining pollen self-incompatibility specificity, possibly by acting as a ligand for the stigmatic receptor.

As a starting point for a phylogenetic study of self-incompatibility (SI) in crucifers and to elucidate the genetic basis of transitions between outcrossing and self-fertilizing mating systems in this family, we investigated the SI system of Arabidopsis lyrata . A. lyrata is an outcrossing close relative of the self-fertile A. thaliana and is thought to have diverged from A. thaliana ‫ف‬ 5 million years ago and from Brassica spp 15 to 20 million years ago. Analysis of two S (sterility) locus haplotypes demonstrates that the A. lyrata S locus contains tightly linked orthologs of the S locus receptor kinase ( SRK ) gene and the S locus cysteine-rich protein ( SCR ) gene, which are the determinants of SI specificity in stigma and pollen, respectively, but lacks an S locus glycoprotein gene. As described previously in Brassica , the S haplotypes of A. lyrata differ by the rearranged order of their genes and by their variable physical sizes. Comparative mapping of the A. lyrata and Brassica S loci indicates that the S locus of crucifers is a dynamic locus that has undergone several duplication events since the Arabidopsis-Brassica split and was translocated as a unit between two distant chromosomal locations during diversification of the two taxa. Furthermore, comparative analysis of the S locus region of A. lyrata and its homeolog in self-fertile A. thaliana identified orthologs of the SRK and SCR genes and demonstrated that self-compatibility in this species is associated with inactivation of SI specificity genes. INTRODUCTIONSelf-incompatibility (SI) is the major outcrossing mechanism in the family Brassicaceae (de Nettancourt, 1977). Species in this family have been grouped into 19 tribes on the basis of morphological criteria (Schultz, 1936), and SI has been described in all tribes analyzed to date. When Bateman (1955) surveyed 182 species distributed in 11 tribes, he found that approximately half of these species included selfincompatible accessions. In a survey of 59 taxa in the subtribe Brassicineae of the tribe Brassiceae (which includes Brassica and Raphanus ), 50 taxa were self-incompatible (Takahata and Hinata, 1980). In all cases analyzed, SI has been shown to be controlled sporophytically by a single S (sterility) locus, with multiple alleles or variants and complex dominance relationships between alleles (Bateman, 1954(Bateman, , 1955Thompson and Taylor, 1966): in self-incompatible plants, pollen will not develop on a stigma that expresses the same S alleles as the pollen parent.Molecular analysis of the Brassica S locus region has shown that this mendelian locus is a gene complex consisting of distinct stigma-expressed and anther-expressed genes that determine SI specificity in stigma and pollen, respectively (reviewed in Nasrallah, 2000). The SRK (for S locus receptor kinase) gene (Stein et al., 1991) encodes a plasma membrane-spanning receptor serine/threonine kinase specific to the stigma epidermis (Stein et al., 1996) and is the determinant of SI specificity in the stigma (Takasaki et al., 2000). ...

The switch from an out-crossing to a self-fertilizing mating system is one of the most prevalent evolutionary trends in plant reproduction and is thought to have occurred repeatedly in flowering plants. However, little is known about the evolution of self-fertility and the genetic architecture of selfing. Here, we establish Arabidopsis thaliana as a model for genetic analysis of the switch to self-fertility in the crucifer family, where the ancestral out-crossing mode of mating is determined by self-incompatibility (SI), a genetic system controlled by the S locus. We show that A. thaliana ecotypes exhibit S-locus polymorphisms and differ in their ability to express the SI trait upon transformation with S-locus genes derived from the obligate out-crosser Arabidopsis lyrata. Remarkably, at least one ecotype was reverted to a stable, self-incompatible phenotype identical to that of naturally self-incompatible species. These ecotype differences are heritable and reflect the fixation in different A. thaliana populations of independent mutations that caused or enforced the switch to self-fertility. Their continued analysis promises to identify the loci that were the targets of natural selection for selfing and to contribute to a mechanistic understanding of the SI response.

Transitions from cross-fertilizing to self-fertilizing mating systems have occurred frequently in natural and domesticated plant populations, but the underlying genetic causes are unknown. We show that gene transfer of the stigma receptor kinase SRK and its pollen-borne ligand SCR from one S-locus haplotype of the self-incompatible and cross-fertilizing Arabidopsis lyrata is sufficient to impart self-incompatibility phenotype in self-fertile Arabidopsis thaliana, which lacks functional orthologs of these genes. This successful complementation demonstrates that the signaling cascade leading to inhibition of self-related pollen was maintained in A. thaliana. Analysis of self-incompatibility will be facilitated by the tools available in this species.