In vivo imaging of the S-locus receptor kinase,the female specificity determinant of self-incompatibility,in transgenic self-incompatible Arabidopsis thaliana

Background and Aims The S-locus receptor kinase (SRK), which is expressed in stigma epidermal cells, is responsible for the recognition and inhibition of ‘self’ pollen in the self-incompatibility (SI) response of the Brassicaceae. The allele-specific interaction of SRK with its cognate pollen coat-localized ligand, the S-locus cysteine-rich (SCR) protein, is thought to trigger a signalling cascade within the stigma epidermal cell that leads to the arrest of ‘self’ pollen at the stigma surface. In addition to the full-length signalling SRK receptor, stigma epidermal cells express two other SRK protein species that lack the kinase domain and whose role in the SI response is not understood: a soluble version of the SRK ectodomain designated eSRK and a membrane-tethered form designated tSRK. The goal of this study was to describe the sub-cellular distribution of the various SRK protein species in stigma epidermal cells as a prelude to visualizing receptor dynamics in response to SCR binding.Methods The Arabidopsis lyrata SRKb variant was tagged with the Citrine variant of yellow fluorescent protein (cYFP) and expressed in A. thaliana plants of the C24 accession, which had been shown to exhibit a robust SI response upon transformation with the SRKb–SCRb gene pair. The transgenes used in this study were designed for differential production and visualization of the three SRK protein species in stigma epidermal cells. Transgenic stigmas were analysed by pollination assays and confocal microscopy.Key Results and Conclusions Pollination assays demonstrated that the cYFP-tagged SRK proteins are functional and that the eSRK is not required for SI. Confocal microscopic analysis of cYFP-tagged SRK proteins in live stigma epidermal cells revealed the differential sub-cellular localization of the three SRK protein species but showed no evidence for redistribution of these proteins subsequent to incompatible pollination.     

Brassica self-incompatibility: A glimpse below the surface

Self-incompatibility (SI) has emerged as an evolutionary strategy to enhance the genetic variability of plant species. In Brassica, it is controlled by a single multiallelic locus, the S-locus, encoding a receptor kinase (SRK) expressed in the stigma papilla cells and its ligand, a small protein (SCR) located in the pollen coat. Pollen rejection is achieved only when the receptor recognizes SCR coming from the same S-allele. If a single papilla cell is simultaneously pollinated by a self- and a cross-pollen grain, it is capable of distinguishing between the two and responding accordingly, rejecting self while accepting cross pollen. This phenomenon reveals that SI response is strictly localized and does not involve the whole papilla cell. It also suggests that the distribution of SRK inside the cell may play an important role in regulating this dual response. We have recently demonstrated that SRK is mostly intracellular, only small amounts being present in distinct domains of the plasma membrane (PM), where interaction with SCR occurs. Following ligand recognition, the receptor-ligand complex is endocytosed and degraded. Based on this, we propose a model of the significance of SRK intracellular trafficking for the functioning and specificity of SI response.Key words: self-incompatibility, S-receptor kinase, internalization, SI domains     

Molecular mechanism of self-recognition in Brassica self-incompatibility

In most self-incompatible plant species, recognition of self-pollen is controlled by a single locus, termed the S-locus. In Brassica, genetic dissection of the S-locus has revealed the presence of three highly-polymorphic genes: S-receptor kinase (SRK), S-locus protein 11 (SP11) (also known as S-locus cysteine-rich protein; SCR) and S-locus glycoprotein (SLG). SRK encodes a membrane-spanning serine/threonine kinase that determines the S-haplotype specificity of the stigma. SP11 encodes a small cysteine-rich protein that determines the S-haplotype specificity of pollen. SLG encodes a secreted form of stigma protein similar to the extracellular domain of SRK. Recent biochemical studies have revealed that SP11 functions as the sole ligand for its cognate SRK receptor complex. Their interaction induces the autophosphorylation of SRK, which is expected to trigger the signalling cascade that results in the rejection of self-pollen. This so-called ligand-receptor complex interaction and receptor activation occur in an S-haplotype-specific manner, and this specificity is almost certainly the basis for self-pollen recognition.     

Ligand-Mediated cis-Inhibition of Receptor Signaling in the Self-Incompatibility Response of the Brassicaceae

The inhibition of self-pollination in self-incompatible Brassicaceae is based on allele-specific trans-activation of the highly polymorphic S-locus receptor kinase (SRK), which is displayed at the surface of stigma epidermal cells, by its even more polymorphic pollen coat-localized ligand, the S-locus cysteine-rich (SCR) protein. In an attempt to achieve constitutive activation of SRK and thus facilitate analysis of self-incompatibility (SI) signaling, we coexpressed an Arabidopsis lyrata SCR variant with its cognate SRK receptor in the stigma epidermal cells of Arabidopsis (Arabidopsis thaliana) plants belonging to the C24 accession, in which expression of SRK and SCR had been shown to exhibit a robust SI response. Contrary to expectation, however, coexpression of SRK and SCR was found to inhibit SRK-mediated signaling and to disrupt the SI response. This phenomenon, called cis-inhibition, is well documented in metazoans but has not as yet been reported for plant receptor kinases. We demonstrate that cis-inhibition of SRK, like its trans-activation, is based on allele-specific interaction between receptor and ligand. We also show that stigma-expressed SCR causes entrapment of its SRK receptor in the endoplasmic reticulum, thus disrupting the proper targeting of SRK to the plasma membrane, where the receptor would be available for productive interaction with its pollen coat-derived SCR ligand. Although based on an artificial cis-inhibition system, the results suggest novel strategies of pollination control for the generation of hybrid cultivars and large-scale seed production from hybrid plants in Brassicaceae seed crops and, more generally, for inhibiting cell surface receptor function and manipulating signaling pathways in plants.Ligand receptor signaling plays important roles in cell-cell communication between neighboring cells in a variety of developmental and physiological processes. This communication typically relies on the interaction of transmembrane receptors displayed on the surface of signal-receiving cells with their cognate ligands derived from signal-sending neighboring cells, which, in turn, leads to the activation of receptor-mediated signaling cascades that modify intracellular activities of the signal-receiving cell. Such is the case with communication between pollen grains and stigma epidermal cells, a process that has an important role in directing reproductive success and determining pollination modes (i.e. selfing or outcrossing) in the Brassicaceae. In this family, outcrossing is enforced by self-incompatibility (SI), a mechanism controlled by haplotypes of the S locus, by which the stigma epidermal cells of a plant recognize and reject self pollen grains (i.e. those derived from the same flower, the same plant, or plants expressing the same S-locus haplotype), thus preventing self-pollination, while allowing the growth of tubes from nonself pollen grains (i.e. those derived from plants expressing a different S-locus haplotype; Nasrallah and Nasrallah, 2014a). Inhibition of self pollen in the SI response is initiated by allele-specific interaction between two highly polymorphic proteins encoded at the S locus: the S-locus receptor kinase (SRK), which is localized at the plasma membrane of stigma epidermal cells (Stein et al., 1991, 1996), and its ligand, the S-locus cysteine-rich protein (SCR), which accumulates in the pollen coat and diffuses onto the stigma surface upon pollen-stigma contact (Schopfer et al., 1999; Takayama et al., 2000; Shiba et al., 2001). The interaction of the SRK extracellular domain, or S domain, with its cognate SCR ligand is thought to activate downstream signaling cascades in stigma epidermal cells, which lead to inhibition of pollen germination on the stigma surface and/or pollen tube penetration through the stigma epidermal cell wall. The SRK and SCR genes are the primary determinants of the transition between the outcrossing and selfing modes of mating in the Brassicaceae, as demonstrated by the observation that transformation of SRK and SCR gene pairs derived from self-incompatible Arabidopsis lyrata or Capsella grandiflora restored SI in several accessions of the normally self-fertile Arabidopsis (Arabidopsis thaliana; Nasrallah et al., 2002, 2004; Boggs et al., 2009).Tight regulation of the SI response is critical for ensuring reproductive success in self-incompatible plants. Activation of SRK signaling must be triggered only by pollen-derived cognate SCR ligand upon interaction of stigma epidermal cells with self pollen grains, because constitutive activation of SI signaling in stigma epidermal cells would result in inhibition of nonself as well as self pollen grains and would result in female sterility. This adverse outcome is averted by tight regulation of the SCR gene, which is expressed exclusively in the anthers of self-incompatible plants and whose protein products are localized exclusively in the pollen coat (Schopfer and Nasrallah, 2000; Shiba et al., 2001). For experimental studies of SI, however, constitutive activation of SRK-mediated signaling in stigma epidermal cells would be useful, as it might provide a convenient means of identifying components of the poorly understood SRK-mediated signaling pathway.A reaction that resembles a constitutive SI response, in which stigma epidermal cells inhibit both self and nonself pollen grains, has been obtained by manual application of purified recombinant SCR proteins produced in bacteria (Kachroo et al., 2001; Chookajorn et al., 2004) or synthetic SCR (Takayama et al., 2001) to stigmas that express their cognate SRK receptors. Unlike the highly localized activation induced by pollen-derived SCR at the site of pollen-stigma contact, treatment of the stigma surface with SCR protein can clearly cause global activation of SRK in most, if not all, epidermal cells of a stigma. However, treating stigmas in the numbers required for analysis of SRK signaling is extremely laborious, can damage stigmas, and produces inconsistent results. Therefore, a method that circumvents these problems would be advantageous. In metazoans, constitutive activation of receptor kinases has been shown to result not only from receptor mutations that cause constitutive kinase activity (Webster and Donoghue, 1996; Hirota et al., 1998) and mutations in signaling components that cause ligand-independent activation of downstream cascades (Wang et al., 2012, 2014; Roberts et al., 2013; Han, 2014), but also from ectopic expression of ligands within the same cells as their receptors, as occurs in several pathological conditions (Sporn and Roberts, 1985; Castellano et al., 2006; Krasagakis et al., 2011).Accordingly, an attempt was made to generate Arabidopsis plants having a stable constitutive stigma SI response by coexpressing an SRK variant and its cognate SCR in stigma epidermal cells, which should, in principle, constitutively activate the SI response in these cells. This report shows that, while pollen-derived SCR trans-activates the SRK-mediated SI response, stigma-expressed SCR inhibits the activity of its cognate SRK by causing entrapment of the receptor in the endoplasmic reticulum (ER). This phenomenon is similar in its outcome to the ligand-mediated cis-inhibition phenomenon that had previously been observed in metazoans for some signaling systems that use transmembrane proteins as ligands (Yaron and Sprinzak, 2012) but had not been described for plant receptor-like kinases. The results suggest novel strategies for control of receptor-like kinase activity and manipulation of signaling pathways in plants and for pollination control in hybrid breeding programs and seed production from hybrid plants in the Brassicaceae.     

Evolution of the S-locus region in Arabidopsis relatives

The S locus, a single polymorphic locus, is responsible for self-incompatibility (SI) in the Brassicaceae family and many related plant families. Despite its importance, our knowledge of S-locus evolution is largely restricted to the causal genes encoding the S-locus receptor kinase (SRK) receptor and S-locus cysteine-rich protein (SCR) ligand of the SI system. Here, we present high-quality sequences of the genomic region of six S-locus haplotypes: Arabidopsis (Arabidopsis thaliana; one haplotype), Arabidopsis lyrata (four haplotypes), and Capsella rubella (one haplotype). We compared these with reference S-locus haplotypes of the self-compatible Arabidopsis and its SI congener A. lyrata. We subsequently reconstructed the likely genomic organization of the S locus in the most recent common ancestor of Arabidopsis and Capsella. As previously reported, the two SI-determining genes, SCR and SRK, showed a pattern of coevolution. In addition, consistent with previous studies, we found that duplication, gene conversion, and positive selection have been important factors in the evolution of these two genes and appear to contribute to the generation of new recognition specificities. Intriguingly, the inactive pseudo-S-locus haplotype in the self-compatible species C. rubella is likely to be an old S-locus haplotype that only very recently became fixed when C. rubella split off from its SI ancestor, Capsella grandiflora.     

SRK-SCR转基因拟南芥自交不亲和性的研究进展

十字花科植物自交不亲和性（SI）受墨位点（S-locus）编码的sRK和sCR控制,它们分别是柱头和花粉中的sI特异识别因子。野生型拟南芥不具有sI,而近来通过转基因手段将外源艘K—scR基因转入野生型拟南芥可以使其表现sI,由此建立了一个可用于十字花科sI研究的新型模式植物。本文综述了利用这种转基因拟南芥在SI机制及进化方面取得的进展,包括sI新基因的挖掘、候选基因功能分析和拟南芥生殖模式的转变等。     

Endocytosis and Endosomal Regulation of the S-Receptor Kinase during the Self-Incompatibility Response in Brassica oleracea

Intracellular trafficking of plant receptor kinases (PRKs) is a key step in regulation of cellular signaling. Our current knowledge in this field is based on systems that address signaling pathways affecting the whole cell. There are, however, signaling phenomena that add a further layer of complexity. In the Brassica self-incompatibility response, a single cell can adequately respond to two opposite stimuli: accepting cross-pollen and rejecting self-pollen simultaneously. To understand how PRK signaling can influence the coexistence of two seemingly exclusive states of the cell, we investigated the subcellular localization and internalization of the S-receptor kinase (SRK) involved in the self-incompatibility response of Brassica oleracea. Here, we describe the unusual subcellular distribution of SRK₃, which localizes predominantly to intracellular compartments and to a much lesser extent to the plasma membrane. Using an anti-SRK antibody that fully substitutes for the natural ligand, we demonstrate that the interaction with the receptor takes place at the plasma membrane and is followed by SRK internalization in endosomes that are enriched in the SRK negative regulator Thioredoxin-h-like1.     

Exploring the role of a stigma-expressed plant U-box gene in the pollination responses of transgenic self-incompatible Arabidopsis thaliana

Recognition of “self” pollen in the self-incompatibility (SI) response of the Brassicaceae is determined by allele-specific interaction between the S-locus receptor kinase (SRK), a transmembrane protein of the stigma epidermis, and its ligand, the pollen coat-localized S-locus cysteine-rich (SCR) protein. The current model for SRK-mediated signaling proposes a central role for the plant U-box (PUB) Armadillo repeat-containing protein ARC1, an E3 ligase that interacts with, and is phosphorylated by, the kinase domain of SRK. According to the model, activated ARC1 causes the degradation of factors required for successful pollen tube growth. However, Arabidopsis thaliana plants transformed with functional SRK and SCR genes isolated from self-incompatible A. lyrata can express an intense SI response despite lacking a functional ARC1 gene. Here, we tested the possibility that a different member of the A. thaliana PUB protein family might have assumed the role of ARC1 in SI. Toward this end, we analyzed the AtPUB2 gene, which is annotated as being highly expressed in stigmas. Our functional analysis of a T-DNA insertion pub2 allele, together with yeast two-hybrid interaction assays and reporter analysis of AtPUB2 promoter activity, demonstrates that AtPUB2 does not function in SI. The results leave open the question of whether the proposed model of ARC1-mediated signaling applies to transgenic SRK–SCR self-incompatible A. thaliana plants.     

S cysteine-rich (SCR) binding domain analysis of the Brassica self-incompatibility S-locus receptor kinase

Brassica self-incompatibility, a highly discriminating outbreeding mechanism, has become a paradigm for the study of plant cell-cell communications. When self-pollen lands on a stigma, the male ligand S cysteine-rich (SCR), which is present in the pollen coat, is transmitted to the female receptor, S-locus receptor kinase (SRK). SRK is a membrane-spanning serine/threonine receptor kinase present in the stigmatic papillar cell membrane. Haplotype-specific binding of SCR to SRK brings about pollen rejection. The extracellular receptor domain of SRK (eSRK) is responsible for binding SCR. Based on sequence homology, eSRK can be divided into three subdomains: B lectin-like, hypervariable, and PAN. Biochemical analysis of these subdomains showed that the hypervariable subdomain is responsible for most of the SCR binding capacity of eSRK, whereas the B lectin-like and PAN domains have little, if any, affinity for SCR. Fine mapping of the SCR binding region of SRK using a peptide array revealed a region of the hypervariable subdomain that plays a key role in binding the SCR molecule. We show that residues within the hypervariable subdomain define SRK binding and are likely to be involved in defining haplotype specificity.     

Evolution of the self-incompatibility system in the Brassicaceae: identification of S-locus receptor kinase (SRK) in self-incompatible Capsella grandiflora

Self-incompatibility (SI) has been well studied in the genera Brassica and Arabidopsis, which have become models for investigation into the SI system. To understand the evolution of the SI system in the Brassicaceae, comparative analyses of the S-locus in genera other than Brassica and Arabidopsis are necessary. We report the identification of six putative S-locus receptor kinase genes (SRK) in natural populations of Capsella grandiflora, an SI species from a genus which is closely related to Arabidopsis. These S-alleles display striking similarities to the Arabidopsis lyrata SRK alleles in sequence and structure. Our phylogenetic analysis supports the scenario of differing SI evolution along the two lineages (The Brassica lineage and Arabidopsis/Capsella lineage). Our results also argue that the ancestral S-locus lacked the SLG gene (S-locus glycoprotein) and that the diversification of S-alleles predates the separation of Arabidopsis and Capsella.     

In Planta Assessment of the Role of Thioredoxin h Proteins in the Regulation of S-Locus Receptor Kinase Signaling in Transgenic Arabidopsis

The self-incompatibility (SI) response of the Brassicaceae is mediated by allele-specific interaction between the stigma-localized S-locus receptor kinase (SRK) and its ligand, the pollen coat-localized S-locus cysteine-rich protein (SCR). Based on work in Brassica spp., the thioredoxin h-like proteins THL1 and THL2, which interact with SRK, have been proposed to function as oxidoreductases that negatively regulate SRK catalytic activity. By preventing the spontaneous activation of SRK in the absence of SCR ligand, these thioredoxins are thought to be essential for the success of cross pollinations in self-incompatible plants. However, the in planta role of thioredoxins in the regulation of SI signaling has not been conclusively demonstrated. Here, we addressed this issue using Arabidopsis thaliana plants transformed with the SRKb-SCRb gene pair isolated from self-incompatible Arabidopsis lyrata. These plants express an intense SI response, allowing us to exploit the extensive tools and resources available in A. thaliana for analysis of SI signaling. To test the hypothesis that SRK is redox regulated by thioredoxin h, we expressed a mutant form of SRKb lacking a transmembrane-localized cysteine residue thought to be essential for the SRK-thioredoxin h interaction. We also analyzed transfer DNA insertion mutants in the A. thaliana orthologs of THL1 and THL2. In neither case did we observe an effect on the pollination responses of SRKb-expressing stigmas toward incompatible or compatible pollen. Our results are consistent with the conclusion that, contrary to their proposed role, thioredoxin h proteins are not required to prevent the spontaneous activation of SRK in the A. thaliana stigma.Many flowering plants possess self-incompatibility (SI), a genetic system that promotes outcrossing by preventing self-fertilization. In the Brassicaceae family, the SI response is controlled by haplotypes of the S locus, each of which contains two genes that encode highly polymorphic proteins, the S-locus receptor kinase (SRK), which is a plasma membrane resident single-pass transmembrane Ser/Thr receptor kinase displayed at the surface of stigma epidermal cells (Stein et al., 1991; Takasaki et al., 2000), and the S-locus Cys-rich protein (SCR), which is the pollen coat-localized ligand for SRK (Schopfer et al., 1999; Kachroo et al., 2001; Takayama et al., 2001). SRK and SCR exhibit allele-specific interactions, whereby only SRK and SCR encoded by the same S-locus haplotype interact. In a self-pollination, the binding of this “self” pollen-borne SCR to the extracellular domain of SRK activates the SRK kinase, thereby triggering a cellular response in stigma epidermal cells that causes inhibition of pollen germination and tube penetration into the stigma epidermal cell wall (for review, see Tantikanjana et al., 2010).Tight regulation of SRK kinase activity and its signaling cascade is critical for productive pollen-stigma interactions because constitutive (i.e. SCR-independent) activity of the receptor is expected to result in sterile stigmas that reject both compatible and incompatible pollen. In the classical view of ligand-activated receptor kinases, the receptor occurs as catalytically inactive monomers in the absence of ligand and only becomes activated upon ligand-induced dimerization (for review, see Lemmon and Schlessinger, 2010). However, some receptor kinases in both animals (Chan et al., 2000; Ehrlich et al., 2011) and plants (Giranton et al., 2000; Wang et al., 2005, 2008; Shimizu et al., 2010; Bücherl et al., 2013) form catalytically inactive dimers or oligomers in the absence of ligand, with receptor activation presumably resulting from ligand-induced higher order oligomerization or conformational changes (Lemmon and Schlessinger, 2010). Similar to the latter receptors, SRK forms oligomers in unpollinated stigmas, i.e. in the absence of SCR (Giranton et al., 2000), at least partly via ligand-independent dimerization domains located within the SRK extracellular domain (Naithani et al., 2007). It has been proposed that these ligand-independent SRK oligomers are maintained in an inactive state by thioredoxins, the ubiquitous small oxidoreductases that reduce disulfide bridges in proteins (Buchanan and Balmer, 2005). This hypothesis is supported by the following observations: (1) two Brassica napus thioredoxins, the Thioredoxin H-Like proteins THL1 and THL2, were identified as SRK interactors in a yeast (Saccharomyces cerevisiae) two-hybrid screen that used the B. napus SRK₉₁₀ kinase domain as bait (Bower et al., 1996); (2) when purified from pistils or insect cells, the Brassica oleracea SRK₃ variant was found to exhibit constitutive autophosphorylation activity in vitro, and this activity was inhibited by Escherichia coli-expressed THL1 proteins and was restored by addition of pollen coat proteins containing self but not of pollen coat proteins containing nonself SCR (Cabrillac et al., 2001); (3) the catalytic activity of THL1 was required for its inhibition of SRK₃ autophosphorylation activity in vitro (Cabrillac et al., 2001); and (4) antisense suppression of THL1/THL2 gene expression in the stigmas of a self-compatible B. napus strain reportedly produced a low-level constitutive incompatibility (Haffani et al., 2004), as might be expected if the THL1/THL2 proteins prevent the spontaneous activation of SRK-mediated signaling in stigmas.These observations notwithstanding, the in planta role of thioredoxin h proteins as negative regulators of SRK activity has not been conclusively demonstrated. To date, this proposed function has only been evaluated in a self-compatible strain of B. napus (Haffani et al., 2004). Consequently, it is not known if the proposed inhibitory effect of these thioredoxins on SRK catalytic activity is manifested in self-incompatible stigmas and if it applies to all SRK variants, be they derived from Brassica spp. or other self-incompatible species of the Brassicaceae such as Arabidopsis lyrata.In this study, we tested the in planta role of thioredoxin h proteins in the regulation of SI signaling using a transgenic self-incompatible Arabidopsis thaliana model that we generated by transforming A. thaliana with the SRKb-SCRb gene pair isolated from the Sb haplotype of self-incompatible A. lyrata (Kusaba et al., 2001; Nasrallah et al., 2002, 2004). We had previously shown that the stigmas of A. thaliana SRKb-SCRb transformants can exhibit an SI response that is as robust as the SI response observed in naturally self-incompatible A. lyrata, demonstrating that A. thaliana, which harbors nonfunctional S-locus haplotypes (Kusaba et al., 2001; Sherman-Broyles et al., 2007; Shimizu et al., 2008; Boggs et al., 2009c), has nevertheless retained all other factors required for SI. In view of the availability in A. thaliana of a highly efficient transformation method and numerous genetic resources, the A. thaliana SRK-SCR transgenic model has enabled the use of experimental approaches that are difficult or impossible to implement in Brassica species and has thus proven to be an invaluable platform for in planta analysis of SRK and SI signaling (Liu et al., 2007; Boggs et al., 2009a, 2009b; Tantikanjana et al., 2009; Tantikanjana and Nasrallah, 2012).We therefore used this transgenic A. thaliana self-incompatible model to determine if abolishing the proposed SRK-thioredoxin h interaction or eliminating expression of the major thioredoxin h proteins expressed in stigmas would affect the outcome of self- or cross pollination. To this end, we expressed a mutant form of SRKb that lacked the Cys residue previously shown to be required for the interaction of SRK with THLs (Mazzurco et al., 2001), and we analyzed plants carrying knockout insertional mutations in thioredoxin h genes. Our results are inconsistent with the proposed role of thioredoxin h proteins as negative regulators of SRK catalytic activity and SI signaling.     

Characterization of the SP11/SCR high-affinity binding site involved in self/nonself recognition in brassica self-incompatibility

In Brassica self-incompatibility, the recognition of self/nonself pollen grains, is controlled by the S-locus, which encodes three highly polymorphic proteins: S-locus receptor kinase (SRK), S-locus protein 11 (SP11; also designated S-locus Cys-rich protein), and S-locus glycoprotein (SLG). SP11, located in the pollen coat, determines pollen S-haplotype specificity, whereas SRK, located on the plasma membrane of stigmatic papilla cells, determines stigmatic S-haplotype specificity. SLG shares significant sequence similarity with the extracellular domain of SRK and is abundant in the stigmatic cell wall, but its function is controversial. We previously showed that SP11 binds directly to its cognate SRK with high affinity (K(d) = 0.7 nM) and induces its autophosphorylation. We also found that an SLG-like, 60-kD protein on the stigmatic membrane forms a high-affinity binding site for SP11. Here, we show that the 60-kD stigmatic membrane protein is a truncated form of SRK containing the extracellular domain, transmembrane domain, and part of the juxtamembrane domain. A transiently expressed, membrane-anchored form of SRK exhibits high-affinity binding to SP11, whereas the soluble SRK (eSRK) lacking the transmembrane domain exhibits no high-affinity binding, as is the case with SLG. The different binding affinities of the membrane-anchored SRK and soluble eSRK or SLG will be significant for the specific perception of SP11 by SRK.     

SRK, the stigma-specific S locus receptor kinase of Brassica, is targeted to the plasma membrane in transgenic tobacco.

The S locus receptor kinase (SRK) gene is one of two S locus genes required for the self-incompatibility response in Brassica. We have identified the product of the SRK6 gene in B. oleracea stigmas and have shown that it has characteristics of an integral membrane protein. When expressed in transgenic tobacco, SRK6 is glycosylated and targeted to the plasma membrane. These results provide definitive biochemical evidence for the existence in plants of a plasma membrane-localized transmembrane protein kinase with a known cell-cell recognition function. The timing of SRK expression in stigmas follows a time course similar to that previously described for another S locus-linked gene, the S locus glycoprotein (SLG) gene, and correlates with the ability of stigmas to mount a self-incompatibility response. Based on SRK6 promoter studies, the site of gene expression overlaps with that of SLG and exhibits predominant expression in the stigmatic papillar cells. Although reporter gene studies indicated that the SRK promoter was active in pollen, SRK protein was not detected in pollen, suggesting that SRK functions as a cell surface receptor exclusively in the papillar cells of the stigma.     

Independent S-Locus Mutations Caused Self-Fertility in Arabidopsis thaliana

A common yet poorly understood evolutionary transition among flowering plants is a switch from outbreeding to an inbreeding mode of mating. The model plant Arabidopsis thaliana evolved to an inbreeding state through the loss of self-incompatibility, a pollen-rejection system in which pollen recognition by the stigma is determined by tightly linked and co-evolving alleles of the S-locus receptor kinase (SRK) and its S-locus cysteine-rich ligand (SCR). Transformation of A. thaliana, with a functional AlSRKb-SCRb gene pair from its outcrossing relative A. lyrata, demonstrated that A. thaliana accessions harbor different sets of cryptic self-fertility–promoting mutations, not only in S-locus genes, but also in other loci required for self-incompatibility. However, it is still not known how many times and in what manner the switch to self-fertility occurred in the A. thaliana lineage. Here, we report on our identification of four accessions that are reverted to full self-incompatibility by transformation with AlSRKb-SCRb, bringing to five the number of accessions in which self-fertility is due to, and was likely caused by, S-locus inactivation. Analysis of S-haplotype organization reveals that inter-haplotypic recombination events, rearrangements, and deletions have restructured the S locus and its genes in these accessions. We also perform a Quantitative Trait Loci (QTL) analysis to identify modifier loci associated with self-fertility in the Col-0 reference accession, which cannot be reverted to full self-incompatibility. Our results indicate that the transition to inbreeding occurred by at least two, and possibly more, independent S-locus mutations, and identify a novel unstable modifier locus that contributes to self-fertility in Col-0.     

The ARC1 E3 Ligase Gene Is Frequently Deleted in Self-Compatible Brassicaceae Species and Has a Conserved Role in Arabidopsis lyrata Self-Pollen Rejection

Self-pollen rejection is an important reproductive regulator in flowering plants, and several different intercellular signaling systems have evolved to elicit this response. In the Brassicaceae, the self-incompatibility system is mediated by the pollen S-locus Cys-Rich/S-locus Protein11 (SCR/SP11) ligand and the pistil S Receptor Kinase (SRK). While the SCR/SP11-SRK recognition system has been identified in several species across the Brassicaceae, less is known about the conservation of the SRK-activated cellular responses in the stigma, following self-pollen contact. The ARM Repeat Containing1 (ARC1) E3 ubiquitin ligase functions downstream of SRK for the self-incompatibility response in Brassica, but it has been suggested that ARC1 is not required in Arabidopsis species. Here, we surveyed the presence of ARC1 orthologs in several recently sequenced genomes from Brassicaceae species that had diversified ∼20 to 40 million years ago. Surprisingly, the ARC1 gene was deleted in several species that had lost the self-incompatibility trait, suggesting that ARC1 may lose functionality in the transition to self-mating. To test the requirement of ARC1 in a self-incompatible Arabidopsis species, transgenic ARC1 RNA interference Arabidopsis lyrata plants were generated, and they exhibited reduced self-incompatibility responses resulting in successful fertilization. Thus, this study demonstrates a conserved role for ARC1 in the self-pollen rejection response within the Brassicaceae.     

植物孢子体自交不亲和信号转导功能分子研究进展

孢子体自交不亲和(SSI)是许多植物采取的一种抵制近亲繁殖的重要措施,受S位点复等位基因控制。近年来,参与其信号转导的许多功能分子及它们的编码基因被分离并得到了充分研究：当自花授粉时,SPlI／SCR与SRK特异识别,造成后的Ser／Thr激酶的磷酸化,引发了一系列由SLG、ARC1及水孔蛋白等因子参与的SSI信号转导途径,最终产生自交不亲和的结果。     

Molecular mechanisms of self-incompatibility in Brassica

In Brassica species, self-incompatibility has been mapped genetically to a single chromosomal location. In this region several closely linked genes have been identified. One of them, S-locus receptor kinase (SRK), determines S haplotype specificity of the stigma and it's the key protein for SI reaction. The role of the S locus glycoprotein (SLG) gene remains unclear. In the last decade approximately 15 additional genes linked to S-locus have been found. Recently, a gene has been identified (SCR) that encodes a small cysteine-rich protein which is a candidate for the pollen ligand. In addition to S locus linked genes there are unlinked SLRgenes (S-locus related genes). In this review, we discuss the role of these genes and the current view on the self-incompatibility mechanism in Brassica.