Non-self Pollen Research Articles

Self-(In)compatibility Systems: Target Traits for Crop-Production, Plant Breeding, and Biotechnology.

Self-incompatibility (SI) mechanisms prevent self-fertilization in flowering plants based on specific discrimination between self- and non-self pollen. Since this trait promotes outcrossing and avoids inbreeding it is a widespread mechanism of controlling sexual plant reproduction. Growers and breeders have effectively exploited SI as a tool for manipulating domesticated crops for thousands of years. However, only within the past thirty years have studies begun to elucidate the underlying molecular features of SI. The specific S-determinants and some modifier factors controlling SI have been identified in the sporophytic system exhibited by Brassica species and in the two very distinct gametophytic systems present in Papaveraceae on one side and in Solanaceae, Rosaceae, and Plantaginaceae on the other. Molecular level studies have enabled SI to SC transitions (and vice versa) to be intentionally manipulated using marker assisted breeding and targeted approaches based on transgene integration, silencing, and more recently CRISPR knock-out of SI-related factors. These scientific advances have, in turn, provided a solid basis to implement new crop production and plant breeding practices. Applications of self-(in)compatibility include widely differing objectives such as crop yield and quality improvement, marker-assisted breeding through SI genotyping, and development of hybrids for overcoming intra- and interspecific reproductive barriers. Here, we review scientific progress as well as patented applications of SI, and also highlight future prospects including further elucidation of SI systems, deepening our understanding of SI-environment relationships, and new perspectives on plant self/non-self recognition.

Aminooxyacetic acid (АОА), inhibitor of 1-aminocyclopropane-1-carboxilic acid (AСС) synthesis, suppresses self-incompatibility-induced programmed cell death in self-incompatible Petunia hybrida L. pollen tubes.

Self-incompatibility (SI) is genetically determined reproductive barrier preventing inbreeding and thereby providing the maintenance of plant species diversity. At present, active studies of molecular bases of SI mechanisms are underway. S-RNAse-based SI in Petunia hybrida L. is a self-/non-self recognition system that allows the pistil to reject self pollen and to accept non-self pollen for outcrossing. In the present work, using fluorescent methods including the TUNEL method allowed us to reveal the presence of markers of programmed cell death (PCD), such as DNA fragmentation, in growing in vivo petunia pollen tubes during the passage of the SI reaction. The results of statistical analysis reliably proved that PCD is the factor of S-RNAse-based SI. It was found that preliminary treatment before self-pollination of stigmas of petunia self-incompatible line with aminooxyacetic acid (AOA), inhibitor of ACC synthesis, led to stimulation of pollen tubes growth when the latter did not exhibit any hallmarks of PCD. These data argue in favor of assumption that ethylene controls the passage of PCD in incompatible pollen tubes in the course of S-RNAse-based SI functioning. The involvement of the hormonal regulation in SI mechanism in P. hybrida L. is the finding observed by us for the first time.

Mate Selection in Self-Compatible Wild Tobacco Results from Coordinated Variation in Homologous Self-Incompatibility Genes

In flowering plants, intraspecific mate preference is frequently related to mating systems: the rejection of self pollen in self-incompatible (SI) plants that prevents inbreeding is one of the best described examples. However, in other mating systems, more nuanced patterns of pollen rejection occur. In the self-compatible (SC) Nicotiana attenuata, in which SI is not found and all crosses are compatible, certain pollen genotypes are consistently selected in mixed pollinations. However, the molecular mechanisms of this polyandrous mate selection remain unknown. Style-expressed NaS-like-RNases and pollen-expressed NaSLF-like genes, homologous to SI factors in Solanaceae, were identified and examined for a role in N.attenuata's mate selection. A comparison of two NaS-like-RNases and six NaSLF-like genes among 26 natural accessions revealed specific combinations of co-expression and direct protein-protein interactions. To evaluate their role in mate selection, we silenced the expression of specific NaS-like-RNases and NaSLF-like proteins and conducted diagnostic binary mixed pollinations and mixed pollinations with 14 different non-self pollen donors. Styles expressing particular combinations of NaS-like-RNases selected mates from plants with corresponding NaS-like-RNase expression patterns, while styles lacking NaS-like-RNase expression were non-selective in their fertilizations, which reflected the genotype ratios of pollen mixtures deposited on the stigmas. DNA methylation could account for some of the observed variation in stylar NaS-like-RNase patterns. We conclude that the S-RNase-SLF recognition mechanism plays a central role in polyandrous mate selection in thisself-compatible species. These results suggest that after the SI-SC transition, natural variation of SI homologous genes was repurposed to mediate intraspecific mate selection.

Transcriptome Analysis Provides Insight into the Molecular Mechanisms Underlying gametophyte factor 2-Mediated Cross-Incompatibility in Maize

In maize (Zea mays L.), unilateral cross-incompatibility (UCI) is controlled by Gametophyte factors (Ga), including Ga1, Ga2, and Tcb1; however, the molecular mechanisms underpinning this process remain unexplored. Here, we report the pollination phenotype of an inbred line, 511L, which carries a near-dominant Ga2-S allele. We performed a high-throughput RNA sequencing (RNA-Seq) analysis of the compatible and incompatible crosses between 511L and B73, to identify the transcriptomic differences associated with Ga2-mediated UCI. An in vivo kinetics analysis revealed that the growth of non-self pollen tubes was blocked at the early stages after pollination in 511L, maintaining the UCI barrier in Ga2. In total, 25,759 genes were expressed, of which, 2063 differentially expressed genes (DEGs) were induced by pollination (G_GG, G_GB, B_BB, B_BG). A gene ontology (GO) enrichment analysis revealed that these genes were specifically enriched in functions involved in cell wall strength and pectic product modification. Moreover, 1839, 4382, and 5041 genes were detected to differentially express under same pollination treatments, including B_G, BG_GG, and BB_GB, respectively. A total of 1467 DEGs were constitutively expressed between the two inbred lines following pollination treatments, which were enriched in metabolic processes, flavonoid biosynthesis, cysteine biosynthesis, and vacuole functions. Furthermore, we confirmed 14 DEGs related to cell wall modification and stress by qRT-PCR, which might be involved in Ga2-S-mediated UCI. Our results provide a comprehensive foundation for the molecular mechanisms involved in silks of UCI mediated by Ga2-S.

Loss of preferred habitat and pollen limitation threatens reproduction in a rare mountain Paeonia delavayi

Paeonia delavayi is a wild tree peony species endemic to high-altitude regions in southwestern China. Recent agricultural land expansion, however, not only causes its dwindling population size, but also poses a severe threat to its long-term persistence. Since our knowledge of the reproductive biology of P. delavayi is very limited, and some management misconceptions have further exacerbated the already worrisome situation, the aim of the present study is to provide some scientific evidence regarding habitat preference and breeding system of P. delavayi, and to help correct some management misconceptions. Specifically, morphological traits of P. delavayi populations growing in two different habitats (i.e., the open area versus forest understory), including basal stem diameter, plant height and flower numbers per plant, were measured and compared with each other. A range of floral manipulation experiments was further conducted to study the extent of self-compatibility and pollen limitation. The results showed that P. delavayi preferred the open habitat over forest understory as it generally grew bigger with thicker stems, and produced a higher number of flowers in the former habitat. Therefore, the open habitat should receive immediate priority for conservation action instead of being converted to agricultural land; also, P. delavayi is pollen limited, as evidenced by the fact that experimental flowers receiving supplemental non-self pollen generally displayed elevated seed production than flowers subjected to other treatments (e.g., non-supplemental self pollen, non-supplemental non-self pollen and supplemental self pollen), which is consistent with reported observations that alpine plants are more likely to be pollen-limited than lowland plants. We suggest that human intervention might be necessary to guarantee the long-term persistence of P. delavayi as harsh alpine environment, intense competition for pollinators and different anthropogenic perturbations co-limit its reproductive success.

Cullin1-P is an Essential Component of Non-Self Recognition System in Self-Incompatibility in Petunia.

Self-incompatibility (SI) in flowering plants is a genetic reproductive barrier to distinguish self- and non-self pollen to promote outbreeding. In Solanaceae, self-pollen is rejected by the ribonucleases expressed in the styles (S-RNases), via its cytotoxic function. On the other side, the male-determinant is the S-locus F-box proteins (SLFs) expressed in pollen. Multiple SLFs collaboratively detoxify non-self S-RNases, therefore, non-self recognition is the mode of self-/non-self discrimination in Solanaceae. It is considered that SLFs function as a substrate-recognition module of the Skp1-Cullin1-F-box (SCF) complex that inactivates non-self S-RNases via their polyubiquitination, which leads to degradation by 26S proteasome. In fact, PhSSK1 (Petunia hybrida SLF-interacting Skp1-like1) was identified as a specific component of SCFSLF and was shown to be essential for detoxification of S-RNase in Petunia However, different molecules are proposed as the candidate Cullin1, another component of SCFSLF, and there is as yet no definite conclusion. Here, we identified five Cullin1s from the expressed sequence tags (ESTs) derived from the male reproductive organ in Petunia Among them, only PhCUL1-P was co-immunoprecipitated with S7-SLF2. In vitro protein-binding assay suggested that PhSSK1 specifically forms a complex with PhCUL1-P in an SLF-dependent manner. Knockdown of PhCUL1-P suppressed fertility of transgenic pollen in cross-compatible pollination in the functional S-RNase-dependent manner. These results suggested that SCFSLF selectively uses PhCUL1-P. Phylogeny of Cullin1s indicates that CUL1-P is recruited into the SI machinery during the evolution of Solanaceae, suggesting that the SI components have evolved differently among species in Solanaceae and Rosaceae, despite both families sharing the S-RNase-based SI.

Pollen S-locus F-box proteins of Petunia involved in S-RNase-based self-incompatibility are themselves subject to ubiquitin-mediated degradation.

Many flowering plants show self-incompatibility, an intra-specific reproductive barrier by which pistils reject self-pollen to prevent inbreeding and accept non-self pollen to promote out-crossing. In Petunia, the polymorphic S-locus determines self/non-self recognition. The locus contains a gene encoding an S-RNase, which controls pistil specificity, and multiple S-locus F-box (SLF) genes that collectively control pollen specificity. Each SLF is a component of an SCF (Skp1/Cullin/F-box) complex that is responsible for mediating degradation of non-self S-RNase(s), with which the SLF interacts, via the ubiquitin-26S proteasome pathway. A complete set of SLFs is required to detoxify all non-self S-RNases to allow cross-compatible pollination. Here, we show that SLF1 of Petunia inflata is itself subject to degradation via the ubiquitin-26S proteasome pathway, and identify an 18 amino acid sequence in the C-terminal region of S2 -SLF1 (SLF1 of S2 haplotype) that contains a degradation motif. Seven of the 18 amino acids are conserved among all 17 SLF proteins of S2 haplotype and S3 haplotype involved in pollen specificity, suggesting that all SLF proteins are probably subject to similar degradation. Deleting the 18 amino acid sequence from S2 -SLF1 stabilized the protein but abolished its function in self-incompatibility, suggesting that dynamic cycling of SLF proteins is an integral part of their function in self-incompatibility.

Insight into S-RNase-based self-incompatibility in Petunia: recent findings and future directions.

S-RNase-based self-incompatibility in Petunia is a self/non-self recognition system that allows the pistil to reject self-pollen to prevent inbreeding and to accept non-self pollen for outcrossing. Cloning of S-RNase in 1986 marked the beginning of nearly three decades of intensive research into the mechanism of this complex system. S-RNase was shown to be the sole female determinant in 1994, and the first male determinant, S-locus F-box protein1 (SLF1), was identified in 2004. It was discovered in 2010 that additional SLF proteins are involved in pollen specificity, and recently two S-haplotypes of Petunia inflata were found to possess 17 SLF genes based on pollen transcriptome analysis, further increasing the complexity of the system. Here, we first summarize the current understanding of how the interplay between SLF proteins and S-RNase in the pollen tube allows cross-compatible pollination, but results in self-incompatible pollination. We then discuss some of the aspects that are not yet elucidated, including uptake of S-RNase into the pollen tube, nature, and assembly of SLF-containing complexes, the biochemical basis for differential interactions between SLF proteins and S-RNase, and fate of non-self S-RNases in the pollen tube.

Gene duplication and genetic exchange drive the evolution of S-RNase-based self-incompatibility in Petunia.

Self-incompatibility (SI) systems in flowering plants distinguish self- and non-self pollen to prevent inbreeding. While other SI systems rely on the self-recognition between specific male- and female-determinants, the Solanaceae family has a non-self recognition system resulting in the detoxification of female-determinants of S-ribonucleases (S-RNases), expressed in pistils, by multiple male-determinants of S-locus F-box proteins (SLFs), expressed in pollen. It is not known how many SLF components of this non-self recognition system there are in Solanaceae species, or how they evolved. We identified 16-20 SLFs in each S-haplotype in SI Petunia, from a total of 168 SLF sequences using large-scale next-generation sequencing and genomic polymerase chain reaction (PCR) techniques. We predicted the target S-RNases of SLFs by assuming that a particular S-allele must not have a conserved SLF that recognizes its own S-RNase, and validated these predictions by transformation experiments. A simple mathematical model confirmed that 16-20 SLF sequences would be adequate to recognize the vast majority of target S-RNases. We found evidence of gene conversion events, which we suggest are essential to the constitution of a non-self recognition system and also contribute to self-compatible mutations.

Self-incompatibility in Papaver: advances in integrating the signalling network

Self-fertilization, which results in reduced fitness of offspring, is a common problem in hermaphrodite angiosperms. To prevent this, many plants utilize SI (self-incompatibility), which is determined by the multi-allelic S-locus, that allows discrimination between self (incompatible) and non-self (compatible) pollen by the pistil. In poppy (Papaver rhoeas), the pistil S-determinant (PrsS) is a small secreted protein which interacts with the pollen S-determinant PrpS, a ~20 kDa novel transmembrane protein. Interaction of matching pollen and pistil S-determinants results in self-recognition, initiating a Ca²⁺-dependent signalling network in incompatible pollen. This triggers several downstream events, including alterations to the cytoskeleton, phosphorylation of sPPases (soluble inorganic pyrophosphatases) and an MAPK (mitogen-activated protein kinase), increases in ROS (reactive oxygen species) and nitric oxide (NO), and activation of several caspase-like activities. This results in the inhibition of pollen tube growth, prevention of self-fertilization and ultimately PCD (programmed cell death) in incompatible pollen. The present review focuses on our current understanding of the integration of these signals with their targets in the SI/PCD network. We also discuss our recent functional expression of PrpS in Arabidopsis thaliana pollen.

A Pollen Coat–Inducible Autoinhibited Ca2+-ATPase Expressed in Stigmatic Papilla Cells Is Required for Compatible Pollination in the Brassicaceae

In the Brassicaceae, intraspecific non-self pollen (compatible pollen) can germinate and grow into stigmatic papilla cells, while self-pollen or interspecific pollen is rejected at this stage. However, the mechanisms underlying this selective acceptance of compatible pollen remain unclear. Here, using a cell-impermeant calcium indicator, we showed that the compatible pollen coat contains signaling molecules that stimulate Ca(2+) export from the papilla cells. Transcriptome analyses of stigmas suggested that autoinhibited Ca(2+)-ATPase13 (ACA13) was induced after both compatible pollination and compatible pollen coat treatment. A complementation test using a yeast Saccharomyces cerevisiae strain lacking major Ca(2+) transport systems suggested that ACA13 indeed functions as an autoinhibited Ca(2+) transporter. ACA13 transcription increased in papilla cells and in transmitting tracts after pollination. ACA13 protein localized to the plasma membrane and to vesicles near the Golgi body and accumulated at the pollen tube penetration site after pollination. The stigma of a T-DNA insertion line of ACA13 exhibited reduced Ca(2+) export, as well as defects in compatible pollen germination and seed production. These findings suggest that stigmatic ACA13 functions in the export of Ca(2+) to the compatible pollen tube, which promotes successful fertilization.

Molecular Determinants and Mechanisms of Gametophytic Self-Incompatibility in Fruit Trees of Rosaceae

Self-incompatibility is an important genetic mechanism that prevents inbreeding and promotes genetic polymorphism and heterosis in flowering plants. Many fruit species in the Rosaceae, including apple, pear, plum, apricot, sweet cherry, Japanese apricot, and almond, exhibit typical gametophytic self-incompatibility (GSI) controlled by an apparently single multi-allelic locus. This locus encodes at least two components from both the pollen and the pistil, and controls recognition of self- and non-self pollen. Recently, the GSI system has been investigated at the molecular and cellular levels in Rosaceae, and findings have provided some important insights as to how these two genes interact within pollen tubes that lead to specific inhibition of germination and/or growth of self-pollen tubes. In this review, molecular features of S-determinants of both pistil and pollen, identification of S-alleles, mechanisms of self-incompatibility break-down, and evolution of S-alleles are presented. Moreover, hypothetical signal transduction models in a self-incompatible system in Rosaceae are proposed based on recent findings that indicate that several signal factors are involved in GSI responses.

The post‐pollination ethylene burst and the continuation of floral advertisement are harbingers of non‐random mate selection in Nicotiana attenuata

The self-compatible plant Nicotiana attenuata grows in genetically diverse populations after fires, and produces flowers that remain open for 3 days and are visited by assorted pollinators. To determine whether and when post-pollination non-random mate selection occurs among self and non-self pollen, seed paternity and semi-in vivo pollen tube growth were determined in controlled single/mixed pollinations. Despite all pollen sources being equally proficient in siring seeds in single-genotype pollinations, self pollen was consistently selected in mixed pollinations, irrespective of maternal genotype. However, clear patterns of mate discrimination occurred amongst non-self pollen when mixed pollinations were performed soon after corollas open, including selection against hygromycin B resistance (transformation selectable marker) in wild-type styles and for it in transformed styles. However, mate choice among pollen genotypes was completely shut down in plants transformed to be unable to produce (irACO) or perceive (ETR1) ethylene. The post-pollination ethylene burst, which originates primarily from the stigma and upper style, was strongly correlated with mate selection in single and mixed hand-pollinations using eight pollen donors in two maternal ecotypes. The post-pollination ethylene burst was also negatively correlated with the continuation of emission of benzylacetone, the most abundant pollinator-attracting corolla-derived floral volatile. We conclude that ethylene signaling plays a pivotal role in mate choice, and the post-pollination ethylene burst and the termination of benzylacetone release are accurate predictors, both qualitatively and quantitatively, of pre-zygotic mate selection and seed paternity.

Primary style protein expression in the self-incompatible/compatible apricot by the 2D-DIGE technique

In order to explore the molecular mechanism underlying self-incompatibility (SI) in the apricot (Prunus armeniaca L.) at the proteome level, we examined the style proteomes at different stages of flower development: small bud, big bud, 24h after self-pollination and 24h after cross-pollination with cultivar Badanshui in the SI apricot cultivar Xinshiji and the self-compatible (SC) apricot cultivar Katy by 2D fluorescence difference gel electrophoresis (2D-DIGE) and mass spectrometry (MS). About 1500 style protein spots were detected; 66 were expressed differently in the four stages in Xinshiji. About 1600 style protein spots were detected; 143 were expressed differently in the four stages of flower development in Katy. In Xinshiji, one protein was expressed specifically, four proteins showed up-regulated expression and twenty-nine proteins showed down-regulated expression in the cross-pollinated style compared to the self-pollinated style. Thirteen proteins were identified unambiguously. In Katy, three proteins were expressed specifically, five proteins showed up-regulated expression and thirteen proteins showed down-regulated expression in the cross-pollinated style compared to self-pollinated style. Seven proteins were identified unambiguously. The different reactions of the style at the proteomic level were triggered in Xinshiji and Katy by self pollen and non-self pollen.

Self‐incompatibility in Petunia: a self/nonself‐recognition mechanism employing S‐locus F‐box proteins and S‐RNase to prevent inbreeding

Many flowering plants producing bisexual flowers have adopted self-incompatibility (SI), a reproductive strategy which allows pistils to distinguish between self and nonself pollen, and to only permit nonself pollen to effect fertilization. To date, three different SI mechanisms have been identified, and this article focuses on the S-RNase-based mechanism using Petunia (Solanaceae) as a model. The genetic basis of this type of SI was established nearly a century ago; the polymorphic S-locus specifies the genetic identity of pollen and the pistil. Molecular genetic studies carried out since the late 1980s have led to the identification of the polymorphic genes at the S-locus that control self/nonself-recognition between pollen and the pistil. The S-RNase gene, which controls pistil specificity, was identified first, and subsequent sequencing of the S-locus region containing S-RNase led to the identification of the S-locus F-box (SLF) gene (now named SLF1). A transgenic approach was used to show that S2-SLF1 (SLF1 of S2-halotype) of Petunia inflata controls pollen specificity. The S-locus contains additional pollen-expressed F-box genes that show sequence similarity with SLF1, and initially they were thought not to be involved in pollen specificity. However, further studies of SLF1 suggested that it is not the only pollen specificity gene. Indeed, it has recently been shown that two previously identified SLF-like genes in P. inflata (now named SLF2 and SLF3) and a yet unknown number of additional SLF-like genes (named SLF4, SLF5, etc.) collaboratively function to control pollen specificity. The significance and implications of this new finding are discussed.

The self-incompatible RNase S-alleles of Brazilian apple cultivars

Apple plants are self-incompatible because a genetic mechanism allows the female reproductive organ to recognize and reject self-pollen or pollen from genetic related individuals and allows non-self pollen to effect fertilization. Thus, there are implications to both breeding strategies and orchard management for fruit production. The purpose of this study was to identify and to characterize the S-RNase alleles of the gametophytic incompatibility among apple cultivars developed in Brazil, seeking to give support for choosing right combinations of parent in the apple breeding programs. It also sought to identify correct combinations of scion/pollinator cultivars of commercial apple orchards. A total of 16 specific S-RNase alleles primers were tested against DNA extracted from 12 Brazilian cultivars and their parents. The molecular analysis confronted to the reference cultivars, showed that the cultivars Daiane, Imperatriz and Princesa have the same incompatibility S3 and S5 alleles, while ‘Lisgala’ showed the alleles S2 and S5; ‘Suprema’, S1 and S9; ‘Catarina’, S1 and S19; ‘Joaquina’ and ‘Fred Hough’, S5 and S19; ‘Baronesa’, S3 and S9; ‘Duquesa’, S2 and S3. For ‘Primicia’ and ‘Condessa’ it was only possible to identify one of the S-alleles, namely S24 and S2, respectively, with the second allele remaining to be identified. Progeny test indicated the Mendelian inheritance for RNase alleles. Results of this study will be helpful to judiciously choose parents in apple breeding programs to improve compatibility.

The amino terminal F-box domain of Petunia inflata S-locus F-box protein is involved in the S-RNase-based self-incompatibility mechanism

Pistils of flowering plants possessing self-incompatibility (SI) can distinguish between self and non-self pollen, and only allow non-self pollen to effect fertilization. For Petunia inflata, the S-RNase gene encodes pistil specificity and multiple S-locus F-box (SLF) genes encode pollen specificity. Each SLF produced in pollen interacts with a subset of non-self S-RNases to mediate their ubiquitination and degradation by the 26S proteasome. S-locus F-box has been proposed to function as a component of the conventional SCF (SKP1-CULLIN-F-box protein) complex, based on the finding that two SKP1-like proteins, AhSSK1 (Antirrhinum hispanicum SLF-interacting SKP1-like1) and PhSSK1 (Petunia hybrida SSK1), interact with the F-box domain of some allelic variants of SLF. However, we previously showed that PiSLF (P. inflata SLF) did not interact with any SKP1 of P. inflata or Arabidopsis thaliana, but instead interacted with a RING-finger protein, PiSBP1 (P. inflata S-RNase-Binding Protein1), which may also play the role of SKP1. Thus, the biochemical nature of the SLF-containing complex is as yet unclear. To examine whether the F-box domain of PiSLF is required for SI function, we expressed a truncated PiSLF(2) (S(2) allelic variant) without this domain in S(2)S(3) plants and showed that, unlike the full-length PiSLF(2), it did not cause breakdown of SI in S(3) pollen. We identified PiSSK1 (P. inflata SSK1) and found that it did not interact with PiSLF(1), PiSLF(2) or PiSLF(3). The finding that the truncated PiSLF(2) did not cause breakdown of SI in S(3) transgenic pollen suggests that the F-box domain of PiSLF(2) is required for mediating degradation of S(3)-RNase, a non-self S-RNase, in S(3) pollen, and thus is required for SI function. The finding that PiSSK1 did not interact with three allelic variants of PiSLF is consistent with our previous finding that PiSLF might not be in a conventional SCF complex.

Contrôle de la fécondation par des mécanismes d'auto-incompatibilité

Flowering plants (angiosperms) are the most prevalent and evolutionarily advanced group of plants. Reproductive strategies that promote cross-fertilization have played an essential role in the success of angiosperms as they contribute to genetic variability among plant species. A major genetic barrier to self-fertilization is self-incompatibility (SI), which allows female reproductive cells to discriminate between self- and non-self pollen and specifically reject self-pollen. In this review, we describe three SI mechanisms showing that different flowering plant families use distinct molecules for recognition of self as well as diverse biochemical pathways to arrest pollen tube growth.

'A life or death decision' for pollen tubes in S-RNase-based self-incompatibility

Mate choice is an essential process during sexual plant reproduction, in which self-incompatibility (SI) is widely adopted as an intraspecific reproductive barrier to inhibit self-fertilization by many flowering plants. Genetic studies show that a single polymorphic S-locus, encoding at least two components from both the pollen and pistil sides, controls the discrimination of self and non-self pollen. In the Solanaceae, Plantaginaceae, and Rosaceae, an S-RNase-based SI mechanism is involved in such a discrimination process. Recent studies have provided some important clues to how a decision is made to accept cross pollen or specifically to reject self pollen. In this review, the molecular features of the pistil and pollen S-specificity factors are briefly summarized and then our current knowledge of the molecular control of cross-pollen compatibility (CPC) and self-pollen incompatibility (SPI) responses, respectively, is presented. The possible biochemical mechanisms of the specificity determinant between the pistil and pollen S factors are discussed and a hypothetical S-RNase endosome sorting model is proposed to illustrate the distinct destinies of pollen tubes following compatible and incompatible pollination.

Cellular pathways regulating responses to compatible and self-incompatible pollen in Brassica and Arabidopsis stigmas intersect at Exo70A1, a putative component of the exocyst complex.

In the Brassicaceae, compatible pollen-pistil interactions result in pollen adhesion to the stigma, while pollen grains from unrelated plant species are largely ignored. There can also be an additional layer of recognition to prevent self-fertilization, the self-incompatibility response, whereby self pollen grains are distinguished from nonself pollen grains and rejected. This pathway is activated in the stigma and involves the ARM repeat-containing 1 (ARC1) protein, an E3 ubiquitin ligase. In a screen for ARC1-interacting proteins, we have identified Brassica napus Exo70A1, a putative component of the exocyst complex that is known to regulate polarized secretion. We show through transgenic studies that loss of Exo70A1 in Brassica and Arabidopsis thaliana stigmas leads to the rejection of compatible pollen at the same stage as the self-incompatibility response. A red fluorescent protein:Exo70A1 fusion rescues this stigmatic defect in Arabidopsis and is found to be mobilized to the plasma membrane concomitant with flowers opening. By contrast, increased expression of Exo70A1 in self-incompatible Brassica partially overcomes the self pollen rejection response. Thus, our data show that the Exo70A1 protein functions at the intersection of two cellular pathways, where it is required in the stigma for the acceptance of compatible pollen in both Brassica and Arabidopsis and is negatively regulated by Brassica self-incompatibility.