The ARC1 E3 ligase gene is frequently deleted in self-compatible Brassicaceae species and has a conserved role in Arabidopsis lyrata self-pollen rejection

Investigating a role for PUB17 and PUB16 in the self-incompatibility signaling pathway in transgenic Arabidopsis thaliana

In Brassicaceae self-incompatibility (SI), self-pollen rejection is initiated by the S-haplotype specific interactions between the pollen S cysteine-rich/S-locus protein 11 (SCR/SP11) ligands and the stigma S receptor kinases (SRK). In Brassica SI, a member of the Plant U-Box (PUB) E3 ubiquitin ligases, ARM-repeat containing 1 (ARC1), is then activated by SRK in this stigma and cellular events downstream of this cause SI pollen rejection by inhibiting pollen hydration and pollen tube growth. During the transition to selfing, Arabidopsis thaliana lost the SI components, SCR, SRK, and ARC1. However, this trait can be reintroduced into A. thaliana by adding back functional copies of these genes from closely related SI species. Both SCR and SRK are required for this, though the degree of SI pollen rejection varies between A. thaliana accessions, and ARC1 is not always needed to produce a strong SI response. For the A. thaliana C24 accession, only transforming with Arabidopsis lyrata SCR and SRK confers a strong SI trait (SI-C24), and so here, we investigated if ARC1-related PUBs were involved in the SI pathway in the transgenic A. thaliana SI-C24 line. Two close ARC1 homologs, PUB17 and PUB16, were selected, and (Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) technology was used to generate pub17 and pub16 mutations in the C24 accession. These mutants were then crossed into the transgenic A. thaliana SI-C24 line and their potential impact on SI pollen rejection was investigated. Overall, we did not observe any significant differences in SI responses to implicate PUB17 and PUB16 functioning in the transgenic A. thaliana SI-C24 stigma to reject SI pollen.

Exo84c-regulated degradation is involved in the normal self-incompatible response in Brassicaceae

The self-incompatibility system evolves in angiosperms to promote cross-pollination by rejecting self-pollination. Here, we show the involvement of Exo84c in the SI response of both Brassica napus and Arabidopsis. The expression of Exo84c is specifically elevated in stigma during the SI response. Knocking out Exo84c in B. napus and SI Arabidopsis partially breaks down the SI response. The SI response inhibits both the protein secretion in papillae and the recruitment of the exocyst complex to the pollen-pistil contact sites. Interestingly, these processes can be partially restored in exo84c SI Arabidopsis. After incompatible pollination, the turnover of the exocyst-labeled compartment is enhanced in papillae. However, this process is perturbed in exo84c SI Arabidopsis. Taken together, our results suggest that Exo84c regulates the exocyst complex vacuolar degradation during the SI response. This process is likely independent of the known SI pathway in Brassicaceae to secure the SI response.

Contrasting self-recognition rejection systems for self-incompatibility in Brassica and Papaver

Self-incompatibility (SI) plays a pivotal role in whether self-pollen is accepted or rejected. Most SI systems employ two tightly linked loci encoding highly polymorphic pollen (male) and pistil (female) S-determinants that control whether self-pollination is successful or not. In recent years our knowledge of the signalling networks and cellular mechanisms involved has improved considerably, providing an important contribution to our understanding of the diverse mechanisms used by plant cells to recognise each other and elicit responses. Here, we compare and contrast two important SI systems employed in the Brassicaceae and Papaveraceae. Both use 'self-recognition' systems, but their genetic control and S-determinants are quite different. We describe the current knowledge about the receptors and ligands, and the downstream signals and responses utilized to prevent self-seed set. What emerges is a common theme involving the initiation of destructive pathways that block the key processes that are required for compatible pollen-pistil interactions.

E3 ubiquitin ligase OsPIE3 destabilises the B-lectin receptor-like kinase PID2 to control blast disease resistance in rice

Previous studies have reported that PID2, which encodes a B-lectin receptor-like kinase, is a key gene in the resistance of rice to Magnaporthe oryzae strain ZB15. However, the PID2-mediated downstream signalling events remain largely unknown. The U-box E3 ubiquitin ligase OsPIE3 (PID2-interacting E3) was isolated and confirmed to play key roles in PID2-mediated rice blast resistance. Yeast two-hybrid analysis showed that the armadillo repeat region of OsPIE3 is required for its interaction with PID2. Further investigation demonstrated that OsPIE3 can modify the subcellular localisation of PID2, thus promoting its nuclear recruitment from the plasma membrane for protein degradation in the ubiquitin-proteasome system. Site-directed mutagenesis of a conserved cysteine site (C230S) within the U-box domain of OsPIE3 reduces PID2 translocation and ubiquitination. Genetic analysis suggested that OsPIE3 loss-of-function mutants exhibited enhanced resistance to M. oryzae isolate ZB15, whereas mutants with overexpressed OsPIE3 exhibited reduced resistance. Furthermore, the OsPIE3/PID2-double mutant displayed a similar blast phenotype to that of the PID2 single mutant, suggesting that OsPIE3 is a negative regulator and functions along with PID2 in blast disease resistance. Our findings confirm that the E3 ubiquitin ligase OsPIE3 is necessary for PID2-mediated rice blast disease resistance regulation.

Emerging roles of the ubiquitin-proteasome pathway in enhancing crop yield by optimizing seed agronomic traits

Ubiquitin-proteasome pathway has the potential to modulate crop productivity by influencing agronomic traits. Being sessile, the plant often uses the ubiquitin-proteasome pathway to maintain the stability of different regulatory proteins to survive in an ever-changing environment. The ubiquitin system influences plant reproduction, growth, development, responses to the environment, and processes that control critical agronomic traits. E3 ligases are the major players in this pathway, and they are responsible for recognizing and tagging the targets/substrates. Plants have a variety of E3 ubiquitin ligases, whose functions have been studied extensively, ranging from plant growth to defense strategies. Here we summarize three agronomic traits influenced by ubiquitination: seed size and weight, seed germination, and accessory plant agronomic traits particularly panicle architecture, tillering in rice, and tassels branch number in maize. This review article highlights some recent progress on how the ubiquitin system influences the stability/modification of proteins that determine seed agronomic properties like size, weight, germination and filling, and ultimately agricultural productivity and quality. Further research into the molecular basis of the aforementioned processes might lead to the identification of genes that could be modified or selected for crop development. Likewise, we also propose advances and future perspectives in this regard.

Generation of Arabidopsis thaliana transformants showing the self-recognition activity of Brassica rapa

Self-incompatibility in the Brassicaceae family is governed by SRK and SCR, which are two highly polymorphic genes located at the S-locus. Previously, the Arabidopsis lyrata SRK and SCR genes were introduced into Arabidopsis thaliana to generate self-incompatible lines. However, there are no reports showing that Brassica SRK and SCR genes confer self-incompatibility in A. thaliana. Doing so would further advance the mechanistic understanding of self-incompatibility in Brassicaceae. Therefore, we attempted to generate A. thaliana transformants showing the self-recognition activity of Brassica rapa by introducing BrSCR along with a chimeric BrSRK (BrSRK chimera, in which the kinase domain of BrSRK was replaced with that of AlSKR-b). We found that the BrSRK chimera and BrSCR of B. rapa S-9 and S-46 haplotypes, but not those of S-29, S-44, and S-60 haplotypes, conferred self-recognition activity in A. thaliana. Analyses of A. thaliana transformants expressing mutant variants of the BrSRK-9 chimera and BrSCR-9 revealed that mutations at the amino acid residues involved in BrSRK9-BrSCR9 interaction caused defects in the self-incompatibility response. The method developed in this study for generating self-incompatible A. thaliana transformants showing B. rapa self-recognition activity will be useful for analysis of self-recognition mechanisms in Brassicaceae.

Review: Exploring possible approaches using ubiquitylation and sumoylation pathways in modifying plant stress tolerance

Ubiquitin and similar proteins, such as SUMO, are utilized by plants to modify target proteins to rapidly change their stability and activity in cells. This review will provide an overview of these crucial protein interactions with a focus on ubiquitylation and sumoylation in plants and how they contribute to stress tolerance. The work will also explore possibilities to use these highly conserved pathways for novel approaches to generate more robust crop plants better fit to cope with abiotic and biotic stress situations.

Cell-cell signaling during the Brassicaceae self-incompatibility response

Self-incompatibility (SI) is a mechanism that many plant families employ to prevent self-fertilization. In the Brassicaceae, the S-haplotype-specific interaction of the pollen-borne ligand, and a stigma-specific receptor protein kinase triggers a signaling cascade that culminates in the rejection of self-pollen. While the upstream molecular components at the receptor level of the signaling pathway have been extensively studied, the intracellular responses beyond receptor activation were not as well understood. Recent research has uncovered several key molecules and signaling events that operate in concert for the manifestation of the self-incompatible responses in Brassicaceae stigmas. Here, we review the recent discoveries in both the compatible and self-incompatible pathways and provide new perspectives on the early stages of Brassicaceae pollen-pistil interactions.

Autophagy is required for self-incompatible pollen rejection in two transgenic Arabidopsis thaliana accessions

Successful reproduction in the Brassicaceae is mediated by a complex series of interactions between the pollen and the pistil, and some species have an additional layer of regulation with the self-incompatibility trait. While the initial activation of the self-incompatibility pathway by the pollen S-locus protein 11/S locus cysteine-rich protein and the stigma S Receptor Kinase is well characterized, the downstream mechanisms causing self-pollen rejection are still not fully understood. In previous studies, we detected the presence of autophagic bodies with self-incompatible (SI) pollinations in Arabidopsis lyrata and transgenic Arabidopsis thaliana lines, but whether autophagy was essential for self-pollen rejection was unknown. Here, we investigated the requirement of autophagy in this response by crossing mutations in the essential AUTOPHAGY7 (ATG7) and ATG5 genes into two different transgenic SI A. thaliana lines in the Col-0 and C24 accessions. By using these previously characterized transgenic lines that express A. lyrata and Arabidopsis halleri self-incompatibility genes, we demonstrated that disrupting autophagy weakened their SI responses in the stigma. When the atg7 or atg5 mutations were present, an increased number of SI pollen was found to hydrate and form pollen tubes that successfully fertilized the SI pistils. Additionally, we confirmed the presence of GFP-ATG8a-labeled autophagosomes in the stigmatic papillae following SI pollinations. Together, these findings support the requirement of autophagy in the self-incompatibility response and add to the growing understanding of the intracellular mechanisms employed in the transgenic A. thaliana stigmas to reject self-pollen.

How Many Faces Does the Plant U-Box E3 Ligase Have?

Ubiquitination is a major type of post-translational modification of proteins in eukaryotes. The plant U-Box (PUB) E3 ligase is the smallest family in the E3 ligase superfamily, but plays a variety of essential roles in plant growth, development and response to diverse environmental stresses. Hence, PUBs are potential gene resources for developing climate-resilient crops. However, there is a lack of review of the latest advances to fully understand the powerful gene family. To bridge the gap and facilitate its use in future crop breeding, we comprehensively summarize the recent progress of the PUB family, including gene evolution, classification, biological functions, and multifarious regulatory mechanisms in plants.

FERONIA receptor kinase-regulated reactive oxygen species mediate self-incompatibility in Brassica rapa

Most plants in the Brassicaceae evolve self-incompatibility (SI) to avoid inbreeding and generate hybrid vigor. Self-pollen is recognized by the S-haplotype-specific interaction of the pollen ligand S-locus protein 11 (SP11) (also known as S-locus cysteine-rich protein [SCR]) and its stigma-specific S-locus receptor kinase (SRK). However, mechanistically much remains unknown about the signaling events that culminate in self-pollen rejection. Here, we show that self-pollen triggers high levels of reactive oxygen species (ROS) in stigma papilla cells to mediate SI in heading Chinese cabbage (Brassica rapa L. ssp. pekinensis). We found that stigmatic ROS increased after self-pollination but decreased after compatible(CP)- pollination. Reducing stigmatic ROS by scavengers or suppressing the expression of respiratory burst oxidase homologs (Rbohs), which encode plant NADPH oxidases that produce ROS, both broke down SI. On the other hand, increasing the level of ROS inhibited the germination and penetration of compatible pollen on the stigma, mimicking an incompatible response. Furthermore, suppressing a B. rapa FERONIA (FER) receptor kinase homolog or Rac/Rop guanosine triphosphatase (GTPase) signaling effectively reduced stigmatic ROS and interfered with SI. Our results suggest that FER-Rac/Rop signaling-regulated, NADPH oxidase-produced ROS is an essential SI response leading to self-pollen rejection.

The molecular signatures of compatible and incompatible pollination in Arabidopsis

Background

Fertilization in flowering plants depends on the early contact and acceptance of pollen grains by the receptive papilla cells of the stigma. Deciphering the specific transcriptomic response of both pollen and stigmatic cells during their interaction constitutes an important challenge to better our understanding of this cell recognition event.

Results

Here we describe a transcriptomic analysis based on single nucleotide polymorphisms (SNPs) present in two Arabidopsis thaliana accessions, one used as female and the other as male. This strategy allowed us to distinguish 80% of transcripts according to their parental origins. We also developed a tool which predicts male/female specific expression for genes without SNP. We report an unanticipated transcriptional activity triggered in stigma upon incompatible pollination and show that following compatible interaction, components of the pattern-triggered immunity (PTI) pathway are induced on the female side.

Conclusions

Our work unveils the molecular signatures of compatible and incompatible pollinations both at the male and female side. We provide invaluable resource and tools to identify potential new molecular players involved in pollen-stigma interaction.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07503-7.

Cross-Species Metabolic Profiling of Floral Specialized Metabolism Facilitates Understanding of Evolutional Aspects of Metabolism Among Brassicaceae Species

Plants produce a variety of floral specialized (secondary) metabolites with roles in several physiological functions, including light-protection, attraction of pollinators, and protection against herbivores. Pigments and volatiles synthesized in the petal have been focused on and characterized as major chemical factors influencing pollination. Recent advances in plant metabolomics have revealed that the major floral specialized metabolites found in land plant species are hydroxycinnamates, phenolamides, and flavonoids albeit these are present in various quantities and encompass diverse chemical structures in different species. Here, we analyzed numerous floral specialized metabolites in 20 different Brassicaceae genotypes encompassing both different species and in the case of crop species different cultivars including self-compatible (SC) and self-incompatible (SI) species by liquid chromatography-mass spectrometry (LC-MS). Of the 228 metabolites detected in flowers among 20 Brassicaceae species, 15 metabolite peaks including one phenylacyl-flavonoids and five phenolamides were detected and annotated as key metabolites to distinguish SC and SI plant species, respectively. Our results provide a family-wide metabolic framework and delineate signatures for compatible and incompatible genotypes thereby providing insight into evolutionary aspects of floral metabolism in Brassicaceae species.

Arabidopsis U-box E3 ubiquitin ligase PUB11 negatively regulates drought tolerance by degrading the receptor-like protein kinases LRR1 and KIN7

Both plant receptor-like protein kinases (RLKs) and ubiquitin-mediated proteolysis play crucial roles in plant responses to drought stress. However, the mechanism by which E3 ubiquitin ligases modulate RLKs is poorly understood. In this study, we showed that Arabidopsis PLANT U-BOX PROTEIN 11 (PUB11), an E3 ubiquitin ligase, negatively regulates abscisic acid (ABA)-mediated drought responses. PUB11 interacts with and ubiquitinates two receptor-like protein kinases, LEUCINE RICH REPEAT PROTEIN 1 (LRR1) and KINASE 7 (KIN7), and mediates their degradation during plant responses to drought stress in vitro and in vivo. pub11 mutants were more tolerant, whereas lrr1 and kin7 mutants were more sensitive, to drought stress than the wild type. Genetic analyses show that the pub11 lrr1 kin7 triple mutant exhibited similar drought sensitivity as the lrr1 kin7 double mutant, placing PUB11 upstream of the two RLKs. Abscisic acid and drought treatment promoted the accumulation of PUB11, which likely accelerates LRR1 and KIN7 degradation. Together, our results reveal that PUB11 negatively regulates plant responses to drought stress by destabilizing the LRR1 and KIN7 RLKs.

Two subgroups of receptor-like kinases promote early compatible pollen responses in the Arabidopsis thaliana pistil

In flowering plants, cell-cell communication between the compatible pollen grain/growing pollen tube and the pistil is an essential component for successful sexual reproduction. In Arabidopsis thaliana, the later stages of this dialogue are mediated by several peptide ligands and receptors that guide pollen tubes to the ovules for the release of sperm cells. Despite a detailed understanding of these processes, a key gap remains regarding the nature of the regulators that function at the earlier stages which are essential steps leading to fertilization. Here, we report on new functions for A. thaliana Receptor-Like Kinase (RLK) genes belonging to the LRR-II and LRR-VIII-2 RLK subgroups in the female reproductive tract to regulate compatible pollen hydration and the early stages of pollen tube growth. Mutant pistils for the A. thaliana RKF1 gene cluster were observed to support reduced wild-type pollen hydration and, when combined with the SERK1 and SERK3/BAK1 mutations, reduced pollen tube travel distances occurred. As these mutant pistils displayed a wild-type morphology, we propose that the observed altered compatible pollen responses result from an impaired pollen-pistil dialogue at these early stages.

Expression of Brassica napus GLO1 is sufficient to breakdown artificial self-incompatibility in Arabidopsis thaliana

Members of the Brassicaceae family have the ability to regulate pollination events occurring on the stigma surface. In Brassica species, self-pollination leads to an allele-specific interaction between the pollen small cysteine-rich peptide ligand (SCR/SP11) and the stigmatic S-receptor kinase (SRK) that activates the E3 ubiquitin ligase ARC1 (Armadillo repeat-containing 1), resulting in proteasomal degradation of various compatibility factors including glyoxalase I (GLO1) which is necessary for successful pollination. In Brassica napus, the suppression of GLO1 was sufficient to reduce compatibility, and overexpression of GLO1 in self-incompatible Brassica napus stigmas resulted in partial breakdown of the self-incompatibility response. Here, we verified if BnGLO1 could function as a compatibility factor in the artificial self-incompatibility system of Arabidopsis thaliana expressing AlSCR_b, AlSRK_b and AlARC1 proteins from A. lyrata. Overexpression of BnGLO1 is sufficient to breakdown self-incompatibility response in A. thaliana stigmas. Therefore, GLO1 has an indisputable role as a compatibility factor in the stigma in regulating pollen attachment and pollen tube growth. Lastly, this study demonstrates the usefulness of an artificial self-incompatibility system in A. thaliana for interspecific self-incompatibility studies.

Comparative study of vegetative and reproductive growth of different tea varieties response to different fluoride concentrations stress

BACKGROUND

The amount of fluoride accumulation in tea leaves was gradually increase as the matures of tea plants, and the excessive fluoride intake can threaten people's health. Based on years of field investigations, a low level of fluoride variety Xiangbo Lǜ (XBL) and a high level of fluoride variety Zhenong 139 (ZN139) were selected.

RESULTS

In this study, the root, 1st and the 5th leaf of the two-year-old tea trees were used for morphological, physiological and comparative transcriptomics analysis to understand the different features of "XBL" and "ZN139" under fluoride stress conditions. The color of the 1st and 5th leaves of XBL were yellower, the activity of peroxidase, catalase and antioxidant enzyme were lower than ZN139 under the high-fluoride stress. Transcriptomics analysis indicated that core genes involved in photosynthesis rates regulation showed no significantly exchanged expression, the co-downregulation of magnesium ions transportation, while the ROS scavenging, vegetative growth and self-compatibility between the two varieties were different. Crucial genes' expression were also identified by the real-time RT-PCR.

CONCLUSION

The tea tree is one of the few plants that has a high-fluoride content, but the different varieties respond differently to fluoride stress. High-fluoride tea tree varieties, such as ZN139, have stronger ROS scavenging abilities through the use of both their non-enzymatic and enzymatic antioxidant systems which act by increasing the expression levels of inositol-1-monophosphatases and peroxidases, among others. ZN139 can also compensate for the decrease in photosynthetic rate that is associated with the ionic imbalance caused by the reduced consumption of light energy during long-periods of high fluoride stress. Reproductive development was protected in ZN139 by the up-regulated expression of S-locus glycoprotein, Mildew resistance locus o and Phospholipase D under fluoride stress, while the vegetative development of low-fluoride varieties such as XBL was retarded. More starch and cellulose were redistributed to glucose by increasing the expression levels of glycosyl transferases and hydrolases to provide more energy for processes involved in the response and tolerance towards fluoride stress.

Pollen-stigma interactions in Brassicaceae: complex communication events regulating pollen hydration

This article comments on: Rozier F, Riglet L, Kodera C, Bayle V, Durand E, Schnabel J, Gaude T, Fobis-Loisy I. 2020. Live-cell imaging of early events following pollen perception in self-incompatible Arabidopsis thaliana. Journal of Experimental Botany 71, 2513–2526.

Boosting autophagy in sexual reproduction: a plant perspective

The key process of sexual reproduction is the successful fusion of the sperm and egg cell. Distinct from dynamic and flagellated animal sperm cells, higher flowering plant sperm cells are immotile. Therefore, plants have evolved a novel reproductive system to achieve fertilization and generate progenies. Plant sexual reproduction consists of multiple steps, mainly including gametophyte development, pollen-pistil recognition, pollen germination, double fertilization and postfertilization. During reproduction, active production, consumption and recycling of cellular components and energy are critically required to achieve fertilization. However, the underlying machinery of cellular degradation and turnover remains largely unexplored. Autophagy, the major catabolic pathway in eukaryotic cells, participates in regulating multiple aspects of plant activities, including abiotic and biotic stress resistance, pathogen response, senescence, nutrient remobilization and plant development. Nevertheless, a key unanswered question is how autophagy regulates plant fertilization and reproduction. Here, we focus on comparing and contrasting autophagy in several key reproductive processes of plant and animal systems to feature important distinctions and highlight future research directions of autophagy in angiosperm reproduction. We further discuss the potential crosstalk between autophagy and programmed cell death, which are often considered as two disconnected events in plant sexual reproduction.

Parallel evolution of dominant pistil-side self-incompatibility suppressors in Arabidopsis

Selfing is a frequent evolutionary trend in angiosperms, and is a suitable model for studying the recurrent patterns underlying adaptive evolution. Many plants avoid self-fertilization by physiological processes referred to as self-incompatibility (SI). In the Brassicaceae, direct and specific interactions between the male ligand SP11/SCR and the female receptor kinase SRK are required for the SI response. Although Arabidopsis thaliana acquired autogamy through loss of these genes, molecular evolution contributed to the spread of self-compatibility alleles requires further investigation. We show here that in this species, dominant SRK silencing genes have evolved at least twice. Different inverted repeat sequences were found in the relic SRK region of the Col-0 and C24 strains. Both types of inverted repeats suppress the functional SRK sequence in a dominant fashion with different target specificities. It is possible that these dominant suppressors of SI contributed to the rapid fixation of self-compatibility in A. thaliana.

In Brassicaceae, interaction between the pollen-derived peptide ligand SP11 and the pistil-expressed receptor kinase SRK leads to self-incompatibility. Here the authors provide evidence that in Arabidopsis dominant self-compatibility inducers evolved at least twice via insertion of inverted repeats in the SRK locus.

Impact of whole genome triplication on the evolutionary history and the functional dynamics of regulatory genes involved in Brassica self-incompatibility signalling pathway

Polyploidy or whole genome duplication is a frequent and recurrent phenomenon in flowering plants that has played a major role in their diversification, adaptation and speciation. The adaptive success of polyploids relates to the different evolutionary fates of duplicated genes. In this study, we explored the impact of the whole genome triplication (WGT) event in the Brassiceae tribe on the genes involved in the self-incompatibility (SI) signalling pathway, a mechanism allowing recognition and rejection of self-pollen in hermaphrodite plants. By taking advantage of the knowledge acquired on this pathway as well as of several reference genomes in Brassicaceae species, we determined copy number of the different genes involved in this pathway and investigated their structural and functional evolutionary dynamics. We could infer that whereas most genes involved in the SI signalling returned to single copies after the WGT event (i.e. ARC1, JDP1, THL1, THL2, Exo70A01) in diploid Brassica species, a few were retained in duplicated (GLO1 and PLDα) or triplicated copies (MLPK). We also carefully studied the gene structure of these latter duplicated genes (including the conservation of functional domains and active sites) and tested their transcription in the stigma to identify which copies seem to be involved in the SI signalling pathway. By taking advantage of these analyses, we then explored the putative origin of a contrasted SI phenotype between two Brassica rapa varieties that have been fully sequenced and shared the same S-allele (S60).

Impact of whole genome triplication on the evolutionary history and the functional dynamics of regulatory genes involved in Brassica self-incompatibility signalling pathway

Polyploidy or whole genome duplication is a frequent and recurrent phenomenon in flowering plants that has played a major role in their diversification, adaptation and speciation. The adaptive success of polyploids relates to the different evolutionary fates of duplicated genes. In this study, we explored the impact of the whole genome triplication (WGT) event in the Brassiceae tribe on the genes involved in the self-incompatibility (SI) signalling pathway, a mechanism allowing recognition and rejection of self-pollen in hermaphrodite plants. By taking advantage of the knowledge acquired on this pathway as well as of several reference genomes in Brassicaceae species, we determined copy number of the different genes involved in this pathway and investigated their structural and functional evolutionary dynamics. We could infer that whereas most genes involved in the SI signalling returned to single copies after the WGT event (i.e. ARC1, JDP1, THL1, THL2, Exo70A01) in diploid Brassica species, a few were retained in duplicated (GLO1 and PLDα) or triplicated copies (MLPK). We also carefully studied the gene structure of these latter duplicated genes (including the conservation of functional domains and active sites) and tested their transcription in the stigma to identify which copies seem to be involved in the SI signalling pathway. By taking advantage of these analyses, we then explored the putative origin of a contrasted SI phenotype between two Brassica rapa varieties that have been fully sequenced and shared the same S-allele (S60).

EbARC1, an E3 Ubiquitin Ligase Gene in Erigeron breviscapus, Confers Self-Incompatibility in Transgenic Arabidopsis thaliana

Erigeron breviscapus (Vant.) Hand.-Mazz. is a famous traditional Chinese medicine that has positive effects on the treatment of cardiovascular and cerebrovascular diseases. With the increase of market demand (RMB 500 million per year) and the sharp decrease of wild resources, it is an urgent task to cultivate high-quality and high-yield varieties of E. breviscapus. However, it is difficult to obtain homozygous lines in breeding due to the self-incompatibility (SI) of E. breviscapus. Here, we first proved that E. breviscapus has sporophyte SI (SSI) characteristics. Characterization of the ARC1 gene in E. breviscapus showed that EbARC1 is a constitutive expression gene located in the nucleus. Overexpression of EbARC1 in Arabidopsis thaliana L. (Col-0) could cause transformation of transgenic lines from self-compatibility (SC) into SI. Yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays indicated that EbARC1 and EbExo70A1 interact with each other in the nucleus, and the EbARC1-ubox domain and EbExo70A1-N are the key interaction regions, suggesting that EbARC1 may ubiquitinate EbExo70A to regulate SI response. This study of the SSI mechanism in E. breviscapus has laid the foundation for further understanding SSI in Asteraceae and breeding E. breviscapus varieties.

Programmed Cell Death in Stigmatic Papilla Cells Is Associated With Senescence-Induced Self-Incompatibility Breakdown in Chinese Cabbage and Radish

Self-incompatibility (SI) is a genetic mechanism flowering plants adopted to reject self-pollen and promote outcrossing. In the Brassicaceae family plants, the stigma tissue plays a key role in self-pollen recognition and rejection. We reported earlier in Chinese cabbage (Brassica rapa) that stigma tissue showed upregulated ethylene responses and programmed cell death (PCD) upon compatible pollination, but not in SI responses. Here, we show that SI is significantly compromised or completely lost in senescent flowers and young flowers of senescent plants. Senescence upregulates senescence-associated genes in B. rapa. Suppressing their expression in young stigmas by antisense oligodeoxyribonucleotide abolishes compatible pollination-triggered PCD and inhibits the growth of compatible pollen tubes. Furthermore, ethylene biosynthesis genes and response genes are upregulated in senescent stigmas, and increasing the level of ethylene or inhibiting its response increases or decreases the expression of senescence-associated genes, respectively. Our results show that senescence causes PCD in stigmatic papilla cells and is associated with the breakdown of SI in Chinese cabbage and in radish.

Identification and characterization of BoPUB3: a novel interaction protein with S-locus receptor kinase in Brassica oleracea L

Armadillo repeat containing 1 (ARC1) is phosphorylated by S-locus receptor kinase (SRK) and functions as a positive regulator in self-incompatibility response of Brassica. However, ARC1 only causes partial breakdown of the self-incompatibility response, and other SRK downstream factors may also participate in the self-incompatibility signaling pathway. In the present study, to search for SRK downstream targets, a plant U-box protein 3 (BoPUB3) was identified from the stigma of Brassica oleracea L. BoPUB3 was highly expressed in the stigma, and its expression was increased with the stigma development and reached to the highest level in the mature-stage stigma. BoPUB3, a 76.8-kDa protein with 697 amino acids, is a member of the PUB-ARM family and contains three domain characteristics of BoARC1, including a U-box N-terminal domain, a U-box motif, and a C-terminal arm repeat domain. The phylogenic tree showed that BoPUB3 was close to BoARC1. The synteny analysis revealed that B. oleracea chromosomal region containing BoPUB3 had high synteny with the Arabidopsis thaliana chromosomal region containing AtPUB3 (At3G54790). In addition, the subcellular localization analysis showed that BoPUB3 primarily localized in the plasma membrane and also in the cytoplasm. The combination of the yeast two-hybrid and in vitro binding assay showed that both BoPUB3 and BoARC1 could interact with SRK kinase domain, and SRK showed much higher level of β-galactosidase activity in its interaction with BoPUB3 than with BoARC1. These results implied that BoPUB3 is a novel interactor with SRK, which lays a basis for further research on whether PUB3 participates in the self-incompatibility signaling pathway.

Self-incompatibility in Papaver pollen: programmed cell death in an acidic environment

Self-incompatibility (SI) is a genetically controlled mechanism that prevents self-fertilization and thus encourages outbreeding and genetic diversity. During pollination, most SI systems utilize cell-cell recognition to reject incompatible pollen. Mechanistically, one of the best-studied SI systems is that of Papaver rhoeas (poppy), which involves the interaction between the two S-determinants, a stigma-expressed secreted protein (PrsS) and a pollen-expressed plasma membrane-localized protein (PrpS). This interaction is the critical step in determining acceptance of compatible pollen or rejection of incompatible pollen. Cognate PrpS-PrsS interaction triggers a signalling network causing rapid growth arrest and eventually programmed cell death (PCD) in incompatible pollen. In this review, we provide an overview of recent advances in our understanding of the major components involved in the SI-induced PCD (SI-PCD). In particular, we focus on the importance of SI-induced intracellular acidification and consequences for protein function, and the regulation of soluble inorganic pyrophosphatase (Pr-p26.1) activity by post-translational modification. We also discuss attempts to identify protease(s) involved in the SI-PCD process. Finally, we outline future opportunities made possible by the functional transfer of the P. rhoeas SI system to Arabidopsis.

Progress on deciphering the molecular aspects of cell-to-cell communication in Brassica self-incompatibility response

The sporophytic system of self-incompatibility is a widespread genetic phenomenon in plant species, promoting out-breeding and maintaining genetic diversity. This phenomenon is of commercial importance in hybrid breeding of Brassicaceae crops and is controlled by single S locus with multiple S haplotypes. The molecular genetic studies of Brassica 'S' locus has revealed the presence of three tightly linked loci viz. S-receptor kinase (SRK), S-locus cysteine-rich protein/S-locus protein 11 (SCR/SP11), and S-locus glycoprotein (SLG). On self-pollination, the allele-specific ligand-receptor interaction activates signal transduction in stigma papilla cells and leads to rejection of pollen tube on stigmatic surface. In addition, arm-repeat-containing protein 1 (ARC1), M-locus protein kinase (MLPK), kinase-associated protein phosphatase (KAPP), exocyst complex subunit (Exo70A1) etc. has been identified in Brassica crops and plays a key role in self-incompatibility signaling pathway. Furthermore, the cytoplasmic calcium (Ca2+) influx in papilla cells also mediates self-incompatibility response in Brassicaceae, but how this cytoplasmic Ca2+ influx triggers signal transduction to inhibit pollen hydration is still obscure. There are many other signaling components which are not well characterized yet. Much progress has been made in elucidating the downstream multiple pathways of Brassica self-incompatibility response. Hence, in this review, we have made an effort to describe the recent advances made on understanding the molecular aspects of genetic mechanism of self-incompatibility in Brassicaceae.

The Molecular and Cellular Regulation of Brassicaceae Self-Incompatibility and Self-Pollen Rejection

In flowering plants, sexual reproduction is actively regulated by cell-cell communication between the male pollen and female pistil, and many species possess self-incompatibility systems for the selective rejection of self-pollen to maintain genetic diversity. The Brassicaceae self-incompatibility pathway acts early on when pollen grains have landed on the stigmatic papillae at the top of the pistil. Extensive studies have revealed that self-pollen rejection in the Brassicaceae is initiated by an S-haplotype-specific interaction between two polymorphic proteins: the pollen S-locus protein 11/S cysteine-rich (SP11/SCR) ligand and the stigma S receptor kinase (SRK). While the different S-haplotypes are typically codominant, there are several examples of dominant-recessive interactions, and a small RNA-based regulation of SP11/SCR expression has been uncovered as a mechanism behind these genetic interactions. Recent research has also added to our understanding of various cellular components in the pathway leading from the SP11/SCR-SRK interaction, including two signaling proteins, the M-locus protein kinase (MLPK) and the ARM-repeat containing 1 (ARC1) E3 ligase, as well as calcium fluxes and induction of autophagy in the stigmatic papillae. Finally, a better understanding of the compatible pollen responses that are targeted by the self-incompatibility pathway is starting to emerge, and this will allow us to more fully understand how the Brassicaceae self-incompatibility pathway causes self-pollen rejection. Here, we provide an overview of the field, highlighting recent contributions to our understanding of Brassicaceae self-incompatibility, and draw comparisons to a recently discovered unilateral incompatibility system.

Patterns of Polymorphism at the Self-Incompatibility Locus in 1,083 Arabidopsis thaliana Genomes

Although the transition to selfing in the model plant Arabidopsis thaliana involved the loss of the self-incompatibility (SI) system, it clearly did not occur due to the fixation of a single inactivating mutation at the locus determining the specificities of SI (the S-locus). At least three groups of divergent haplotypes (haplogroups), corresponding to ancient functional S-alleles, have been maintained at this locus, and extensive functional studies have shown that all three carry distinct inactivating mutations. However, the historical process of loss of SI is not well understood, in particular its relation with the last glaciation. Here, we took advantage of recently published genomic resequencing data in 1,083 Arabidopsis thaliana accessions that we combined with BAC sequencing to obtain polymorphism information for the whole S-locus region at a species-wide scale. The accessions differed by several major rearrangements including large deletions and interhaplogroup recombinations, forming a set of haplogroups that are widely distributed throughout the native range and largely overlap geographically. “Relict” A. thaliana accessions that directly derive from glacial refugia are polymorphic at the S-locus, suggesting that the three haplogroups were already present when glacial refugia from the last Ice Age became isolated. Interhaplogroup recombinant haplotypes were highly frequent, and detailed analysis of recombination breakpoints suggested multiple independent origins. These findings suggest that the complete loss of SI in A. thaliana involved independent self-compatible mutants that arose prior to the last Ice Age, and experienced further rearrangements during postglacial colonization.

News from the PUB: plant U-box type E3 ubiquitin ligases

Plant U-box type E3 ubiquitin ligases (PUBs) are well known for their functions in a variety of stress responses, including immune responses and the adaptation to abiotic stresses. First linked to pollen self-incompatibility, their repertoire of roles has grown to encompass also the regulation of developmental processes. Notably, new studies provide clues to their mode of action, underline the existence of conserved PUB-kinase modules, and suggest new links to G-protein signalling, placing PUBs at the crossroads of major signalling hubs. The frequent association with membranes, by interacting and/or targeting membrane proteins, as well as through a recently reported direct interaction with phospholipids, indicates a general function in the control of vesicle transport and their cargoes. This review aims to give an overview of the most significant advances in the field, while also trying to identify common themes of PUB function.

The genetic breakdown of sporophytic self-incompatibility in Tolpis coronopifolia (Asteraceae)

Angiosperm diversity has been shaped by mating system evolution, with the most common transition from outcrossing to self-fertilizing. To investigate the genetic basis of this transition, we performed crosses between two species endemic to the Canary Islands, the self-compatible (SC) species Tolpis coronopifolia and its self-incompatible (SI) relative Tolpis santosii. We scored self-compatibility as self-seed set of recombinant plants within two F₂ populations. To map and genetically characterize the breakdown of SI, we built a draft genome sequence of T. coronopifolia, genotyped F₂ plants using multiplexed shotgun genotyping (MSG), and located MSG markers to the genome sequence. We identified a single quantitative trait locus (QTL) that explains nearly all variation in self-seed set in both F₂ populations. To identify putative causal genetic variants within the QTL, we performed transcriptome sequencing on mature floral tissue from both SI and SC species, constructed a transcriptome for each species, and then located each predicted transcript to the T. coronopifolia genome sequence. We annotated each predicted gene within the QTL and found two strong candidates for SI breakdown. Each gene has a coding sequence insertion/deletion mutation within the SC species that produces a truncated protein. Homologs of each gene have been implicated in pollen development, pollen germination, and pollen tube growth in other species.

E3 Ubiquitin Ligases: Ubiquitous Actors in Plant Development and Abiotic Stress Responses

Understanding the precise regulatory mechanisms of plant development and stress responses at the post-translational level is currently a topic of intensive research. Protein ubiquitination, including the sequential performances of ubiquitin-activating (E1), ubiquitin-conjugating (E2) and ubiquitin ligase (E3) enzymes, is a refined post-translational modification ubiquitous in all eukaryotes. Plants are an integral part of our ecosystem and, as sessile organisms, the ability to perceive internal and external signals and to adapt well to various environmental challenges is crucial for their survival. Over recent decades, extensive studies have demonstrated that protein ubiquitination plays key roles in multiple plant developmental stages (e.g. seed dormancy and germination, root growth, flowering time control, self-incompatibility and chloroplast development) and several abiotic stress responses (e.g. drought and high salinity), by regulating the abundance, activities or subcellular localizations of a variety of regulatory polypeptides and enzymes. Importantly, diverse E3 ligases are involved in these regulatory pathways by mediating phytohormone and light signaling or other pathways. In this updated review, we mainly summarize recent advances in our understanding of the regulatory roles of protein ubiquitination in plant development and plant-environment interactions, and primarily focus on different types of E3 ligases because they play critical roles in determining substrate specificity.

Plant Lectins and Lectin Receptor-Like Kinases: How Do They Sense the Outside?

Lectins are fundamental to plant life and have important roles in cell-to-cell communication; development and defence strategies. At the cell surface; lectins are present both as soluble proteins (LecPs) and as chimeric proteins: lectins are then the extracellular domains of receptor-like kinases (LecRLKs) and receptor-like proteins (LecRLPs). In this review; we first describe the domain architectures of proteins harbouring G-type; L-type; LysM and malectin carbohydrate-binding domains. We then focus on the functions of LecPs; LecRLKs and LecRLPs referring to the biological processes they are involved in and to the ligands they recognize. Together; LecPs; LecRLKs and LecRLPs constitute versatile recognition systems at the cell surface contributing to the detection of symbionts and pathogens; and/or involved in monitoring of the cell wall structure and cell growth.

Genome sequencing supports a multi-vertex model for Brassiceae species

The economically important Brassica genus is a good system for studying the evolution of polyploids. Brassica genomes have undergone whole genome triplication (WGT). Subgenome dominance phenomena such as biased gene fractionation and dominant gene expression were observed in tripled genomes of Brassica. The genome of radish (Raphanus sativus), another important crop of tribe Brassiceae, was derived from the same WGT event and shows similar subgenome dominance. These findings and molecular dating indicate that radish occupies a similar evolutionary origin as that of Brassica species. Here, we extended the Brassica "triangle of U" to a multi-vertex model. This model describes the relationships or the potential of using more Brassiceae mesohexaploids in the creation of new allotetraploid oil or vegetable crop species.

The N-Terminal UND Motif of the Arabidopsis U-Box E3 Ligase PUB18 Is Critical for the Negative Regulation of ABA-Mediated Stomatal Movement and Determines Its Ubiquitination Specificity for Exocyst Subunit Exo70B1

The Arabidopsis thaliana U-box E3 ligases PUB18/PUB19 and PUB22/PUB23 are negative regulators of drought stress responses. PUB18/PUB19 regulate the drought stress response in an abscisic acid (ABA)-dependent manner, whereas PUB22/PUB23 regulate this response in an ABA-independent manner. A major structural difference between PUB18/PUB19 and PUB22/PUB23 is the presence of the UND (U-box N-terminal domain). Here, we focused on elucidating the molecular mechanism that mediates the functional difference between PUB18 and PUB22 and found that the UND^PUB18 was critically involved in the negative regulation of ABA-mediated stomatal movements. Exo70B1, a subunit of the exocyst complex, was identified as a target of PUB18, whereas Exo70B2 was a substrate of PUB22. However, the ∆UND-PUB18 derivative failed to ubiquitinate Exo70B1, but ubiquitinated Exo70B2. By contrast, the UND^PUB18-PUB22 chimeric protein ubiquitinated Exo70B1 instead of Exo70B2, suggesting that the ubiquitination specificities of PUB18 and PUB22 to Exo70B1 and Exo70B2, respectively, are dependent on the presence or absence of the UND^PUB18 motif. The ABA-insensitive phenotypes of the pub18 pub19 exo70b1 triple mutant were reminiscent of those of exo70b1 rather than pub18 pub19, indicating that Exo70B1 functions downstream of PUB18. Overall, our results suggest that the UND^PUB18 motif is crucial for the negative regulation of ABA-dependent stomatal movement and for determination of its ubiquitination specificity to Exo70B1.

Pollen Acceptance or Rejection: A Tale of Two Pathways

While the molecular and cellular basis of self-incompatibility leading to self-pollen rejection in the Brassicaceae has been extensively studied, relatively little attention has been paid to compatible pollen recognition and the corresponding cellular responses in the stigmatic papillae. This is now changing because research has started to uncover steps in the Brassicaceae 'basal compatible pollen response pathway' in the stigma leading to pollen hydration and germination. Furthermore, recent studies suggest that self-incompatible pollen activates both the basal compatible pathway and the self-incompatibility pathway in the stigma, with the self-incompatibility response ultimately prevailing to reject self-pollen. We review here recent discoveries in both pathways and discuss how compatible pollen is accepted by the stigma versus the rejection of self-incompatible pollen.

What causes mating system shifts in plants? Arabidopsis lyrata as a case study

The genetic breakdown of self-incompatibility (SI) and subsequent mating system shifts to inbreeding has intrigued evolutionary geneticists for decades. Most of our knowledge is derived from interspecific comparisons between inbreeding species and their outcrossing relatives, where inferences may be confounded by secondary mutations that arose after the initial loss of SI. Here, we study an intraspecific breakdown of SI and its consequences in North American Arabidopsis lyrata to test whether: (1) particular S-locus haplotypes are associated with the loss of SI and/or the shift to inbreeding; (2) a population bottleneck may have played a role in driving the transition to inbreeding; and (3) the mutation(s) underlying the loss of SI are likely to have occurred at the S-locus. Combining multiple approaches for genotyping, we found that outcrossing populations on average harbour 5 to 9 S-locus receptor kinase (SRK) alleles, but only two, S1 and S19, are shared by most inbreeding populations. Self-compatibility (SC) behaved genetically as a recessive trait, as expected from a loss-of-function mutation. Bulked segregant analysis in SC × SI F2 individuals using deep sequencing confirmed that all SC plants were S1 homozygotes but not all S1 homozygotes were SC. This was also revealed in population surveys, where only a few S1 homozygotes were SC. Together with crossing data, this suggests that there is a recessive factor that causes SC that is physically unlinked to the S-locus. Overall, our results emphasise the value of combining classical genetics with advanced sequencing approaches to resolve long outstanding questions in evolutionary biology.

Identification of a novel MLPK homologous gene MLPKn1 and its expression analysis in Brassica oleracea

M locus protein kinase, one of the SRK-interacting proteins, is a necessary positive regulator for the self-incompatibility response in Brassica. In B. rapa, MLPK is expressed as two different transcripts, MLPKf1 and MLPKf2, and either isoform can complement the mlpk/mlpk mutation. The AtAPK1B gene has been considered to be the ortholog of BrMLPK, and AtAPK1B has no role in self-incompatibility (SI) response in A. thaliana SRK-SCR plants. Until now, what causes the MLPK and APK1B function difference during SI response in Brassica and A. thaliana SRKb-SCRb plants has remained unknown. Here, in addition to the reported MLPKf1/2, we identified the new MLPKf1 homologous gene MLPKn1 from B. oleracea. BoMLPKn1 and BoMLPKf1 shared nucleotide sequence identity as high as 84.3 %, and the most striking difference consisted in two fragment insertions in BoMLPKn1. BoMLPKn1 and BoMLPKf1 had a similar gene structure; both their deduced amino acid sequences contained a typical plant myristoylation consensus sequence and a Ser/Thr protein kinase domain. BoMLPKn1 was widely expressed in petal, sepal, anther, stigma and leaf. Genome-wide survey revealed that the B. oleracea genome contained three MLPK homologous genes: BoMLPKf1/2, BoMLPKn1 and Bol008343n. The B. rapa genome also contained three MLPK homologous genes, BrMLPKf1/2, BraMLPKn1 and Bra040929. Phylogenetic analysis revealed that BoMLPKf1/2 and BrMLPKf1/2 were phylogenetically more distant from AtAPK1A than Bol008343n, Bra040929, BraMLPKn1 and BoMLPKn1, Synteny analysis revealed that the B. oleracea chromosomal region containing BoMLPKn1 displayed high synteny with the A. thaliana chromosomal region containing APK1B, whereas the B. rapa chromosomal region containing BraMLPKn1 showed high synteny with the A. thaliana chromosomal region containing APK1B. Together, these results revealed that BoMLPKn1/BraMLPKn1, and not the formerly reported BoMLPKf1/2 (BrMLPKf1/2), was the orthologous genes of AtAPK1B, and no ortholog of BoMLPKf1/2 (BrMLPKf1/2) was found in the A. thaliana genome. We speculated that Brassica MLPKf1/2 might have emerged after speciation of Brassica and A. thailiana, and that it was recruited to the SRK-triggered SI signaling cascade in Brassica.

Towards development of new ornamental plants: status and progress in wide hybridization

The present review provides insights into the key findings of the hybridization process, crucial factors affecting the adaptation of new technologies within wide hybridization of ornamental plants and presents perspectives of further development of this strategy. Wide hybridization is one of the oldest breeding techniques that contributed enormously to the development of modern plant cultivars. Within ornamental breeding, it represents the main source of genetic variation. During the long history of wide hybridization, a number of methods were implemented allowing the evolution from a conventional breeding tool into a modern methodology. Nowadays, the research on model plants and crop species increases our understanding of reproductive isolation among distant species and partly explains the background of the traditional approaches previously used for overcoming hybridization barriers. Characterization of parental plants and hybrids is performed using molecular and cytological techniques that strongly facilitate breeding processes. Molecular markers and sequencing technologies are used for the assessment of genetic relationships among plants, as the genetic distance is typically depicted as one of the most important factors influencing cross-compatibility in hybridization processes. Furthermore, molecular marker systems are frequently applied for verification of hybrid state of the progeny. The flow cytometry and genomic in situ hybridization are used in the assessment of hybridization partners and characterization of hybrid progeny in relation to genome stabilization as well as genome recombination and introgression. In the future, new research and technologies are likely to provide more detailed information about genes and pathways responsible for interspecific reproductive isolation. Ultimately, this knowledge will enable development of strategies for obtaining compatible lines for hybrid production. Recent development in sequencing technologies and availability of sequence data will also facilitate creation of new molecular markers that will advance marker-assisted selection in hybridization process.

Yeast two-hybrid interactions between Arabidopsis lyrata S Receptor Kinase and the ARC1 E3 ligase

Here we describe protein-protein interactions between signaling components in the conserved self-incompatibility pathway from Brassica spp. and Arabidopsis lyrata. Previously, we had demonstrated that ARC1 is necessary in A. lyrata for the rejection of self-pollen by the self-incompatibility pathway. The results described here demonstrate that A. lyrata ARC1 interacts with A. lyrata S Receptor Kinase (SRK1) in the yeast 2-hybrid system. A. lyrata ARC1 also interacted with B. napus SRK910 illustrating that interactions in this pathway are conserved across species. Finally, we discuss how the more widely occurring interactions between SRK and ARC1-related family members may be modulated in vivo by expression and subcellular localization patterns resulting in a particular response.

Genome-Wide Identification of Soybean U-Box E3 Ubiquitin Ligases and Roles of GmPUB8 in Negative Regulation of Drought Stress Response in Arabidopsis

Plant U-box (PUB) E3 ubiquitin ligases play important roles in hormone signaling pathways and response to abiotic stresses, but little is known about them in soybean, Glycine max. Here, we identified and characterized 125 PUB genes from the soybean genome, which were classified into eight groups according to their protein domains. Soybean PUB genes (GmPUB genes) are broadly expressed in many tissues and are a little more abundant in the roots than in the other tissues. Nine GmPUB genes, GmPUB1-GmPUB9, showed induced expression patterns by drought, and the expression of GmPUB8 was also induced by exogenous ABA and NaCl. GmPUB8 was localized to post-Golgi compartments, interacting with GmE2 protein as demonstrated by yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) experiments, and showed E3 ubiquitin ligase activity by in vitro ubiquitination assay. Heterogeneous overexpression of GmPUB8 in Arabidopsis showed decreased drought tolerance, enhanced sensitivity with respect to osmotic and salt stress inhibition of seed germination and seedling growth, and inhibited ABA- and mannitol-mediated stomatal closure. Eight drought stress-related genes were less induced in GmPUB8-overexpressing Arabidopsis after drought treatment compared with the wild type and the pub23 mutant. Taken together, our results suggested that GmPUB8 might negatively regulate plant response to drought stress. In addition, Y2H and BiFC showed that GmPUB8 interacted with soybean COL (CONSTANS LIKE) protein. GmPUB8-overexpressing Arabidopsis flowered earlier under middle- and short-day conditions but later under long-day conditions, indicating that GmPUB8 might regulate flowering time in the photoperiod pathway. This study helps us to understand the functions of PUB E3 ubiquitin ligases in soybean.

Evolution of Selfing: Recurrent Patterns in Molecular Adaptation

Selfing has evolved in animals, fungi, and plants, and since Darwin's pioneering study, it is considered one of the most frequent evolutionary trends in flowering plants. Generally, the evolution of selfing is characterized by a loss of self-incompatibility, the selfing syndrome, and changes in genome-wide polymorphism patterns. Recent interdisciplinary studies involving molecular functional experiments, genome-wide data, experimental evolution, and evolutionary ecology using Arabidopsis thaliana, Caenorhabditis elegans, and other species show that the evolution of selfing is not merely a degradation of outcrossing traits but a model for studying the recurrent patterns underlying adaptive molecular evolution. For example, in wild Arabidopsis relatives, self-compatibility evolved from mutations in the male specificity gene, S-LOCUS CYSTEINE-RICH PROTEIN/S-LOCUS PROTEIN 11 (SCR/SP11), rather than the female specificity gene, S-LOCUS RECEPTOR KINASE (SRK), supporting the theoretical prediction of sexual asymmetry. Prevalence of dominant self-compatible mutations is consistent with Haldane's sieve, which acts against recessive adaptive mutations. Time estimates based on genome-wide polymorphisms and self-incompatibility genes generally support the recent origin of selfing.

RNA Silencing of Exocyst Genes in the Stigma Impairs the Acceptance of Compatible Pollen in Arabidopsis

Initial pollen-pistil interactions in the Brassicaceae are regulated by rapid communication between pollen grains and stigmatic papillae and are fundamentally important, as they are the first step toward successful fertilization. The goal of this study was to examine the requirement of exocyst subunits, which function in docking secretory vesicles to sites of polarized secretion, in the context of pollen-pistil interactions. One of the exocyst subunit genes, EXO70A1, was previously identified as an essential factor in the stigma for the acceptance of compatible pollen in Arabidopsis (Arabidopsis thaliana) and Brassica napus. We hypothesized that EXO70A1, along with other exocyst subunits, functions in the Brassicaceae dry stigma to deliver cargo-bearing secretory vesicles to the stigmatic papillar plasma membrane, under the pollen attachment site, for pollen hydration and pollen tube entry. Here, we investigated the functions of exocyst complex genes encoding the remaining seven subunits, SECRETORY3 (SEC3), SEC5, SEC6, SEC8, SEC10, SEC15, and EXO84, in Arabidopsis stigmas following compatible pollinations. Stigma-specific RNA-silencing constructs were used to suppress the expression of each exocyst subunit individually. The early postpollination stages of pollen grain adhesion, pollen hydration, pollen tube penetration, seed set, and overall fertility were analyzed in the transgenic lines to evaluate the requirement of each exocyst subunit. Our findings provide comprehensive evidence that all eight exocyst subunits are necessary in the stigma for the acceptance of compatible pollen. Thus, this work implicates a fully functional exocyst complex as a component of the compatible pollen response pathway to promote pollen acceptance.

Calcium signalling mediates self-incompatibility response in the Brassicaceae

Self-incompatibility in the Brassicaceae is controlled by multiple haplotypes encoding the pollen ligand (S-locus protein 11, SP11, also known as S-locus cysteine-rich protein, SCR) and its stigmatic receptor (S-receptor kinase, SRK). A haplotype-specific interaction between SP11/SCR and SRK triggers the self-incompatibility response that leads to self-pollen rejection, but the signalling pathway remains largely unknown. Here we show that Ca(2+) influx into stigma papilla cells mediates self-incompatibility signalling. Using self-incompatible Arabidopsis thaliana expressing SP11/SCR and SRK, we found that self-pollination specifically induced an increase in cytoplasmic Ca(2+) ([Ca(2+)]cyt) in papilla cells. Direct application of SP11/SCR to the papilla cell protoplasts induced Ca(2+) increase, which was inhibited by D-(-)-2-amino-5-phosphonopentanoic acid (AP-5), a glutamate receptor channel blocker. An artificial increase in [Ca(2+)]cyt in papilla cells arrested wild-type (WT) pollen hydration. Treatment of papilla cells with AP-5 interfered with self-incompatibility, and Ca(2+) increase on the self-incompatibility response was reduced in the glutamate receptor-like channel (GLR) gene mutants. These results suggest that Ca(2+) influx mediated by GLR is the essential self-incompatibility response leading to self-pollen rejection.

Overexpression of the Arabidopsis thaliana signalling peptide TAXIMIN1 affects lateral organ development

Lateral organ boundary formation is highly regulated by transcription factors and hormones such as auxins and brassinosteroids. However, in contrast to many other developmental processes in plants, no role for signalling peptides in the regulation of this process has been reported yet. The first characterization of the secreted cysteine-rich TAXIMIN (TAX) signalling peptides in Arabidopsis is presented here. TAX1 overexpression resulted in minor alterations in the primary shoot and root metabolome, abnormal fruit morphology, and fusion of the base of cauline leaves to stems forming a decurrent leaf attachment. The phenotypes at the paraclade junction match TAX1 promoter activity in this region and are similar to loss of LATERAL ORGAN FUSION (LOF) transcription factor function. Nevertheless, TAX1 expression was unchanged in lof1lof2 paraclade junctions and, conversely, LOF gene expression was unchanged in TAX1 overexpressing plants, suggesting TAX1 may act independently. This study identifies TAX1 as the first plant signalling peptide influencing lateral organ separation and implicates the existence of a peptide signal cascade regulating this process in Arabidopsis.

S-locus receptor kinase signalling

SRK (S-locus receptor kinase) is the receptor that allows stigma epidermal cells to discriminate between genetically related ('self') and genetically unrelated ('non-self') pollen in the self-incompatibility response of the Brassicaceae. SRK and its ligand, the pollen coat-localized SCR (S-locus cysteine-rich protein), are highly polymorphic, and their allele-specific interaction explains specificity in the self-incompatibility response. The present article reviews current knowledge of the role of SRK in the recognition and response phases of self-incompatibility, and highlights the new insights provided by analysis of a transgenic self-incompatible Arabidopsis thaliana model.