Ubiquitination and auxin signaling: a degrading story

Identification and fine mapping of Brmmd1 gene controlling recessive genic male sterility in Brassica rapa L

Recessive genic male sterility (RGMS) provides an effective approach for the commercial exploitation of heterosis, especially in Brassica crops. Although some artificial RGMS mutants have been reported in B. rapa, no causal genes derived from these natural mutants have been identified so far. In this study, a spontaneous RGMS mutant Bcajh97-01A derived from the 'Aijiaohuang' line traced back to the 1980 s was identified. Genetic analysis revealed that the RGMS trait was controlled by a single locus in the Bcajh97-01A/B system. Bulk segregant analysis (BSA) in combination with linkage analysis was employed to delimit the causal gene to an approximate 129 kb interval on chromosome A02. The integrated information of transcriptional levels and the predicted genes in the target region indicated that the Brmmd1 (BraA02g017420) encoding a PHD-containing nuclear protein was the most likely candidate gene. A 374 bp miniature inverted-repeat transposable element (MITE) was inserted into the first exon to prematurely stop the Brmmd1 gene translation, thus blocking the normal expression of this gene at the tetrad stage in the Bcajh97-01A. Additionally, a co-segregating structure variation (SV) marker was developed to rapidly screen the RGMS progenies from Bcajh97-01A/B system. Our findings reveal that BraA02g017420 is the causal gene responsible for the RGMS trait. This study lays a foundation for marker-assisted selection and further molecular mechanism exploration of pollen development in B. rapa.

Genome-Wide Identification and Expression Analysis of the PHD Finger Gene Family in Pea (Pisum sativum)

The plant homeodomain finger (PHD finger) protein, a type of zinc finger protein extensively distributed in eukaryotes, plays diverse roles in regulating plant growth and development. While PHD finger proteins have been identified in various species, their functions remain largely unexplored in pea (Pisum sativum). In this study, we identified 84 members of the PHD finger gene family in pea, which displayed an uneven distribution across seven chromosomes. Through a comprehensive analysis using data from Arabidopsis thaliana and Medicago truncatula, we categorized the PHD finger proteins into 20 subfamilies via phylogenetic tree analysis. Each subfamily exhibited distinct variations in terms of quantity, genetic structure, conserved domains, and physical and chemical properties. Collinearity analysis revealed conserved evolutionary relationships among the PHD finger genes across the three different species. Furthermore, we identified the conserved and important roles of the subfamily M members in anther development. RT-qPCR and in situ hybridization revealed high expression of the pea subfamily M members PsPHD11 and PsPHD16 in microspores and the tapetum layer. In conclusion, this analysis of the PHD finger family in pea provides valuable guidance for future research on the biological roles of PHD finger proteins in pea and other leguminous plants.

The transcription factors and pathways underpinning male reproductive development in Arabidopsis

As Arabidopsis flowers mature, specialized cells within the anthers undergo meiosis, leading to the production of haploid microspores that differentiate into mature pollen grains, each containing two sperm cells for double fertilization. During pollination, the pollen grains are dispersed from the anthers to the stigma for subsequent fertilization. Transcriptomic studies have identified a large number of genes expressed over the course of male reproductive development and subsequent functional characterization of some have revealed their involvement in floral meristem establishment, floral organ growth, sporogenesis, meiosis, microsporogenesis, and pollen maturation. These genes encode a plethora of proteins, ranging from transcriptional regulators to enzymes. This review will focus on the regulatory networks that control male reproductive development, starting from flower development and ending with anther dehiscence, with a focus on transcription factors and some of their notable target genes.

The scaffold nucleoporins SAR1 and SAR3 are essential for proper meiotic progression in Arabidopsis thaliana

Nuclear Pore Complexes (NPCs) are embedded in the nuclear envelope (NE), regulating macromolecule transport and physically interacting with chromatin. The NE undergoes dramatic breakdown and reformation during plant cell division. In addition, this structure has a specific meiotic function, anchoring and positioning telomeres to facilitate the pairing of homologous chromosomes. To elucidate a possible function of the structural components of the NPCs in meiosis, we have characterized several Arabidopsis lines with mutations in genes encoding nucleoporins belonging to the outer ring complex. Plants defective for either SUPPRESSOR OF AUXIN RESISTANCE1 (SAR1, also called NUP160) or SAR3 (NUP96) present condensation abnormalities and SPO11-dependent chromosome fragmentation in a fraction of meiocytes, which is increased in the double mutant sar1 sar3. We also observed these meiotic defects in mutants deficient in the outer ring complex protein HOS1, but not in mutants affected in other components of this complex. Furthermore, our findings may suggest defects in the structure of NPCs in sar1 and a potential link between the meiotic role of this nucleoporin and a component of the RUBylation pathway. These results provide the first insights in plants into the role of nucleoporins in meiotic chromosome behavior.

Subgenome dominance shapes novel gene evolution in the decaploid pitcher plant Nepenthes gracilis

Subgenome dominance after whole-genome duplication generates distinction in gene number and expression at the level of chromosome sets, but it remains unclear how this process may be involved in evolutionary novelty. Here we generated a chromosome-scale genome assembly of the Asian pitcher plant Nepenthes gracilis to analyse how its novel traits (dioecy and carnivorous pitcher leaves) are linked to genomic evolution. We found a decaploid karyotype and a clear indication of subgenome dominance. A male-linked and pericentromerically located region on the putative sex chromosome was identified in a recessive subgenome and was found to harbour three transcription factors involved in flower and pollen development, including a likely neofunctionalized LEAFY duplicate. Transcriptomic and syntenic analyses of carnivory-related genes suggested that the paleopolyploidization events seeded genes that subsequently formed tandem clusters in recessive subgenomes with specific expression in the digestive zone of the pitcher, where specialized cells digest prey and absorb derived nutrients. A genome-scale analysis suggested that subgenome dominance likely contributed to evolutionary innovation by permitting recessive subgenomes to diversify functions of novel tissue-specific duplicates. Our results provide insight into how polyploidy can give rise to novel traits in divergent and successful high-ploidy lineages.

Resequencing of global Lotus corniculatus accessions reveals population distribution and genetic loci, associated with cyanogenic glycosides accumulation and growth traits

BACKGROUND

Lotus corniculatus is a widely distributed perennial legume whose great adaptability to different environments and resistance to barrenness make it an excellent forage and ecological restoration plant. However, its molecular genetics and genomic relationships among populations are yet to be uncovered.

RESULT

Here we report on a genomic variation map from worldwide 272 L. corniculatus accessions by genome resequencing. Our analysis suggests that L. corniculatus accessions have high genetic diversity and could be further divided into three subgroups, with the genetic diversity centers were located in Transcaucasia. Several candidate genes and SNP site associated with CNglcs content and growth traits were identified by genome-wide associated study (GWAS). A non-synonymous in LjMTR was responsible for the decreased expression of CNglcs synthesis genes and LjZCD was verified to positively regulate CNglcs synthesis gene CYP79D3. The LjZCB and an SNP in LjZCA promoter were confirmed to be involved in plant growth.

CONCLUSION

This study provided a large number of genomic resources and described genetic relationship and population structure among different accessions. Moreover, we attempt to provide insights into the molecular studies and breeding of CNglcs and growth traits in L. corniculatus.

Genotyping by Sequencing for SNP-Based Linkage Analysis and the Development of KASPar Markers for Male Sterility and Polyembryony in Citrus

Polyembryony and male sterility (MS) are essential characters for citrus breeding. MS, coupled with parthenocarpy, allows for addressing the diversification of diploid seedless mandarin varieties, and nucleocytoplasmic MS is the most prevalent system. Polyembryony limits the use of seed parents in scion breeding programs, and the recovery of monoembryonic hybrids to be used as female parents is a crucial pre-breeding component. The objectives of this work were the identification of SNPs closely linked with the genes implied in these traits for marker-assisted selection. Genotyping by sequencing was used to genotype 61 diploid hybrids from an F1 progeny recovered from crossing 'Kiyomi' and 'Murcott' tangors. A total of 6444 segregating markers were identified and used to establish the two parental genetic maps. They consisted of 1374 and 697 markers encompassing 1416.287 and 1339.735 cM for 'Kiyomi' and 'Murcott', respectively. Phenotyping for MS and polyembryony was performed. The genotype-trait association study identified a genomic region on LG8 which was significantly associated with MS, and a genomic region on LG1 which was significantly associated with polyembryony. Annotation of the identified region for MS revealed 19 candidate genes. One SNP KASPar marker was developed and fully validated for each trait.

Genomic convergence underlying high-altitude adaptation in alpine plants

Evolutionary convergence is one of the most striking examples of adaptation driven by natural selection. However, genomic evidence for convergent adaptation to extreme environments remains scarce. Here, we assembled reference genomes of two alpine plants, Saussurea obvallata (Asteraceae) and Rheum alexandrae (Polygonaceae), with 37,938 and 61,463 annotated protein-coding genes. By integrating an additional five alpine genomes, we elucidated genomic convergence underlying high-altitude adaptation in alpine plants. Our results detected convergent contractions of disease-resistance genes in alpine genomes, which might be an energy-saving strategy for surviving in hostile environments with only a few pathogens present. We identified signatures of positive selection on a set of genes involved in reproduction and respiration (e.g., MMD1, NBS1, and HPR), and revealed signatures of molecular convergence on genes involved in self-incompatibility, cell wall modification, DNA repair and stress resistance, which may underlie adaptation to extreme cold, high ultraviolet radiation and hypoxia environments. Incorporating transcriptomic data, we further demonstrated that genes associated with cuticular wax and flavonoid biosynthetic pathways exhibit higher expression levels in leafy bracts, shedding light on the genetic mechanisms of the adaptive "greenhouse" morphology. Our integrative data provide novel insights into convergent evolution at a high-taxonomic level, aiding in a deep understanding of genetic adaptation to complex environments.

PHD-finger family genes in wheat (Triticum aestivum L.): Evolutionary conservatism, functional diversification, and active expression in abiotic stress

Plant homeodomain (PHD) transcription factors (TFs) are a class of proteins with conserved Cys4-His-Cys3 domains that play important roles in plant growth and development and in response to abiotic stresses. Although characterization of PHDs has been performed in plants, little is known about their function in wheat (Triticum aestivum L.), especially under stress conditions. In the present study, 244 TaPHDs were identified in wheat using comparative genomics. We renamed them TaPHD1-244 based on their chromosomal distribution, and almost all PHD proteins were predicted to be located in the nucleus. According to the unrooted neighbor-joining phylogenetic tree, gene structure, and motif analyses, PHD genes were divided into four clades. A total of 149 TaPHD genes were assigned to arise from duplication events. Furthermore, 230 gene pairs came from wheat itself, and 119, 186, 168, 7, 2, and 6 gene pairs came from six other species (Hordeum vulgareto, Zea mays, Oryza sativa, Arabidopsis thaliana, Brassica rapa, and Gossypium raimondii, respectively). A total of 548 interacting protein branches were identified to be involved in the protein interaction network. Tissue-specific expression pattern analysis showed that TaPHDs were highly expressed in the stigma and ovary during flowering, suggesting that the TaPHD gene plays an active role in the reproductive growth of wheat. In addition, the qRT-PCR results further confirmed that these TaPHD genes are involved in the abiotic stress response of wheat. In conclusion, our study provides a theoretical basis for deciphering the molecular functions of TaPHDs, particularly in response to abiotic stress.

MS1/MMD1 homologues in the moss Physcomitrium patens are required for male and female gametogenesis

The Arabidopsis Plant HomeoDomain (PHD) proteins AtMS1 and AtMMD1 provide chromatin-mediated transcriptional regulation essential for tapetum-dependent pollen formation. This pollen-based male gametogenesis is a derived trait of seed plants. Male gametogenesis in the common ancestors of land plants is instead likely to have been reminiscent of that in extant bryophytes where flagellated sperms are produced by an elaborate gametophyte generation. Still, also bryophytes possess MS1/MMD1-related PHD proteins. We addressed the function of two MS1/MMD1-homologues in the bryophyte model moss Physcomitrium patens by the generation and analysis of reporter and loss-of-function lines. The two genes are together essential for both male and female fertility by providing functions in the gamete-producing inner cells of antheridia and archegonia. They are furthermore expressed in the diploid sporophyte generation suggesting a function during sporogenesis, a process proposed related by descent to pollen formation in angiosperms. We propose that the moss MS1/MMD1-related regulatory network required for completion of male and female gametogenesis, and possibly for sporogenesis, represent a heritage from ancestral land plants.

Genetic Structure and Molecular Mechanisms Underlying the Formation of Tassel, Anther, and Pollen in the Male Inflorescence of Maize (Zea mays L.)

Maize tassel is the male reproductive organ which is located at the plant's apex; both its morphological structure and fertility have a profound impact on maize grain yield. More than 40 functional genes regulating the complex tassel traits have been cloned up to now. However, the detailed molecular mechanisms underlying the whole process, from male inflorescence meristem initiation to tassel morphogenesis, are seldom discussed. Here, we summarize the male inflorescence developmental genes and construct a molecular regulatory network to further reveal the molecular mechanisms underlying tassel-trait formation in maize. Meanwhile, as one of the most frequently studied quantitative traits, hundreds of quantitative trait loci (QTLs) and thousands of quantitative trait nucleotides (QTNs) related to tassel morphology have been identified so far. To reveal the genetic structure of tassel traits, we constructed a consensus physical map for tassel traits by summarizing the genetic studies conducted over the past 20 years, and identified 97 hotspot intervals (HSIs) that can be repeatedly mapped in different labs, which will be helpful for marker-assisted selection (MAS) in improving maize yield as well as for providing theoretical guidance in the subsequent identification of the functional genes modulating tassel morphology. In addition, maize is one of the most successful crops in utilizing heterosis; mining of the genic male sterility (GMS) genes is crucial in developing biotechnology-based male-sterility (BMS) systems for seed production and hybrid breeding. In maize, more than 30 GMS genes have been isolated and characterized, and at least 15 GMS genes have been promptly validated by CRISPR/Cas9 mutagenesis within the past two years. We thus summarize the maize GMS genes and further update the molecular regulatory networks underlying male fertility in maize. Taken together, the identified HSIs, genes and molecular mechanisms underlying tassel morphological structure and male fertility are useful for guiding the subsequent cloning of functional genes and for molecular design breeding in maize. Finally, the strategies concerning efficient and rapid isolation of genes controlling tassel morphological structure and male fertility and their application in maize molecular breeding are also discussed.

MALE STERILITY 3 encodes a plant homeodomain-finger protein for male fertility in soybean

Male-sterile plants are used in hybrid breeding to improve yield in soybean (Glycine max (L.) Merr.). Developing the capability to alter fertility under different environmental conditions could broaden germplasm resources and simplify hybrid production. However, molecular mechanisms potentially underlying such a system in soybean were unclear. Here, using positional cloning, we identified a gene, MALE STERILITY 3 (MS3), which encodes a nuclear-localized protein containing a plant homeodomain (PHD)-finger domain. A spontaneous mutation in ms3 causing premature termination of MS3 translation and partial loss of the PHD-finger. Transgenetic analysis indicated that MS3 knockout resulted in nonfunctional pollen and no self-pollinated pods, and RNA-seq analysis revealed that MS3 affects the expression of genes associated with carbohydrate metabolism. Strikingly, the fertility of mutant ms3 can restore under long-d conditions. The mutant could thus be used to create a new, more stable photoperiod-sensitive genic male sterility line for two-line hybrid seed production, with significant impact on hybrid breeding and production.

A cascade of bHLH-regulated pathways programs maize anther development

The spatiotemporal development of somatic tissues of the anther lobe is necessary for successful fertile pollen production. This process is mediated by many transcription factors acting through complex, multi-layered networks. Here, our analysis of functional knockout mutants of interacting basic helix-loop-helix genes Ms23, Ms32, basic helix-loop-helix 122 (bHLH122), and bHLH51 in maize (Zea mays) established that male fertility requires all four genes, expressed sequentially in the tapetum (TP). Not only do they regulate each other, but also they encode proteins that form heterodimers that act collaboratively to guide many cellular processes at specific developmental stages. MS23 is confirmed to be the master factor, as the ms23 mutant showed the earliest developmental defect, cytologically visible in the TP, with the most drastic alterations in premeiotic gene expression observed in ms23 anthers. Notably, the male-sterile ms23, ms32, and bhlh122-1 mutants lack 24-nt phased secondary small interfering RNAs (phasiRNAs) and the precursor transcripts from the corresponding 24-PHAS loci, while the bhlh51-1 mutant has wild-type levels of both precursors and small RNA products. Multiple lines of evidence suggest that 24-nt phasiRNA biogenesis primarily occurs downstream of MS23 and MS32, both of which directly activate Dcl5 and are required for most 24-PHAS transcription, with bHLH122 playing a distinct role in 24-PHAS transcription.

Identifying small RNAs and Analyzing Their Association with Gene Expression Using Isolated Arabidopsis Male Meiocytes

Meiosis is a specialized cell division that generates gametes and is essential for sexual reproduction. Studying meiosis in plants, like the model flowering plant Arabidopsis thaliana, contributes to our understanding of the fundamental biology of reproductive biology and has practical implications for improving economically important crop species. In this chapter, we provide a detailed protocol for capillary collection of Arabidopsis male meiocytes followed by total RNA extraction, RNA-Seq, and bioinformatics analysis of small-RNAs (sRNAs) including analysis of sRNA cluster that correlate with genomic features.

Genic male sterility increases rice drought tolerance

Plant fertility and resistance to stress environments are antagonistic to each other. At booting stage, fertility is often sacrificed for survive in rice under abiotic stress. However, the relationship between fertility and resistance at molecular level remains elusive. Here, we identified a transcription factor, OsAlfin like 5, which regulates the OsTMS5 and links both the drought stress response and thermosensitive genic male sterility. The OsAL5 overexpression plants (OE-OsAL5) became sensitive to temperature owning to the OsTMS5 that the OE-OsAL5 plants were fertile under low temperature (23 °C) and sterile under high temperature (28 °C). Significantly, the survival rate of OE-OsAL5 lines was higher than that of the wide-type (WT) under drought stress. Further experiments confirmed that the OsAL5 regulated both of the OsTMS5 and the down-stream drought-related genes by binding to the 'GTGGAG' element in vivo, revealing that the OsAL5 participated both in the drought stress response and thermosensitive genic male sterility in rice. These findings open up the possibility of breeding elite TGMS lines with strong drought tolerance by manipulating the expression of OsAL5.

CRISPR/Cas9-based discovery of maize transcription factors regulating male sterility and their functional conservation in plants

Identifying genic male-sterility (GMS) genes and elucidating their roles are important to unveil plant male reproduction and promote their application in crop breeding. However, compared with Arabidopsis and rice, relatively fewer maize GMS genes have been discovered and little is known about their regulatory pathways underlying anther and pollen development. Here, by sequencing and analysing anther transcriptomes at 11 developmental stages in maize B73, Zheng58 and M6007 inbred lines, 1100 transcription factor (TF) genes were identified to be stably differentially expressed among different developmental stages. Among them, 14 maize TF genes (9 types belonging to five TF families) were selected and performed CRISPR/Cas9-mediated gene mutagenesis, and then, 12 genes in eight types, including ZmbHLH51, ZmbHLH122, ZmTGA9-1/-2/-3, ZmTGA10, ZmMYB84, ZmMYB33-1/-2, ZmPHD11 and ZmLBD10/27, were identified as maize new GMS genes by using DNA sequencing, phenotypic and cytological analyses. Notably, ZmTGA9-1/-2/-3 triple-gene mutants and ZmMYB33-1/-2 double-gene mutants displayed complete male sterility, but their double- or single-gene mutants showed male fertility. Similarly, ZmLBD10/27 double-gene mutant displayed partial male sterility with 32.18% of aborted pollen grains. In addition, ZmbHLH51 was transcriptionally activated by ZmbHLH122 and their proteins were physically interacted. Molecular markers co-segregating with these GMS mutations were developed to facilitate their application in maize breeding. Finally, all 14-type maize GMS TF genes identified here and reported previously were compared on functional conservation and diversification among maize, rice and Arabidopsis. These findings enrich GMS gene and mutant resources for deeply understanding the regulatory network underlying male fertility and for creating male-sterility lines in maize.

MS1 is essential for male fertility by regulating the microsporocyte cell plate expansion in soybean

Male sterility is an essential trait in hybrid seed production, especially for monoclinous and autogamous food crops. Soybean male-sterile ms1 mutant has been known for more than 50 years and could be instrumental in making hybrid seeds. However, the gene responsible for the male-sterile phenotype has remained unknown. Here, we report the map-based cloning and characterization of the MS1 gene in soybean. MS1 encodes a kinesin protein and localizes to the nucleus, where it is required for the male meiotic cytokinesis after telophase II. We further substantiated that MS1 colocalizes with microtubules and is essential for cell plate formation in soybean male gametogenesis through immunostaining. Both ms1 and CRISPR/Cas9 knockout mutants show complete male sterility but are otherwise phenotypically normal, making them perfect tools for producing hybrid seeds. The identification of MS1 has the practical potential for assembling the sterility system and speeding up hybrid soybean breeding.

Comparison of Metabolic Profiling of Arabidopsis Inflorescences Between Landsberg erecta and Columbia, and Meiosis-Defective Mutants by 1H-NMR Spectroscopy

With the rapid development of omics technologies during the last several decades, genomics, transcriptomics, and proteomics have been extensively used to characterize gene or protein functions in many organisms at the cell or tissue level. However, metabolomics has not been conducted in reproductive organs, with a focus on meiosis in plants. In this study, we adopted a nuclear magnetic resonance (NMR)-based metabolomics approach to reveal the metabolic profile of inflorescences from two Arabidopsis accessions, Columbia (Col) and Landsberg erecta (Ler), and several sterile mutants caused by meiosis defects. We identified 68 dominant metabolites in the samples. Col and Ler displayed distinct metabolite profiles. Interestingly, mutants with similar meiotic defects, such as Atrad51-3, Atrfc1-2, and Atpol2a-2, exhibited similar alterations in metabolites, including upregulation of energy metabolites and promotion of compounds related to maintenance of genomic stability, cytoplasmic homeostasis, and membrane integrity. The collective data reveal distinct changes in metabolites in Arabidopsis inflorescences between the Col and Ler wild type accessions. NMR-based metabolomics could be an effective tool for molecular phenotyping in studies of aspects of plant reproductive development such as meiosis.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s43657-021-00012-3.

A Hypomorphic Mutant of PHD Domain Protein Male Meiocytes Death 1

Meiosis drives reciprocal genetic exchanges and produces gametes with halved chromosome number, which is important for the genetic diversity, plant viability, and ploidy consistency of flowering plants. Alterations in chromosome dynamics and/or cytokinesis during meiosis may lead to meiotic restitution and the formation of unreduced microspores. In this study, we isolated an Arabidopsis mutant male meiotic restitution 1 (mmr1), which produces a small subpopulation of diploid or polyploid pollen grains. Cytological analysis revealed that mmr1 produces dyads, triads, and monads indicative of male meiotic restitution. Both homologous chromosomes and sister chromatids in mmr1 are separated normally, but chromosome condensation at metaphase I is slightly affected. The mmr1 mutant displayed incomplete meiotic cytokinesis. Supportively, immunostaining of the microtubular cytoskeleton showed that the spindle organization at anaphase II and mini-phragmoplast formation at telophase II are aberrant. The causative mutation in mmr1 was mapped to chromosome 1 at the chromatin regulator Male Meiocyte Death 1 (MMD1/DUET) locus. mmr1 contains a C-to-T transition at the third exon of MMD1/DUET at the genomic position 2168 bp from the start codon, which causes an amino acid change G618D that locates in the conserved PHD-finger domain of histone binding proteins. The F1 progenies of mmr1 crossing with knockout mmd1/duet mutant exhibited same meiotic defects and similar meiotic restitution rate as mmr1. Taken together, we here report a hypomorphic mmd1/duet allele that typically shows defects in microtubule organization and cytokinesis.

Response of phytohormone mediated plant homeodomain (PHD) family to abiotic stress in upland cotton (Gossypium hirsutum spp.)

BACKGROUND

The sequencing and annotations of cotton genomes provide powerful theoretical support to unravel more physiological and functional information. Plant homeodomain (PHD) protein family has been reported to be involved in regulating various biological processes in plants. However, their functional studies have not yet been carried out in cotton.

RESULTS

In this study, 108, 55, and 52 PHD genes were identified in G. hirsutum, G. raimondii, and G. arboreum, respectively. A total of 297 PHD genes from three cotton species, Arabidopsis, and rice were divided into five groups. We performed chromosomal location, phylogenetic relationship, gene structure, and conserved domain analysis for GhPHD genes. GhPHD genes were unevenly distributed on each chromosome. However, more GhPHD genes were distributed on At_05, Dt_05, and At_07 chromosomes. GhPHD proteins depicted conserved domains, and GhPHD genes exhibiting similar gene structure were clustered together. Further, whole genome duplication (WGD) analysis indicated that purification selection greatly contributed to the functional maintenance of GhPHD gene family. Expression pattern analysis based on RNA-seq data showed that most GhPHD genes showed clear tissue-specific spatiotemporal expression patterns elucidating the multiple functions of GhPHDs in plant growth and development. Moreover, analysis of cis-acting elements revealed that GhPHDs may respond to a variety of abiotic and phytohormonal stresses. In this regard, some GhPHD genes showed good response against abiotic and phytohormonal stresses. Additionally, co-expression network analysis indicated that GhPHDs are essential for plant growth and development, while GhPHD genes response against abiotic and phytohormonal stresses may help to improve plant tolerance in adverse environmental conditions.

CONCLUSION

This study will provide useful information to facilitate further research related to the vital roles of GhPHD gene family in plant growth and development.

Differential gene expression analysis and cytological evidence reveal a sexual stage of an amoeba with multiparental cellular and nuclear fusion

Sex is a hallmark of eukaryotes but its evolution in microbial eukaryotes is poorly elucidated. Recent genomic studies revealed microbial eukaryotes possess a genetic toolkit necessary for sexual reproduction. However, the mechanism of sexual development in a majority of microbial eukaryotes including amoebozoans is poorly characterized. The major hurdle in studying sex in microbial eukaryotes is a lack of observational evidence, primarily due to its cryptic nature. In this study, we used a tractable fusing amoeba, Cochliopodium, to investigate sexual development using stage-specific Differential Gene Expression (DGE) and cytological analyses. Both DGE and cytological results showed that most of the meiosis and sex-related genes are upregulated in Cochliopodium undergoing fusion in laboratory culture. Relative gene ontology (GO) category representations in unfused and fused cells revealed a functional skew of the fused transcriptome toward DNA metabolism, nucleus and ligases that are suggestive of a commitment to sexual development. However, the GO categories of unfused cells were dominated by metabolic pathways and other processes indicative of a vegetative phase. Our study provides strong evidence that the fused cells represent a sexual stage in Cochliopodium. Our findings have further implications in understanding the evolution and mechanism of inheritance involving multiparents in other eukaryotes with a similar reproductive strategy.

A study of male fertility control in Medicago truncatula uncovers an evolutionarily conserved recruitment of two tapetal bHLH subfamilies in plant sexual reproduction

Male sterility is an important tool for plant breeding and hybrid seed production. Male-sterile mutants are largely due to an abnormal development of either the sporophytic or gametophytic anther tissues. Tapetum, a key sporophytic tissue, provides nutrients for pollen development, and its delayed degeneration induces pollen abortion. Numerous bHLH proteins have been documented to participate in the degeneration of the tapetum in angiosperms, but relatively little attention has been given to the evolution of the involved developmental pathways across the phylogeny of land plants. A combination of cellular, molecular, biochemical and evolutionary analyses was used to investigate the male fertility control in Medicago truncatula. We characterized the male-sterile mutant empty anther1 (ean1) and identified EAN1 as a tapetum-specific bHLH transcription factor necessary for tapetum degeneration. Our study uncovered an evolutionarily conserved recruitment of bHLH subfamily II and III(a + c)1 in the regulation of tapetum degeneration. EAN1 belongs to the subfamily II and specifically forms heterodimers with the subfamily III(a + c)1 members, which suggests a heterodimerization mechanism conserved in angiosperms. Our work suggested that the pathway of two tapetal-bHLH subfamilies is conserved in all land plants, and likely was established before the divergence of the spore-producing land plants.

Cell-type-dependent histone demethylase specificity promotes meiotic chromosome condensation in Arabidopsis

Histone demethylation is crucial for proper chromatin structure and to ensure normal development, and requires the large family of Jumonji C (JmjC)-containing demethylases; however, the molecular mechanisms that regulate the substrate specificity of these JmjC-containing demethylases remain largely unknown. Here, we show that the substrate specificity of the Arabidopsis histone demethylase JMJ16 is broadened from Lys 4 of histone H3 (H3K4) alone in somatic cells to both H3K4 and H3K9 when it binds to the meiocyte-specific histone reader MMD1. Consistent with this, the JMJ16 catalytic domain exhibits both H3K4 and H3K9 demethylation activities. Moreover, the JMJ16 C-terminal FYR domain interacts with the JMJ16 catalytic domain and probably restricts its substrate specificity. By contrast, MMD1 can compete with the N-terminal catalytic domain of JMJ16 for binding to the FYR-C domain, thereby expanding the substrate specificity of JMJ16 by preventing the FYR domain from binding to the catalytic domain. We propose that MMD1 and JMJ16 together in male meiocytes promote gene expression in an H3K9me3-dependent manner and thereby contribute to meiotic chromosome condensation.

A Tale of Two Families: Whole Genome and Segmental Duplications Underlie Glutamine Synthetase and Phosphoenolpyruvate Carboxylase Diversity in Narrow-Leafed Lupin (Lupinus angustifolius L.)

Narrow-leafed lupin (Lupinus angustifolius L.) has recently been supplied with advanced genomic resources and, as such, has become a well-known model for molecular evolutionary studies within the legume family—a group of plants able to fix nitrogen from the atmosphere. The phylogenetic position of lupins in Papilionoideae and their evolutionary distance to other higher plants facilitates the use of this model species to improve our knowledge on genes involved in nitrogen assimilation and primary metabolism, providing novel contributions to our understanding of the evolutionary history of legumes. In this study, we present a complex characterization of two narrow-leafed lupin gene families—glutamine synthetase (GS) and phosphoenolpyruvate carboxylase (PEPC). We combine a comparative analysis of gene structures and a synteny-based approach with phylogenetic reconstruction and reconciliation of the gene family and species history in order to examine events underlying the extant diversity of both families. Employing the available evidence, we show the impact of duplications on the initial complement of the analyzed gene families within the genistoid clade and posit that the function of duplicates has been largely retained. In terms of a broader perspective, our results concerning GS and PEPC gene families corroborate earlier findings pointing to key whole genome duplication/triplication event(s) affecting the genistoid lineage.

Comparative transcript profiling and cytological observation of the newly bred recessive genic male sterility non-heading Chinese cabbage (Brassica rapa ssp. chinensis) line WS24-3A

BACKGROUND

WS24-3A is a newly bred non-heading Chinese cabbage genic male-sterile line, in which sterility is controlled by a recessive gene, designated as Bra2ms. WS24-3A has been used for hybrid breeding.

OBJECTIVE

To reveal the underlying molecular mechanisms responsible for the sterility of WS24-3A.

METHODS

Cytological observation of the process of sterile/fertile anther development was performed to determine the tissue and stage in which sterility occurs. Phenotyping and transcriptomic analyses were performed to identify differentially expressed genes (DEGs) between sterile and fertile flower buds at different stages.

RESULTS

Cytological analysis revealed no tetrads at stage 7 or at later stages of anther development, and the degradation of callose was delayed. Abnormal meiocytes were surrounded by sustaining callose that degenerated gradually in WS24-3A. Comparative transcript profiling identified 3282 DEGs during three anther developmental stages, namely, pre-meiotic anther, meiotic anther, and anthers with single-celled pollen stage. The difference in DEG percentage between up-regulated and down-regulated at meiotic anther stage was obviously larger than at the other two stages; further, most DEGs are important for male meiosis, callose synthesis and dissolution, and tapetum development. Ten DEGs were found to be involved in anther and pollen development, which were analyzed by quantitative PCR.

CONCLUSION

Bra2ms affected gene expression in meiocytes and associated with callose synthesis, degradation and tapetum development. Our results provide clues to elucidate the molecular mechanism of genic male sterility in non-heading Chinese cabbage.

Comparative development of staminate and pistillate flowers in the dioecious cactus Opuntia robusta

PCD role in unisexual flowers. The developmental processes underlying the transition from hermaphroditism to unisexuality are key to understanding variation and evolution of floral structure and function. A detailed examination of the cytological and histological patterns involved in pollen and ovule development of staminate and pistillate flowers in the dioecious Opuntia robusta was undertaken, and the potential involvement of programmed cell death in the abortion of the sex whorls was explored. Flowers initiated development as hermaphrodites and became functionally unisexual by anthesis. Female individuals have pistillate flowers with a conspicuous stigma, functional ovary, collapsed stamens and no pollen grains. Male individuals have staminate flowers, with large yellow anthers, abundant pollen grains, underdeveloped stigma, style and an ovary that rarely produced ovules. In pistillate flowers, anther abortion resulted from the premature degradation of the tapetum by PCD, followed by irregular deposition of callose wall around the microsporocytes, and finally by microspore degradation. In staminate flowers, the stigma could support pollen germination; however, the ovaries were reduced, with evidence of placental arrest and ovule abortion through PCD, when ovules were present. We demonstrate that PCD is recruited in both pistillate and staminate flower development; however, it occurs at different times of floral development. This study contributes to the understanding of the nature of the O. robusta breeding system and identifies developmental landmarks that contribute to sexual determination in Cactaceae.

TDR INTERACTING PROTEIN 3, encoding a PHD-finger transcription factor, regulates Ubisch bodies and pollen wall formation in rice

Male reproductive development involves a complex series of biological events and precise transcriptional regulation is essential for this biological process in flowering plants. Several transcriptional factors have been reported to regulate tapetum and pollen development, however the transcriptional mechanism underlying Ubisch bodies and pollen wall formation remains less understood. Here, we characterized and isolated a male sterility mutant of TDR INTERACTING PROTEIN 3 (TIP3) in rice. The tip3 mutant displayed smaller and pale yellow anthers without mature pollen grains, abnormal Ubisch body morphology, no pollen wall formation, as well as delayed tapetum degeneration. Map-based cloning demonstrated that TIP3 encodes a conserved PHD-finger protein and further study confirmed that TIP3 functioned as a transcription factor with transcriptional activation activity. TIP3 is preferentially expressed in the tapetum and microspores during anther development. Moreover, TIP3 can physically interact with TDR, which is a key component of the transcriptional cascade in regulating tapetum development and pollen wall formation. Furthermore, disruption of TIP3 changed the expression of several genes involved in tapetum development and degradation, biosynthesis and transport of lipid monomers of sporopollenin in tip3 mutant. Taken together, our results revealed an unprecedented role for TIP3 in regulating Ubisch bodies and pollen exine formation, and presents a potential tool to manipulate male fertility for hybrid rice breeding.

Mutation in a PHD-finger protein MS4 causes male sterility in soybean

BACKGROUND

Male sterility has tremendous scientific and economic importance in hybrid seed production. Identification and characterization of a stable male sterility gene will be highly beneficial for making hybrid seed production economically feasible. In soybean, eleven male-sterile, female-fertile mutant lines (ms1, ms2, ms3, ms4, ms5, ms6, ms7, ms8, ms9, msMOS, and msp) have been identified and mapped onto various soybean chromosomes, however the causal genes responsible for male sterility are not isolated. The objective of this study was to identify and functionally characterize the gene responsible for the male sterility in the ms4 mutant.

RESULTS

The ms4 locus was fine mapped to a 216 kb region, which contains 23 protein-coding genes including Glyma.02G243200, an ortholog of Arabidopsis MALE MEIOCYTE DEATH 1 (MMD1), which is a Plant Homeodomain (PHD) protein involved in male fertility. Isolation and sequencing of Glyma.02G243200 from the ms4 mutant line showed a single base insertion in the 3rd exon causing a premature stop codon resulting in truncated protein production. Phylogenetic analysis showed presence of a homolog protein (MS4_homolog) encoded by the Glyma.14G212300 gene. Both proteins were clustered within legume-specific clade of the phylogenetic tree and were likely the result of segmental duplication during the paleoploidization events in soybean. The comparative expression analysis of Ms4 and Ms4_homologs across the soybean developmental and reproductive stages showed significantly higher expression of Ms4 in early flowering (flower bud differentiation) stage than its homolog. The functional complementation of Arabidopsis mmd1 mutant with the soybean Ms4 gene produced normal stamens, successful tetrad formation, fertile pollens and viable seeds, whereas the Ms4_homolog was not able to restore male fertility.

CONCLUSIONS

Overall, this is the first report, where map based cloning approach was employed to isolate and characterize a gene responsible for the male-sterile phenotype in soybean. Characterization of male sterility genes may facilitate the establishment of a stable male sterility system, highly desired for the viability of hybrid seed production in soybean. Additionally, translational genomics and genome editing technologies can be utilized to generate new male-sterile lines in other plant species.

Maize Genic Male-Sterility Genes and Their Applications in Hybrid Breeding: Progress and Perspectives

As one of the most important crops, maize not only has been a source of the food, feed, and industrial feedstock for biofuel and bioproducts, but also became a model plant system for addressing fundamental questions in genetics. Male sterility is a very useful trait for hybrid vigor utilization and hybrid seed production. The identification and characterization of genic male-sterility (GMS) genes in maize and other plants have deepened our understanding of the molecular mechanisms controlling anther and pollen development, and enabled the development and efficient use of many biotechnology-based male-sterility (BMS) systems for crop hybrid breeding. In this review, we summarize main advances on the identification and characterization of GMS genes in maize, and construct a putative regulatory network controlling maize anther and pollen development by comparative genomic analysis of GMS genes in maize, Arabidopsis, and rice. Furthermore, we discuss and appraise the features of more than a dozen BMS systems for propagating male-sterile lines and producing hybrid seeds in maize and other plants. Finally, we provide our perspectives on the studies of GMS genes and the development of novel BMS systems in maize and other plants. The continuous exploration of GMS genes and BMS systems will enhance our understanding of molecular regulatory networks controlling male fertility and greatly facilitate hybrid vigor utilization in breeding and field production of maize and other crops.

A rice PHD-finger protein OsTITANIA, is a growth regulator that functions through elevating expression of transporter genes for multiple metals

Essential metal absorption for plant growth is mediated predominantly by metal-specific transporters, with expression that responds to the environmental or cellular conditions of specific metals. Differing from metal-specific regulation, we describe a constitutively expressed transcription factor that regulates the transport of several metals in rice. We characterized the rice mutant LOW CADMIUM 5 (LC5), which exhibited reduced growth and accumulation of essential metals (e.g., copper [Cu], zinc [Zn] and manganese [Mn]) in shoots. LC5 was dwarf and developed less tillers than the wild type, but the structure of vasculature was apparently normal. Molecular genetic analysis revealed that the causal gene of LC5 is an ortholog of the transcriptional regulator Arabidopsis thaliana TITANIA (TTA), known as a transcriptional regulator. Expression analyses demonstrated that the OsTTA gene encodes a nucleus-localized protein containing a plant homeodomain-finger (PHD-finger) domain and is expressed ubiquitously in rice plants. RNA sequencing and quantitative PCR analyses revealed that the mRNA accumulation of transporter genes for essential metals, including iron (Fe), Zn, or Mn, were substantially lower in LC5 roots than in the wild type. Unlike known transcription factors of metal transport regulation, OsTTA transcript accumulation was not affected by metal availability. In addition, the growth defect of LC5 was partially rescued by Fe, Zn, or Mn supplementation, respectively. Taken together, OsTTA is a constitutively expressed regulator of multiple metal transporter genes responsible for essential metals delivery to shoots for their normal growth.

Insights Into the Role of Ubiquitination in Meiosis: Fertility, Adaptation and Plant Breeding

Ubiquitination is a post-translational modification process that plays a central role in protein degradation in eukaryotic cell cell division, including meiosis. This modification affects different cellular processes on a global scale by its pleiotropic ability to modify numerous proteins. Meiosis is essential for sexual reproduction and involves two rounds of nuclear division following a single round of DNA replication to produce haploid gametes. Unlike mitosis, meiosis has a unique prophase I, which involves homologous chromosome interaction including pairing, synapsis, recombination and segregation. Over the last several decades, molecular genetic studies have identified many proteins that participate in meiotic progression. In this review, we focus on the recent advances regarding the role of ubiquitination during plant meiotic cell cycle progression and recombination, especially the role played by the Anaphase-Promoting Complex and E3 ligases in modulating crossover formation and its impact on evolution and plant breeding.

MePHD1 as a PHD-Finger Protein Negatively Regulates ADP-Glucose Pyrophosphorylase Small Subunit1a Gene in Cassava

ADP-glucose pyrophosphorylase (AGPase) is an important enzyme in the starch synthesis pathway. Its enzyme activity can determine the efficiency of starch biosynthesis. Cassava (Manihot esculenta Crantz) is the main staple crop worldwide and has a high starch content in its storage root. However, the inner regulatory mechanism of AGPase gene family is unclear. MePHD1; a plant homeodomain transcription factor; was isolated through a yeast one-hybrid screening using the promoter of ADP-glucose pyrophosphorylase small subunit1a (MeAGPS1a) as bait, and cassava storage root cDNA library as prey. This factor could bind to the MeAGPS1a promoter in vitro and in vivo, and its predicted binding region ranged from −400 bp to −201 bp, at the translation initiation site. The transcript level of MePHD1 could be induced by five plant hormones, and a temperature of 42 °C. This was down-regulated during the maturation process of the storage root. MePHD1 protein could repress the promoter activity of MeAGPS1a gene by a dual-luciferase assay; which indicated that MePHD1 is a negative regulator for the transcript level of MeAGPS1a gene.

Meiotic Recombination: Mixing It Up in Plants

Meiosis halves diploid chromosome numbers to haploid levels that are essential for sexual reproduction in most eukaryotes. Meiotic recombination ensures the formation of bivalents between homologous chromosomes (homologs) and their subsequent proper segregation. It also results in genetic diversity among progeny that influences evolutionary responses to selection. Moreover, crop breeding depends upon the action of meiotic recombination to rearrange elite traits between parental chromosomes. An understanding of the molecular mechanisms that drive meiotic recombination is important for both fundamental research and practical applications. This review emphasizes advances made during the past 5 years, primarily in Arabidopsis and rice, by summarizing newly characterized genes and proteins and examining the regulatory mechanisms that modulate their action.

Systematic analysis and comparison of the PHD-Finger gene family in Chinese pear (Pyrus bretschneideri) and its role in fruit development

PHD-finger proteins, which belongs to the type of zinc finger family, and that play an important role in the regulation of both transcription and the chromatin state in eukaryotes. Currently, PHD-finger proteins have been well studied in animals, while few studies have been carried out on their function in plants. In the present study, 129 non-redundant PHD-finger genes were identified from 5 Rosaceae species (pear, apple, strawberry, mei, and peach); among them, 31 genes were identified in pear. Subsequently, we carried out a bioinformatics analysis of the PHD-finger genes. Thirty-one PbPHD genes were divided into 7 subfamilies based on the phylogenetic analysis, which are consistent with the intron-exon and conserved motif analyses. In addition, we identified five segmental duplication events, implying that the segmental duplications might be a crucial role in the expansion of the PHD-finger gene family in pear. The microsynteny analysis of five Rosaceae species showed that there were independent duplication events in addition to the genome-wide duplication of the pear genome. Subsequently, ten expressed PHD-finger genes of pear fruit were identified using qRT-PCR, and one of these genes, PbPHD10, was identified as an important candidate gene for the regulation of lignin synthesis. Our research provides useful information for the further analysis of the function of PHD-finger gene family in pear.

Identification of cyp703a3-3 and analysis of regulatory role of CYP703A3 in rice anther cuticle and pollen exine development

Anther cuticle and pollen exine are two elaborated lipid-soluble barriers protecting pollen grains from environmental and biological stresses. However, less is known about the mechanisms underlying the synthesis of these lipidic polymers. Here, we identified a no-pollen male-sterility mutant cyp703a3-3 from the indica restorer line Zhonghui 8015 (Zh8015) mutant library treated with ⁶⁰Coγ-ray radiation. Histological analysis indicated that cyp703a3-3 underwent abnormal tapetal cells development, produced few orbicules and secreted less sporopollenin precursors to anther locule, as well as cutin monomers on anther. Genetic analysis revealed that cyp703a3-3 was controlled by a single recessive gene. Map-based cloning was performed to narrow down the mutant gene to a 47.78-kb interval on the chromosome 8 between two markers S15-29 and S15-30. Sequence analysis detected three bases (GAA) deletion in the first exon of LOC_Os08g03682, annotated as CYP703A3 with homologous sequences related to male sterility in Arabidopsis, causing the Asparagine deletion in the mutant site. Moreover, we transformed genomic fragment of CYP703A3 into cyp703a3-3, which male-sterility phenotype was recovered. Both the wild-type and cyp703a3-3 mutant 3D structure of CYP703A3 protein were modeled. Results of qPCR suggested CYP703A3 mainly expressed in anthers with greatest abundance at microspore stage, and genes involved in sporopollenin precursors formation and transportation, such as GAMYB, TDR, CYP704B2, DPW2, OsABCG26 and OsABCG15, were significantly reduced in cyp703a3-3. Collectively, our results further elaborated CYP703A3 plays vital role in anther cuticle and pollen exine development in rice (Oryza sativa L.).

Fine mapping of a male sterility gene ms-3 in a novel cucumber (Cucumis sativus L.) mutant

The cucumber male sterility gene ms - 3 was fine mapped in a 76 kb region harboring an MMD1 -like gene Csa3M006660 that may be responsible for the male sterile in cucumber. A cucumber (Cucumis sativus L.) male sterile mutant (ms-3) in an advanced-generation inbred line was identified, and genetic analysis revealed that the male sterility trait was controlled by a recessive nuclear gene, ms-3, which was stably inherited. Histological studies suggested that the main cause of the male sterility was defective microsporogenesis, resulting in no tetrad or microspores being formed. Bulked segregant analysis (BSA) and genotyping of an F₂ population of 2553 individuals were employed used to fine map ms-3, which was delimited to a 76 Kb region. In this region, a single non-synonymous SNP was found in the Csa3M006660 gene locus, which was predicted to result in an amino acid change. Quantitative RT-PCR analysis of Csa3M006660 was consistent with the fact that it plays a role in the early development of cucumber pollen. The protein encoded by Csa3M006660 is predicted to be homeodomain (PHD) finger protein, and the high degree of sequence conservation with homologs from a range of plant species further suggested the importance of the ms-3 non-synonymous mutation. The data presented here provide support for Csa3M006660 as the most likely candidate gene for Ms-3.

Genome-wide identification of PHD-finger genes and expression pattern analysis under various treatments in moso bamboo (Phyllostachys edulis)

Plant homeodomain (PHD)-finger proteins are a class of important zinc-finger transcription factors responsible for regulating transcription and the chromatin state and responsive to various stresses. The family genes have been reported in many plants, but there is little information about PHD-finger genes in moso bamboo. In this study, 60 PHD-finger genes (PePHD1-60) were identified in moso bamboo and classified into 11 subfamilies (A-K) based on phylogenetic analysis. Gene structure and conserved motif analysis showed that these genes contained different numbers of introns but had similar motif organizations within each subfamily. Multiple sequence alignment revealed that the PHD-finger proteins possessed conserved structural domain sequences. In addition, the family underwent purifying selection during evolution and experienced a large-scale duplication event around 7.69-15.4 million years ago. Most importantly, the expression profiles of young leaves (YL), mature leaves (L), roots (R), stems (S), shoots (Sh) and rhizomes (Rh) displayed that they might involve in the formation of these tissues. Based on promoter analysis of 16 putative stress-related genes, quantitative real-time PCR assays were performed using moso bamboo leaves and showed that these genes were differentially regulated under abscisic acid (ABA), drought, low temperature and NaCl treatments. Therefore, the results reveal that PePHD genes play crucial roles in organ formation and response to multiple environmental stress conditions of moso bamboo, which will make for further function analysis of PHD-finger genes in plants.

Be my baby: patterning toward plant germ cells

In flowering plants, germ cells are formed via tightly coordinated patterning processes that facilitate specification of spore mother cells and meiosis during sporogenesis, as well as functional differentiation of germ cells in gametogenesis. Studies using the conventional Arabidopsis system and the newly emerged bryophyte system have revealed novel interactions between regulatory factors that restrict the number of spore mother cells, and evolutionarily conserved factors that promote germ cell differentiation. This short review summarizes recent advances in our understanding of the cellular events that lead to the formation of germ cells in plants, and highlights questions that remain to be addressed in the field.

De novo transcriptome assembly from flower buds of dioecious, gynomonoecious and chemically masculinized female Coccinia grandis reveals genes associated with sex expression and modification

BACKGROUND

Coccinia grandis (ivy gourd), is a dioecious member of Cucurbitaceae having heteromorphic sex chromosomes. Chromosome constitution of male and female plants of C. grandis is 22A + XY and 22A + XX respectively. Earlier we showed that a unique gynomonoecious form of C. grandis (22A + XX) also exists in nature bearing morphologically hermaphrodite flowers (GyM-H). Additionally, application of silver nitrate (AgNO₃) on female plants induces stamen development leading to the formation of morphologically hermaphrodite flowers (Ag-H) despite the absence of Y-chromosome. Due to the unavailability of genome sequence and the slow pace at which sex-linked genes are identified, sex expression and modification in C. grandis are not well understood.

RESULTS

We have carried out a comprehensive RNA-Seq study from early-staged male, female, GyM-H, and Ag-H as well as middle-staged male and GyM-H flower buds. A de novo transcriptome was assembled using Trinity and annotated by BLAST2GO and Trinotate pipelines. The assembled transcriptome consisted of 467,233 'Trinity Transcripts' clustering into 378,860 'Trinity Genes'. Female_Early_vs_Male_Early, Ag_Early_vs_Female_Early, and GyM-H_Middle_vs_Male_Middle comparisons exhibited 35,694, 3574, and 14,954 differentially expressed transcripts respectively. Further, qRT-PCR analysis of selected candidate genes validated digital gene expression profiling results. Interestingly, ethylene response-related genes were found to be upregulated in female buds compared to male buds. Also, we observed that AgNO₃ treatment suppressed ethylene responses in Ag-H flowers by downregulation of ethylene-responsive transcription factors leading to stamen development. Further, GO terms related to stamen development were enriched in early-staged male, GyM-H, and Ag-H buds compared to female buds supporting the fact that stamen growth gets arrested in female flowers.

CONCLUSIONS

Suppression of ethylene responses in both male and Ag-H compared to female buds suggests a probable role of ethylene in stamen suppression similar to monoecious cucurbits such as melon and cucumber. Also, pollen fertility associated GO terms were depleted in middle-staged GyM-H buds compared to male buds indicating the necessity of Y-chromosome for pollen fertility. Overall, this study would enable identification of new sex-biased genes for further investigation of stamen arrest, pollen fertility, and AgNO₃-mediated sex modification.

Genome-wide analysis of the auxin/indoleacetic acid (Aux/IAA) gene family in allotetraploid rapeseed (Brassica napus L.)

BACKGROUND

Auxin/Indoleacetic acid (Aux/IAA) genes participate in the auxin signaling pathway and play key roles in plant growth and development. Although the Aux/IAA gene family has been identified in many plants, within allotetraploid Brassica napus little is known.

RESULTS

In this study, a total of 119 Aux/IAA genes were found in the genome of B. napus. They were distributed non-randomly across all 19 chromosomes and other non-anchored random scaffolds, with a symmetric distribution in the A and C subgenomes. Evolutionary and comparative analysis revealed that 111 (94.1%) B. napus Aux/IAA genes were multiplied due to ancestral Brassica genome triplication and recent allotetraploidy from B. rapa and B. oleracea. Phylogenetic analysis indicated seven subgroups containing 29 orthologous gene sets and two Brassica-specific gene sets. Structures of genes and proteins varied across different genes but were conserved among homologous genes in B. napus. Furthermore, analysis of transcriptional profiles revealed that the expression patterns of Aux/IAA genes in B. napus were tissue dependent. Auxin-responsive elements tend to be distributed in the proximal region of promoters, and are significantly associated with early exogenous auxin up-regulation.

CONCLUSIONS

Members of the Aux/IAA gene family were identified and analyzed comprehensively in the allotetraploid B. napus genome. This analysis provides a deeper understanding of diversification of the Aux/IAA gene family and will facilitate further dissection of Aux/IAA gene function in B. napus.

Doubled Haploid 'CUDH2107' as a Reference for Bulb Onion (Allium cepa L.) Research: Development of a Transcriptome Catalogue and Identification of Transcripts Associated with Male Fertility

Researchers working on model plants have derived great benefit from developing genomic and genetic resources using ‘reference’ genotypes. Onion has a large and highly heterozygous genome making the sharing of germplasm and analysis of sequencing data complicated. To simplify the discovery and analysis of genes underlying important onion traits, we are promoting the use of the homozygous double haploid line ‘CUDH2107’ by the onion research community. In the present investigation, we performed transcriptome sequencing on vegetative and reproductive tissues of CUDH2107 to develop a multi-organ reference transcriptome catalogue. A total of 396 million 100 base pair paired reads was assembled using the Trinity pipeline, resulting in 271,665 transcript contigs. This dataset was analysed for gene ontology and transcripts were classified on the basis of putative biological processes, molecular function and cellular localization. Significant differences were observed in transcript expression profiles between different tissues. To demonstrate the utility of our CUDH2107 transcriptome catalogue for understanding the genetic and molecular basis of various traits, we identified orthologues of rice genes involved in male fertility and flower development. These genes provide an excellent starting point for studying the molecular regulation, and the engineering of reproductive traits.

Long non-coding RNAs are major contributors to transcriptome changes in sunflower meiocytes with different recombination rates

BACKGROUND

Meiosis is a form of specialized cell division that marks the transition from diploid meiocyte to haploid gamete, and provides an opportunity for genetic reassortment through recombination. Experimental data indicates that, relative to their wild ancestors, cultivated sunflower varieties show a higher recombination rate during meiosis. To better understand the molecular basis for this difference, we compared gene expression in male sunflower meiocytes in prophase I isolated from a domesticated line, a wild relative, and a F1 hybrid of the two.

RESULTS

Of the genes that showed differential expression between the wild and domesticated genotypes, 63.62 % could not be identified as protein-coding genes, and of these genes, 70.98 % passed stringent filters to be classified as long non-coding RNAs (lncRNAs). Compared to the sunflower somatic transcriptome, meiocytes express a higher proportion of lncRNAs, and the majority of genes with exclusive expression in meiocytes were lncRNAs. Around 40 % of the lncRNAs showed sequence similarity with small RNAs (sRNA), while 1.53 % were predicted to be sunflower natural antisense transcripts (NATs), and 9.18 % contained transposable elements (TE). We identified 6895 lncRNAs that are exclusively expressed in meiocytes, these lncRNAs appear to have higher conservation, a greater degree of differential expression, a higher proportion of sRNA similarity, and higher TE content relative to lncRNAs that are also expressed in the somatic transcriptome.

CONCLUSIONS

lncRNAs play important roles in plant meiosis and may participate in chromatin modification processes, although other regulatory functions cannot be excluded. lncRNAs could also be related to the different recombination rates seen for domesticated and wild sunflowers.

Structure and function of histone methylation-binding proteins in plants

Post-translational modifications of histones play important roles in modulating many essential biological processes in both animals and plants. These covalent modifications, including methylation, acetylation, phosphorylation, ubiquitination, SUMOylation and so on, are laid out and erased by histone-modifying enzymes and read out by effector proteins. Recent studies have revealed that a number of developmental processes in plants are under the control of histone post-translational modifications, such as floral transition, seed germination, organogenesis and morphogenesis. Therefore, it is critical to identify those protein domains, which could specifically recognize these post-translational modifications to modulate chromatin structure and regulate gene expression. In the present review, we discuss the recent progress in understanding the structure and function of the histone methylation readers in plants, by focusing on Arabidopsis thaliana proteins.

Transcriptional Framework of Male Gametogenesis in the Liverwort Marchantia polymorpha L

In land plants, there are two types of male gametes: one is a non-motile sperm cell which is delivered to the egg cell by a pollen tube, and the other is a motile sperm cell with flagella. The molecular mechanism underlying the sexual reproduction with the egg and pollen-delivered sperm cell is well understood from studies using model plants such as Arabidopsis and rice. On the other hand, the sexual reproduction with motile sperm has remained poorly characterized, due to the lack of suitable models. Marchantia polymorpha L. is a model basal land plant with sexual reproduction involving an egg cell and bi-flagellated motile sperm. To understand the differentiation process of plant motile sperm, we analyzed the gene expression profile of developing antheridia of M. polymorpha. We performed RNA-sequencing experiments and compared transcript profiles of the male sexual organ (antheridiophore and antheridium contained therein), female sexual organ (archegoniophore) and a vegetative organ (thallus). Transcriptome analysis showed that the antheridium expresses nearly half of the protein-coding genes predicted in the genome, but it also has unique features. The antheridium transcriptome shares some common features with male gamete transcriptomes of angiosperms and animals, and homologs of genes involved in male gamete formation and function in angiosperms and animals were identified. In addition, we showed that some of them had distinct expression patterns in the spermatogenous tissue of developing antheridia. This study provides a transcriptional framework on which to study the molecular mechanism of plant motile sperm development in M. polymorpha as a model.

A Transcriptome Atlas of Physcomitrella patens Provides Insights into the Evolution and Development of Land Plants

Identifying the genetic mechanisms that underpin the evolution of new organ and tissue systems is an aim of evolutionary developmental biology. Comparative functional genetic studies between angiosperms and bryophytes can define those genetic changes that were responsible for developmental innovations. Here, we report the generation of a transcriptome atlas covering most phases in the life cycle of the model bryophyte Physcomitrella patens, including detailed sporophyte developmental progression. We identified a comprehensive set of sporophyte-specific transcription factors, and found that many of these genes have homologs in angiosperms that function in developmental processes such as flowering and shoot branching. Deletion of the PpTCP5 transcription factor results in development of supernumerary sporangia attached to a single seta, suggesting that it negatively regulates branching in the moss sporophyte. Given that TCP genes repress branching in angiosperms, we suggest that this activity is ancient. Finally, comparison of P. patens and Arabidopsis thaliana transcriptomes led us to the identification of a conserved core of transcription factors expressed in tip-growing cells. We identified modifications in the expression patterns of these genes that could account for developmental differences between P. patens tip-growing cells and A. thaliana pollen tubes and root hairs.

Transcriptome Profiling of Petal Abscission Zone and Functional Analysis of an Aux/IAA Family Gene RhIAA16 Involved in Petal Shedding in Rose

Roses are one of the most important cut flowers among ornamental plants. Rose flower longevity is largely dependent on the timing of petal shedding occurrence. To understand the molecular mechanism underlying petal abscission in rose, we performed transcriptome profiling of the petal abscission zone during petal shedding using Illumina technology. We identified a total of 2592 differentially transcribed genes (DTGs) during rose petal shedding. Gene ontology term enrichment and pathway analysis revealed that major biochemical pathways the DTGs were involved in included ethylene biosynthesis, starch degradation, superpathway of cytosolic glycolysis, pyruvate dehydrogenase and TCA cycle, photorespiration and the lactose degradation III pathway. This suggests that alterations in carbon metabolism are an important part of rose petal abscission. Among these DTGs, approximately 150 genes putatively encoding transcription factors were identified in rose abscission zone. These included zinc finger, WRKY, ERF, and Aux/IAA gene families, suggesting that petal abscission involves complex transcriptional reprogramming. Approximately 108 DTGs were related to hormone pathways, of which auxin and ethylene related DTGs were the largest groups including 52 and 41 genes, respectively. These also included 12 DTGs related to gibberellin and 6 DTGs in jasmonic acid pathway. Surprisingly, no DTGs involved in the biosynthesis/signaling of abscisic acid, cytokinin, brassinosteroid, and salicylic acid pathways were detected. Moreover, among DTGs related to auxin, we identified an Aux/IAA gene RhIAA16 that was up-regulated in response to petal shedding. Down-regulation of RhIAA16 by virus-induced gene silencing in rose promoted petal abscission, suggesting that RhIAA16 plays an important role in rose petal abscission.

Identification of genes associated with male sterility in a mutant of white birch (Betula platyphylla Suk.)

White birch (Betula platyphylla Suk.) is a monoecious tree species with unisexual flowers. In this study, we used a spontaneous mutant genotype that produced normal-like male (NLM) inflorescences and mutant male (MM) inflorescences at different locations within the tree to investigate the genes necessary for pollen development. A cDNA-amplified fragment length polymorphism (cDNA-AFLP) analysis was used to identify genes differentially expressed between the two types of inflorescences. Of approximately 5000 transcript-derived fragments (TDFs) obtained, 323 were significantly differentially expressed, of which 141 were successfully sequenced. BLAST analyses revealed 51.8% of the sequenced TDFs showed significant homology with proteins of known or predicted functions, 10.6% showed significant homology with putative proteins without any known or predicted function, and the remaining 37.6% had no hits in the NCBI database. Further, in a functional categorization based on the BLAST analyses, the protein fate, metabolism, energy categories had in order the highest percentages of the proteins. A Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that the known TDFs were mainly involved in metabolic (28.4%), signal transduction (23.5%) and folding, sorting and degradation (13.6%) pathways. Ten genes from the NLM and MM development stages in the mutant were analyzed by quantitative real-time reverse transcriptase-polymerase chain reaction (qRT-PCR). The information generated in this study can provide some useful clues to help understand male sterility in B. platyphylla.

ABORTED GAMETOPHYTE 1 is required for gametogenesis in Arabidopsis

In flowering plants, the male and female gametogenesis is a crucial step of sexual reproduction. Although many genes have been identified as being involved in the gametogenesis process, the genetic mechanisms underlying gametogenesis remains poorly understood. We reported here characterization of the gene, ABORTED GAMETOPHYTE 1 (AOG1) that is newly identified as essential for gametogenesis in Arabidopsis thaliana. AOG1 is expressed predominantly in reproductive tissues including the developing pollen grains and ovules. The AOG1 protein shares no significant amino acid sequence similarity with other documented proteins and is located mainly in nuclei of the cells. Mutation in AOG1 caused degeneration of pollen at the uninucleate microspore stage and severe defect in embryo sacs, leading to a significant reduction in male and female fertility. Furthermore, the molecular analyses showed that the aog1 mutant significantly affected the expression of several genes, which are required for gametogenesis. Our results suggest that AOG1 plays important roles in gametogenesis at the stage prior to pollen mitosis I (PMI) in Arabidopsis, possibly through collaboration with other genes.

The plant-specific protein FEHLSTART controls male meiotic entry, initializing meiotic synchronization in Arabidopsis

Meiosis marks the transition from the sporophyte to the gametophyte generation in the life cycle of flowering plants, and creates genetic variations through homologous recombination. In most flowering plants, meiosis is highly synchronized within each anther, which is significant for efficient fertilization. To date, little is known about the molecular mechanisms of entry into meiosis and exit from it, and only a few genes in Arabidopsis have been characterized with a role in regulating meiotic progression. In this study, we report the functional characterization of a plant-specific basic helix-loop-helix (bHLH) protein, FEHLSTART (FST), a defect in which leads to premature meiotic entry and asynchronous meiosis, and results in decreased seed yield. Investigation of the time course of meiosis showed that the onset of leptotene, the first stage of prophase I, frequently occurred earlier in fst-1 than in the wild type. Asynchronous meiosis followed, which could manifest in the disruption of regular spindle structures and symmetric cell divisions in fst-1 mutants during the meiosis I/II transition. In accordance with frequently accelerated meiotic entry, whole-transcriptome analysis of fst-1 anthers undergoing meiosis revealed that 19 circadian rhythm genes were affected and 47 pollen-related genes were prematurely expressed at a higher level. Taken together, we propose that FST is required for normal meiotic entry and the establishment of meiotic synchrony.