CYP704B1 is a long-chain fatty acid omega-hydroxylase essential for sporopollenin synthesis in pollen of Arabidopsis

Cytochromes P450 evolution in the plant terrestrialization context

Plants started to colonize land around 500 million years ago. It meant dealing with new challenges like absence of buoyancy, water and nutrients shortage, increased light radiation, reproduction on land, and interaction with new microorganisms. This obviously required the acquisition of novel functions and metabolic capacities. Cytochrome P450 (CYP) monooxygenases form the largest superfamily of enzymes and are present to catalyse critical and rate-limiting steps in most plant-specific pathways. The different families of CYP enzymes are typically associated with specific functions. CYP family emergence and evolution in the green lineage thus offer the opportunity to obtain a glimpse into the timing of the evolution of the critical functions that were required (or became dispensable) for the plant transition to land. Based on the analysis of currently available genomic data, this review provides an evolutionary history of plant CYPs in the context of plant terrestrialization and describes the associated functions in the different lineages. Without surprise it highlights the relevance of the biosynthesis of antioxidants and UV screens, biopolymers, and critical signalling pathways. It also points to important unsolved questions that would deserve to be answered to improve our understanding of plant adaptation to challenging environments and the management of agricultural traits. This article is part of the theme issue 'The evolution of plant metabolism'.

The Residual Activity of Fatty Acyl-CoA Reductase Underlies Thermo-Sensitive Genic Male Sterility in Rice

Photoperiod/thermo-sensitive genic male sterility (P/TGMS) is critical for rice two-line hybrid system. Previous studies showed that slow development of pollen is a general mechanism for sterility-to-fertility conversion of TGMS in Arabidopsis. However, whether this mechanism still exists in rice is unknown. Here, we identified a novel rice TGMS line, ostms16, which exhibits abnormal pollen exine under high temperature and fertility restoration under low temperature. In mutant, a single base mutation of OsTMS16, a fatty acyl-CoA reductase (FAR), reduced its enzyme activity, leading to defective pollen wall. Under high temperature, the mOsTMS16M549I couldn't provide sufficient protection for the microspores. Under low temperature, the enzyme activity of mOsTMS16M549I is closer to that of OsTMS16, so that the imperfect exine could still protect microspore development. These results indicated whether the residual enzyme activity in mutant could meet the requirement in different temperature is a determinant factor for fertility conversion of P/TGMS lines. Additionally, we previously found that res2, the mutant of a polygalacturonase for tetrad pectin wall degradation, restored multiple TGMS lines in Arabidopsis. In this study, we proved that the osres2 in rice restored the fertility of ostms16, indicating the slow development is also suitable for the fertility restoration in rice.

Transcriptome Remodeling in Arabidopsis: A Response to Heterologous Poplar MSL-lncRNAs Overexpression

Stamens are vital reproductive organs in angiosperms, essential for plant growth, reproduction, and development. The genetic regulation and molecular mechanisms underlying stamen development are, however, complex and varied among different plant species. MSL-lncRNAs, a gene specific to the Y chromosome of Populus deltoides, is predominantly expressed in male flower buds. Heterologous expression of MSL-lncRNAs in Arabidopsis thaliana resulted in an increase in both stamen and anther count, without affecting pistil development or seed set. To reveal the molecular regulatory network influenced by MSL-lncRNAs on stamen development, we conducted transcriptome sequencing of flowers from both wild-type and MSL-lncRNAs-overexpressing Arabidopsis. A total of 678 differentially expressed genes were identified between wild-type and transgenic Arabidopsis. Among these, 20 were classified as transcription factors, suggesting a role for these regulatory proteins in stamen development. GO enrichment analysis revealed that the differentially expressed genes were significantly associated with processes such as pollen formation, polysaccharide catabolic processes, and secondary metabolism. KEGG pathway analysis indicated that MSL-lncRNAs might promote stamen development by upregulating genes involved in the phenylpropanoid biosynthesis pathway. The top three upregulated genes, all featuring the DUF295 domain, were found to harbor an F-box motif at their N-termini, which is implicated in stamen development. Additionally, in transgenic Arabidopsis flowers, genes implicated in tapetum formation and anther development were also observed to be upregulated, implying a potential role for MSL-lncRNAs in modulating pollen development through the positive regulation of these genes. The findings from this study establish a theoretical framework for elucidating the genetic control exerted by MSL-lncRNAs over stamen and pollen development.

Cloning and functional verification of the male sterile gene BrQRT3 in Chinese cabbage

Chinese cabbage is a cross-pollinated crop with significant heterosis, and male sterile lines are an important way to produce hybrid seeds. In this study, a male sterile mutant msm0795 was identified in an EMS-mutagenized population of Chinese cabbage. Cytological observations revealed that the microspores failed to separate after the tetrad stage, and thus developed into abnormal pollen grains, resulting in anther abortion. MutMap combined with Kompetitive Allele Specific PCR genotyping showed that BraA01g011280.3.5 C was identified as the candidate gene, which encodes polygalacturonase QRT3 and plays a direct role in the degradation of pollen mother cell wall during microspore development, named BrQRT3. Subcellular localization and expression analyses demonstrated that BrQRT3 was localized in the cell membrane and was ubiquitously expressed in roots, stems, leaves, flower buds, and flowers, but the expression of BrQRT3 was gradually suppressed with the anther development. Ectopic expression confirmed that over-expression of BrQRT3 in qrt3 background Arabidopsis mutant can rescue the pollen defects caused by loss of AtQRT3 function. It is the first time to achieve a male sterile mutant caused by the mutation of BrQRT3 in Chinese cabbage. These findings contribute to elucidate the mechanism of BrQRT3 in regulating stamen development of Chinese cabbage.

OsSNDP4, a Sec14-nodulin Domain Protein, is Required for Pollen Development in Rice

Pollen is encased in a robust wall that shields the male gametophyte from various stresses and aids in pollination. The pollen wall consists of gametophyte-derived intine and sporophyte-derived exine. The exine is mainly composed of sporopollenin, which is biopolymers of aliphatic lipids and phenolics. The process of exine formation has been the subject of extensive research, yet the underlying molecular mechanisms remain elusive. In this study, we identified a rice mutant of the OsSNDP4 gene that is impaired in pollen development. We demonstrated that OsSNDP4, a putative Sec14-nodulin domain protein, exhibits a preference for binding to phosphatidylinositol (3)-phosphate [PI(3)P], a lipid primarily found in endosomal and vacuolar membranes. The OsSNDP4 protein was detected in association with the endoplasmic reticulum (ER), vacuolar membranes, and the nucleus. OsSNDP4 expression was detected in all tested organs but was notably higher in anthers during exine development. Loss of OsSNDP4 function led to abnormal vacuole dynamics, inhibition in Ubisch body development, and premature degradation of cellular contents and organelles in the tapetal cells. Microspores from the ossndp4 mutant plant displayed abnormal exine formation, abnormal vacuole enlargement, and ultimately, pollen abortion. RNA-seq assay revealed that genes involved in the biosynthesis of fatty acid and secondary metabolites, the biosynthesis of lipid polymers, and exosome formation were enriched among the down-regulated genes in the mutant anthers, which correlated with the morphological defects observed in the mutant anthers. Base on these findings, we propose that OsSNDP4 regulates pollen development by binding to PI(3)P and influencing the dynamics of membrane systems. The involvement of membrane systems in the regulation of sporopollenin biosynthesis, Ubisch body formation, and exine formation provides a novel mechanism regulating pollen wall development.

Identification and Expression Analysis of Cytochrome P450 Genes Probably Involved in Triterpenoid Saponins Biosynthesis in Astragalus mongholicus

Cytochromes P450 (P450s) are one of the largest enzymatic protein families and play critical roles in the synthesis and metabolism of plant secondary metabolites. Astragaloside IV (AS-IV) is one of the primary active components in Astragalus herbs, exhibiting diverse biological activities and pharmacological effects. However, P450s involved in the astragaloside biosynthesis have not been systematically analyzed in Astragalus mongholicus (A. mongholicus). In this study, we identified 209 P450 genes from the genome of A. mongholicus (AmP450s), which were classified into nine clans and 47 families and performed a systematic overview of their physical and chemical properties, phylogeny, gene structures and conserved motifs. Weighted gene co-expression network analysis (WGCNA) revealed that AmP450s are critical in the astragaloside biosynthesis pathway. The expression levels of these AmP450s were verified by quantitative real-time PCR (qRT-PCR) analysis in the root, stem and leaf, showing that most AmP450s are abundant in the root. Additionally, the correlation analysis between gene expressions and AS-IV content showed that twelve AmP450s, especially CYP71A28, CYP71D16 and CYP72A69, may have significant potential in the biosynthesis of astragaloside. This study systematically investigates the P450s of A. mongholicus and offers valuable insights into further exploring the functions of CYP450s in the astragaloside biosynthesis pathway.

Comparative Proteomic Analysis of Floral Buds before and after Opening in Walnut (Juglans regia L.)

The walnut (Juglans regia L.) is a typical and an economically important tree species for nut production with heterodichogamy. The absence of female and male flowering periods seriously affects both the pollination and fruit setting rates of walnuts, thereby affecting the yield and quality. Therefore, studying the characteristics and processes of flower bud differentiation helps in gaining a deeper understanding of the regularity of the mechanism of heterodichogamy in walnuts. In this study, a total of 3540 proteins were detected in walnut and 885 unique differentially expressed proteins (DEPs) were identified using the isobaric tags for the relative and absolute quantitation (iTRAQ)-labeling method. Among all DEPs, 12 common proteins were detected in all four of the obtained contrasts. GO and KEGG analyses of 12 common DEPs showed that their functions are distributed in the cytoplasm metabolic pathways, photosynthesis, glyoxylate and dicarboxylate metabolism, and the biosynthesis of secondary metabolites, which are involved in energy production and conversion, synthesis, and the breakdown of proteomes. In addition, a function analysis was performed, whereby the DEPs were classified as involved in photosynthesis, morphogenesis, metabolism, or the stress response. A total of eight proteins were identified as associated with the morphogenesis of stamen development, such as stamen-specific protein FIL1-like (XP_018830780.1), putative leucine-rich repeat receptor-like serine/threonine-protein kinase At2g24130 (XP_018822513.1), cytochrome P450 704B1-like isoform X2 (XP_018845266.1), ervatamin-B-like (XP_018824181.1), probable glucan endo-1,3-beta-glucosidase A6 (XP_018844051.1), pathogenesis-related protein 5-like (XP_018835774.1), GDSL esterase/lipase At5g22810-like (XP_018833146.1), and fatty acyl-CoA reductase 2 (XP_018848853.1). Our results predict several crucial proteins and deepen the understanding of the biochemical mechanism that regulates the formation of male and female flower buds in walnuts.

Two Cytochrome P450s, CYP709B1 and CYP704C1, Play Essential Roles in Metabolism-Based Multiple Herbicide Resistance in American Sloughgrass (Beckmannia syzigachne (Steud.) Fernald)

This study confirmed a field population of American sloughgrass (Beckmannia syzigachne (Steud.) Fernald) that developed simultaneously high levels of resistance (resistance index >10) to three divergent modes of action herbicides: fenoxaprop-P-ethyl, mesosulfuron-methyl, and isoproturon. The resistance phenotype observed in this population was not attributed to target-site alterations; rather, the resistant plants exhibited a significant increase in the activity of cytochrome P450s (P450s) and enhanced metabolism rates for all three herbicides. RNA sequencing revealed significant upregulation of two P450s, CYP709B1 and CYP704C1, in the resistant plants both before and after herbicide treatments. Molecular docking predicted that the homology models of these P450s should exhibit a binding affinity for a range of herbicides. The heterologous expression of the identified P450s in yeast cells indicated improved growth in the presence of all three of the aforementioned herbicides. Collectively, the increased expression of CYP709B1 and CYP704C1 likely contributed to the P450s-mediated enhanced metabolism, thereby conferring multiple herbicide resistance in B. syzigachne.

Integrated transcriptomic and metabolomic analysis provides insight into the pollen development of CMS-D1 rice

BACKGROUND

Cytoplasmic male sterility (CMS) has greatly improved the utilization of heterosis in crops due to the absence of functional male gametophyte. The newly developed sporophytic D1 type CMS (CMS-D1) rice exhibits unique characteristics compared to the well-known sporophytic CMS-WA line, making it a valuable resource for rice breeding.

RESULTS

In this research, a novel CMS-D1 line named Xingye A (XYA) was established, characterized by small, transparent, and shriveled anthers. Histological and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) assays conducted on anthers from XYA and its maintainer line XYB revealed that male sterility in XYA is a result of delayed degradation of tapetal cells and abnormal programmed cell death (PCD) of microspores. Transcriptome analysis of young panicles revealed that differentially expressed genes (DEGs) in XYA, compared to XYB, were significantly enriched in processes related to chromatin structure and nucleosomes during the microspore mother cell (MMC) stage. Conversely, processes associated with sporopollenin biosynthesis, pollen exine formation, chitinase activity, and pollen wall assembly were enriched during the meiosis stage. Metabolome analysis identified 176 specific differentially accumulated metabolites (DAMs) during the meiosis stage, enriched in pathways such as α-linoleic acid metabolism, flavone and flavonol biosynthesis, and linolenic acid metabolism. Integration of transcriptomic and metabolomic data underscored the jasmonic acid (JA) biosynthesis pathway was significant enriched in XYA during the meiosis stage compared to XYB. Furthermore, levels of JA, MeJA, OPC4, OPDA, and JA-Ile were all higher in XYA than in XYB at the meiosis stage.

CONCLUSIONS

These findings emphasize the involvement of the JA biosynthetic pathway in pollen development in the CMS-D1 line, providing a foundation for further exploration of the molecular mechanisms involved in CMS-D1 sterility.

GhTKPR1_8 functions to inhibit anther dehiscence and reduce pollen viability in cotton

Sporopollenin, as the main component of the pollen exine, is a highly resistant polymer that provides structural integrity under unfavourable environmental conditions. Tetraketone α-pyrone reductase 1 (TKPR1) is essential for sporopollenin formation, catalyzing the reduction of tetraketone carbonyl to hydroxylated α-pyrone. The functional role of TKPR1 in male sterility has been reported in flowering plants such as maize, rice, and Arabidopsis. However, the molecular cloning and functional characterization of TKPR1 in cotton remain unaddressed. In this study, we identified 68 TKPR1s from four cotton species, categorized into three clades. Transcriptomics and RT-qPCR demonstrated that GhTKPR1_8 exhibited typical expression patterns in the tetrad stage of the anther. GhTKPR1_8 was localized to the endoplasmic reticulum. Moreover, ABORTED MICROSPORES (GhAMS) transcriptionally activated GhTKPR1_8 as indicated by luciferase complementation tests. GhTKPR1_8-knockdown inhibited anther dehiscence and reduced pollen viability in cotton. Additionally, overexpression of GhTKPR1_8 in the attkpr1 mutant restored its male sterile phenotype. This study offers novel insights into the investigation of TKPR1 in cotton while providing genetic resources for studying male sterility.

Gene co-expression network analysis in areca floral organ and the potential role of the AcMADS17 and AcMADS23 in transgenic Arabidopsis

Areca catechu L., a monocot belonging to the palm family, is monoecious, with female and male flowers separately distributed on the same inflorescence. To discover the molecular mechanism of flower development in Areca, we sequenced different floral samples to generate tissue-specific transcriptomic profiles. We conducted a comparative analysis of the transcriptomic profiles of apical sections of the inflorescence with male flowers and the basal section of the inflorescence with female flowers. Based on the RNA sequencing dataset, we applied weighted gene co-expression network analysis (WGCNA) to identify sepal, petal, stamen, stigma and other specific modules as well as hub genes involved in specific floral organ development. The syntenic and expression patterns of AcMADS-box genes were analyzed in detail. Furthermore, we analyzed the open chromatin regions and transcription factor PI binding sites in male and female flowers by assay for transposase-accessible chromatin sequencing (ATAC-seq) assay. Heterologous expression revealed the important role of AcMADS17 and AcMADS23 in floral organ development. Our results provide a valuable genomic resource for the functional analysis of floral organ development in Areca.

Lipid and sugar metabolism play an essential role in pollen development and male sterility: a case analysis in Brassica napus

AIMS

The genic male sterility (GMS) system is an important strategy for generating heterosis in plants. To better understand the essential role of lipid and sugar metabolism and to identify additional candidates for pollen development and male sterility, transcriptome and metabolome analysis of a GMS line of 1205AB in B. napus was used as a case study.

DATA RESOURCES GENERATED

To characterize the GMS system, the transcriptome and metabolome profiles were generated for 24 samples and 48 samples of 1205AB in B. napus, respectively. Transcriptome analysis yielded a total of 156.52 Gb of clean data and revealed the expression levels of 109,541 genes and 8,501 novel genes. In addition, a total of 1,353 metabolites were detected in the metabolomic analysis, including 784 in positive ion mode and 569 in negative ion mode.

KEY RESULTS

A total of 15,635 differentially expressed genes (DEGs) and 83 differential metabolites (DMs) were identified from different comparison groups, most of which were involved in lipid and sugar metabolism. The combination of transcriptome and metabolome analysis revealed 49 orthologous GMS genes related to lipid metabolism and 46 orthologous GMS genes related to sugar metabolism, as well as 45 novel genes.

UTILITY OF THE RESOURCE

The transcriptome and metabolome profiles and their analysis provide useful reference data for the future discovery of additional GMS genes and the development of more robust male sterility breeding systems for use in the production of plant hybrids.

Broad Chain-Length Specificity of the Alkane-Forming Enzymes NoCER1A and NoCER3A/B in Nymphaea odorata

Many terrestrial plants produce large quantities of alkanes for use in epicuticular wax and the pollen coat. However, their carbon chains must be long to be useful as fuel or as a petrochemical feedstock. Here, we focus on Nymphaea odorata, which produces relatively short alkanes in its anthers. We identified orthologs of the Arabidopsis alkane biosynthesis genes AtCER1 and AtCER3 in N. odorata and designated them NoCER1A, NoCER3A and NoCER3B. Expression analysis of NoCER1A and NoCER3A/B in Arabidopsis cer mutants revealed that the N. odorata enzymes cooperated with the Arabidopsis enzymes and that the NoCER1A produced shorter alkanes than AtCER1, regardless of which CER3 protein it interacted with. These results indicate that AtCER1 frequently uses a C30 substrate, whereas NoCER1A, NoCER3A/B and AtCER3 react with a broad range of substrate chain lengths. The incorporation of shorter alkanes disturbed the formation of wax crystals required for water-repellent activity in stems, suggesting that chain-length specificity is important for surface cleaning. Moreover, cultured tobacco cells expressing NoCER1A and NoCER3A/B effectively produced C19-C23 alkanes, indicating that the introduction of the two enzymes is sufficient to produce alkanes. Taken together, our findings suggest that these N. odorata enzymes may be useful for the biological production of alkanes of specific lengths. 3D modeling revealed that CER1s and CER3s share a similar structure that consists of N- and C-terminal domains, in which their predicted active sites are respectively located. We predicted the complex structure of both enzymes and found a cavity that connects their active sites.

Characterization of CYP82 genes involved in the biosynthesis of structurally diverse benzylisoquinoline alkaloids in Corydalis yanhusuo

Benzylisoquinoline alkaloids (BIAs) represent a significant class of secondary metabolites with crucial roles in plant physiology and substantial potential for clinical applications. CYP82 genes are involved in the formation and modification of various BIA skeletons, contributing to the structural diversity of compounds. In this study, Corydalis yanhusuo, a traditional Chinese medicine rich in BIAs, was investigated to identify the catalytic function of CYP82s during BIA formation. Specifically, 20 CyCYP82-encoding genes were cloned, and their functions were identified in vitro. Ten of these CyCYP82s were observed to catalyze hydroxylation, leading to the formation of protopine and benzophenanthridine scaffolds. Furthermore, the correlation between BIA accumulation and the expression of CyCYP82s in different tissues of C. yanhusuo was assessed their. The identification and characterization of CyCYP82s provide novel genetic elements that can advance the synthetic biology of BIA compounds such as protopine and benzophenanthridine, and offer insights into the biosynthesis of BIAs with diverse structures in C. yanhusuo.

Revealing the role of CCoAOMT1: fine-tuning bHLH transcription factors for optimal anther development

The tapetum, a crucial innermost layer encompassing male reproductive cells within the anther wall, plays a pivotal role in normal pollen development. The transcription factors (TFs) bHLH010/089/091 redundantly facilitate the rapid nuclear accumulation of DYSFUNCTIONAL TAPETUM 1, a gatekeeper TF in the tapetum. Nevertheless, the regulatory mechanisms governing the activity of bHLH010/089/091 remain unknown. In this study, we reveal that caffeoyl coenzyme A O-methyltransferase 1 (CCoAOMT1) is a negative regulator affecting the nuclear localization and function of bHLH010 and bHLH089, probably through their K259 site. Our findings underscore that CCoAOMT1 promotes the nuclear export and degradation of bHLH010 and bHLH089. Intriguingly, elevated CCoAOMT1 expression resulted in defective pollen development, mirroring the phenotype observed in bhlh010 bhlh089 mutants. Moreover, our investigation revealed that the K259A mutation in the bHLH089 protein disrupted its translocation from the nucleus to the cytosol and impeded its degradation induced by CCoAOMT1. Importantly, transgenic plants with the probHLH089::bHLH089K259A construct failed to rescue proper pollen development or gene expression in bhlh010 bhlh089 mutants. Collectively, these findings emphasize the need to maintain balanced TF homeostasis for male fertility. They firmly establish CCoAOMT1 as a pivotal regulator that is instrumental in achieving equilibrium between the induction of the tapetum transcriptional network and ensuring appropriate anther development.

Mutations in BrABCG26, encoding an ATP-binding cassette transporter, are responsible for male sterility in Chinese cabbage (Brassica rapa L. ssp. pekinensis)

KEY MESSAGE

The gene BrABCG26 responsible for male sterility of Chinese cabbage was confirmed by two allelic mutants. Male-sterile lines are an important way of heterosis utilization in Chinese cabbage. In this study, two allelic male-sterile mutants msm3-1 and msm3-2 were obtained from a Chinese cabbage double haploid (DH) line 'FT' by using EMS-mutagenesis. Compared to the wild-type 'FT,' the stamens of mutants were completely degenerated and had no pollen, and other characters had no obvious differences. Cytological observation revealed that the failure of vacuolation of the mononuclear microspore, accompanied by abnormal tapetal degradation, resulted in anther abortion in mutants. Genetic analysis showed that a recessive gene controlled the mutant trait. MutMap combined with kompetitive allele specific PCR genotyping analyses showed that BraA01g038270.3C, encoding a transporter ABCG26 that played a vital role in pollen wall formation, was the candidate gene for msm3-1, named BrABCG26. Compared with wild-type 'FT,' the mutations existed on the second exon (C to T) and the sixth exon (C to T) of BrABCG26 gene in mutants msm3-1 and msm3-2, leading to the loss-of-function truncated protein, which verified the BrABCG26 function in stamen development. Subcellular localization and expression pattern analysis indicated that BrABCG26 was localized in the nucleus and was expressed in all organs, with the highest expression in flower buds. Compared to the wild-type 'FT,' the expressions of BrABCG26 were significantly reduced in flower buds and anthers of mutants. Promoter activity analysis showed that a strong GUS signal was detected in flower buds. These results indicated that BrABCG26 is responsible for the male sterility of msm3 mutants in Chinese cabbage.

Development and genetic regulation of pollen intine in Arabidopsis and rice

Pollen intine serves as a protective layer situated between the pollen exine and the plasma membrane. It performs essential functions during pollen development, including maintaining the morphological structure of the pollen, preventing the loss of pollen contents, and facilitating pollen germination. The formation of the intine layer commences at the bicellular pollen stage. Pectin, cellulose, hemicellulose and structural proteins are the key constituents of the pollen intine. In Arabidopsis and rice, numerous regulatory factors associated with polysaccharide metabolism and material transport have been identified, which regulate intine development. In this review, we elucidate the developmental processes of the pollen wall and provide a concise summary of the research advancements in the development and genetic regulation of the pollen intine in Arabidopsis and rice. A comprehensive understanding of intine development and regulation is crucial for unraveling the genetic network underlying intine development in higher plants.

Pollen wall development in Impatiens glandulifera: exine substructure and underlying mechanisms

The aim of this study was to investigate in detail the pollen wall ontogeny in Impatiens glandulifera, with emphasis on the substructure and the underlying mechanisms of development. Sporopollenin-containing pollen wall, the exine, consists of two parts, ectexine and endexine. By determining the sequence of developing substructures with TEM, we have in mind to understand in which way the exine substructure is connected with function. We have shown earlier that physical processes of self-assembly and phase separation are universally involved in ectexine development; currently, we try to clear up whether these processes participate in endexine development. The data received were compared with those on other species. The ectexine ontogeny of I. glandulifera followed the main stages observed in many other species, including the late tetrad stage named "Golden gates". It turned out that the same physico-chemical processes act in endexine development, especially expressed in aperture sites. Another peculiar phenomenon observed in exine development was the recurrency of micellar sequence at near-aperture and aperture sites where the periplasmic space is widened. It should be noted that, in the whole, the developmental substructures observed during the tetrad and early post-tetrad period are similar in species with columellate exines. Evidently, these basic physical processes proceed, reiterating again and again in different species, resulting in an enormous variety of exine structures on the base of a relatively modest number of genes. Granular and alveolar exines emerge on the base of the same basic processes but are arrested at spherical and cylindrical micelle mesophases correspondingly.

CRISPR/Cas9-based genome editing of 14 lipid metabolic genes reveals a sporopollenin metabolon ZmPKSB-ZmTKPR1-1/-2 required for pollen exine formation in maize

Lipid biosynthesis and transport are essential for plant male reproduction. Compared with Arabidopsis and rice, relatively fewer maize lipid metabolic genic male-sterility (GMS) genes have been identified, and the sporopollenin metabolon in maize anther remains unknown. Here, we identified two maize GMS genes, ZmTKPR1-1 and ZmTKPR1-2, by CRISPR/Cas9 mutagenesis of 14 lipid metabolic genes with anther stage-specific expression patterns. Among them, tkpr1-1/-2 double mutants displayed complete male sterility with delayed tapetum degradation and abortive pollen. ZmTKPR1-1 and ZmTKPR1-2 encode tetraketide α-pyrone reductases and have catalytic activities in reducing tetraketide α-pyrone produced by ZmPKSB (polyketide synthase B). Several conserved catalytic sites (S128/130, Y164/166 and K168/170 in ZmTKPR1-1/-2) are essential for their enzymatic activities. Both ZmTKPR1-1 and ZmTKPR1-2 are directly activated by ZmMYB84, and their encoded proteins are localized in both the endoplasmic reticulum and nuclei. Based on protein structure prediction, molecular docking, site-directed mutagenesis and biochemical assays, the sporopollenin biosynthetic metabolon ZmPKSB-ZmTKPR1-1/-2 was identified to control pollen exine formation in maize anther. Although ZmTKPR1-1/-2 and ZmPKSB formed a protein complex, their mutants showed different, even opposite, defective phenotypes of anther cuticle and pollen exine. Our findings discover new maize GMS genes that can contribute to male-sterility line-assisted maize breeding and also provide new insights into the metabolon-regulated sporopollenin biosynthesis in maize anther.

Characterization and expression analysis of chalcone synthase gene family members suggested their roles in the male sterility of a wheat temperature-sensitive genic male sterile (TGMS) line

Chalcone synthase (CHS), also known as the plants-specific type III polyketide synthases (PKSs), catalyzes the first key step in the biosynthesis of plant flavonoids. Flavonoids are one of the most important secondary metabolites which participate in flower pigmentation and pollen fertility. Recent reports have demonstrated the role of the CHS family in plant pollen exine formation. This study focused on the potential roles of CHS in the pollen exine formation of wheat. In the present study, a genome-wide investigation of the CHS family was carried out, and 87 CHS genes were identified in wheat. TaCHS3, TaCHS10, and TaCHS13 are wheat orthologs of Arabidopsis LESS ADHESIVE POLLEN (LAP5); TaCHS58, TaCHS64, and TaCHS67 are wheat orthologs of AtLAP6. TaCHS3, TaCHS10, and TaCHS67 showed anther-specific patterns. The expression of TaCHS3, TaCHS10, and TaCHS67 was positively co-expressed with sporopollenin biosynthetic genes, including TaCYP703A2, TaCYP704B1, TaDRL1, TaTKPR2, and TaMS2. Coincidently, the expression of TaCHS3, TaCHS10, and TaCHS67, together with those sporopollenin biosynthetic genes, were repressed at the tetrads and uninucleate stages in the temperature-sensitive genic male-sterile (TGMS) line BS366 under sterile conditions. Wheat anther-specific CHS genes might participate in the exine formation of BS366 through co-expressing with sporopollenin biosynthetic genes, which will undoubtedly provide knowledge of the roles of CHS in wheat pollen development.

Multiplex CRISPR-Cas9 knockout of EIL3, EIL4, and EIN2L advances soybean flowering time and pod set

BACKGROUND

Ethylene inhibitor treatment of soybean promotes flower bud differentiation and early flowering, suggested that there is a close relationship between ethylene signaling and soybean growth and development. The short-lived ETHYLENE INSENSITIVE2 (EIN2) and ETHYLENE INSENSITIVE3 (EIN3) proteins play central roles in plant development. The objective of this study was carried out gene editing of EIL family members in soybeans and to examine the effects on soybean yield and other markers of growth.

METHODS AND RESULTS

By editing key-node genes in the ethylene signaling pathway using a multi-sgRNA-in-one strategy, we obtained a series of gene edited lines with variable edit combinations among 15 target genes. EIL3, EIL4, and EIN2L were editable genes favored by the T0 soybean lines. Pot experiments also show that the early flowering stage R1 of the EIL3, EIL4, and EIN2L triple mutant was 7.05 d earlier than that of the wild-type control. The yield of the triple mutant was also increased, being 1.65-fold higher than that of the control. Comparative RNA-seq revealed that sucrose synthase, AUX28, MADS3, type-III polyketide synthase A/B, ABC transporter G family member 26, tetraketide alpha-pyrone reductase, and fatty acyl-CoA reductase 2 may be involved in regulating early flowering and high-yield phenotypes in triple mutant soybean plants.

CONCLUSION

Our results provide a scientific basis for genetic modification to promote the development of earlier-flowering and higher-yielding soybean cultivars.

OsTKPR1 proteins with a single amino acid substitution fail the synthesis of a specific sporopollenin precursor and cause abnormal exine and pollen development in rice

Fatty acid derivatives are key components of rice pollen exine. The synthesis of aliphatic sporopollenin precursors are initiated in the plastids of the tapetal cells, followed by multiple-step reactions conducted in the endoplasmic reticulum (ER). However, the relative contribution of different precursors to the precise structure of sporopollenin remains largely elusive, let alone the underlying mechanism. Here, we report that two complete male sterile mutants ostkpr1-3 (Tetraketide α-pyrone reductase 1-3, with OsTKPR1P124S substitution) and ostkpr1-4 (with truncated OsTKPR1stop) are defective in pollen exine, Ubisch body and anther cuticle development where ostkpr1-4 display severer phenotypes. Remarkably, OsTKPR1 could produce reduced hydroxylated tetraketide α-pyrone and reduced tetraketide α-pyrone, whereas OsTKPR1P124S fails to produce the latter. Pairwise interaction assays show that mutated OsTKPR1P124S is able to integrate into a recently characterized metabolon, thus its altered catalytic activity is not due to dis-integrity of the metabolon. In short, we find that reduced tetraketide α-pyrone is a key sporopollenin precursor required for normal exine formation, and the conserved 124th proline of OsTKPR1 is essential for the reduction activity. Therefore, this study provided new insights into the sporopollenin precursor constitution critical for exine formation.

Transcriptome analysis provides insight into the regulatory mechanisms underlying pollen germination recovery at normal high ambient temperature in wild banana (Musa itinerans)

Introduction

Cultivated banana are polyploid, with low pollen fertility, and most cultivars are male sterile, which leads to difficulties in banana breeding research. The selection of male parent with excellent resistance and pollen fertility is therefore essential for banana breeding. Wild banana (Musa itinerans) have developed many good characteristics during natural selection and constitute an excellent gene pool for breeding. Therefore, research on wild banana breeding is very important for banana breeding.

Results

In the current analysis, we examined the changes in viability of wild banana pollens at different temperatures by in vitro germination, and found that the germination ability of wild banana pollens cultured at 28°C for 2 days was higher than that of pollens cultured at 23°C (pollens that could not germinate normally under low temperature stress), 24°C (cultured at a constant temperature for 2 days) and 32°C (cultured at a constant temperature for 2 days). To elucidate the molecular mechanisms underlying the germination restoration process in wild banana pollens, we selected the wild banana pollens that had lost its germination ability under low temperature stress (23°C) as the control group (CK) and the wild banana pollens that had recovered its germination ability under constant temperature incubation of 28°C for 2 days as the treatment group (T) for transcriptome sequencing. A total of 921 differentially expressed genes (DEGs) were detected in CK vs T, of which 265 were up-regulated and 656 were down-regulated. The combined analysis of Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) revealed that the activation, metabolism of various substances (lipids, sugars, amino acids) play a major role in restoring pollen germination capacity. TCA cycle and the sesquiterpenoid and triterpenoid biosynthetic pathways were also significantly enriched in the KEGG pathway. And we found that some DEGs may be associated with pollen wall formation, DNA methylation and DNA repair. The cysteine content, free fatty acid (FFA) content, H2O2 content, fructose content, and sucrose content of pollen were increased at treatment of 28°C, while D-Golactose content was decreased. Finally, the GO pathway was enriched for a total of 24 DEGs related to pollen germination, of which 16 DEGs received targeted regulation by 14 MYBs.

Discussions

Our study suggests that the balance between various metabolic processes, pollen wall remodelling, DNA methylation, DNA repairs and regulation of MYBs are essential for germination of wild banana pollens.

Highly sex specific gene expression in Jojoba

BACKGROUND

Dioecious plants have male and female flowers on separate plants. Jojoba is a dioecious plant that is drought-tolerant and native to arid areas. The genome sequence of male and female plants was recently reported and revealed an X and Y chromosome system, with two large male-specific insertions in the Y chromosome.

RESULTS

A total of 16,923 differentially expressed genes (DEG) were identified between the flowers of the male and female jojoba plants. This represented 40% of the annotated genes in the genome. Many genes, including those responsible for plant environmental responses and those encoding transcription factors (TFs), were specific to male or female reproductive organs. Genes involved in plant hormone metabolism were also found to be associated with flower and pollen development. A total of 8938 up-regulated and 7985 down-regulated genes were identified in comparison between male and female flowers, including many novel genes specific to the jojoba plant. The most differentially expressed genes were associated with reproductive organ development. The highest number of DEG were linked with the Y chromosome in male plants. The male specific parts of the Y chromosome encoded 12 very highly expressed genes including 9 novel genes and 3 known genes associated with TFs and a plant hormone which may play an important role in flower development.

CONCLUSION

Many genes, largely with unknown functions, may explain the sexual dimorphisms in jojoba plants and the differentiation of male and female flowers.

CRISPR/Cas9-mediated disruption of CjACOS5 confers no-pollen formation on sugi trees (Cryptomeria japonica D. Don)

Sugi (Cryptomeria japonica D. Don) is an economically important coniferous tree in Japan. However, abundant sugi pollen grains are dispersed and transported by the wind each spring and cause a severe pollen allergy syndrome (Japanese cedar pollinosis). The use of pollen-free sugi that cannot produce pollen has been thought as a countermeasure to Japanese cedar pollinosis. The sugi CjACOS5 gene is an ortholog of Arabidopsis ACOS5 and rice OsACOS12, which encode an acyl-CoA synthetase that is involved in the synthesis of sporopollenin in pollen walls. To generate pollen-free sugi, we mutated CjACOS5 using the CRISPR/Cas9 system. As a result of sugi transformation mediated by Agrobacterium tumefaciens harboring the CjACOS5-targeted CRISPR/Cas9 vector, 1 bp-deleted homo biallelic mutant lines were obtained. Chimeric mutant lines harboring both mutant and wild-type CjACOS5 genes were also generated. The homo biallelic mutant lines had no-pollen in male strobili, whereas chimeric mutant lines had male strobili with or without pollen grains. Our results suggest that CjACOS5 is essential for the production of pollen in sugi and that its disruption is useful for the generation of pollen-free sugi. In addition to conventional transgenic technology, genome editing technology, including CRISPR/Cas9, can confer new traits on sugi.

OsFAR1 is involved in primary fatty alcohol biosynthesis and promotes drought tolerance in rice

MAIN CONCLUSION

OsFAR1 encodes a fatty acyl-CoA reductase involved in biosynthesis of primary alcohols and plays an important role in drought stress response in rice. Cuticular waxes cover the outermost surface of terrestrial plants and contribute to inhibiting nonstomatal water loss and improving plant drought resistance. Primary alcohols are the most abundant components in the leaf cuticular waxes of rice (Oryza sativa), but the biosynthesis and regulation of primary alcohol remain largely unknown in rice. Here, we identified and characterized an OsFAR1 gene belonging to the fatty acyl-CoA reductases (FARs) via a homology-based approach in rice. OsFAR1 was activated by abiotic stresses and abscisic acid, resulting in increased production of primary alcohol in rice. Heterologous expression of OsFAR1 enhanced the amounts of C22:0 and C24:0 primary alcohols in yeast (Saccharomyces cerevisiae) and C24:0 to C32:0 primary alcohols in Arabidopsis. Similarly, OsFAR1 overexpression significantly increased the content of C24:0 to C30:0 primary alcohols on rice leaves. Finally, OsFAR1 overexpression lines exhibited reduced cuticle permeability and enhanced drought tolerance in rice and Arabidopsis. Taken together, our results demonstrate that OsFAR1 is involved in rice primary alcohol biosynthesis and plays an important role in responding to drought and other environmental stresses.

Biosynthesis and transport of pollen coat precursors in angiosperms

The pollen coat is a hydrophobic mixture on the pollen grain surface, which plays an important role in protecting male gametes from various environmental stresses and microorganism attacks, and in pollen-stigma interactions during pollination in angiosperms. An abnormal pollen coat can result in humidity-sensitive genic male sterility (HGMS), which can be used in two-line hybrid crop breeding. Despite the crucial functions of the pollen coat and the application prospect of its mutants, few studies have focused on pollen coat formation. In this Review, the morphology, composition and function of different types of pollen coat are assessed. On the basis of the ultrastructure and development process of the anther wall and exine found in rice and Arabidopsis, the genes and proteins involved in the biosynthesis of pollen coat precursors and the possible transport and regulation process are sorted. Additionally, current challenges and future perspectives, including potential strategies utilizing HGMS genes in heterosis and plant molecular breeding, are highlighted.

Rice anther tapetum: a vital reproductive cell layer for sporopollenin biosynthesis and pollen exine patterning

The tapetum is the innermost layer of the four layers of the rice anther that provides protection and essential nutrients to pollen grain development and delivers precursors for pollen exine formation. The tapetum has a key role in the normal development of pollen grains and tapetal programmed cell death (PCD) that is linked with sporopollenin biosynthesis and transport. Recently, many genes have been identified that are involved in tapetum formation in rice and Arabidopsis. Genetic mutation in PCD-associated genes could affect normal tapetal PCD, which finally leads to aborted pollen grains and male sterility in rice. In this review, we discuss the most recent research on rice tapetum development, including genomic, transcriptomic and proteomic studies. Furthermore, tapetal PCD, sporopollenin biosynthesis, ROS activity for tapetum function and its role in male reproductive development are discussed in detail. This will improve our understanding of the role of the tapetum in male fertility using rice as a model system, and provide information that can be applied in rice hybridization and that of other major crops.

Genome-Wide Identification of the Cytochrome P450 Superfamily Genes and Targeted Editing of BnCYP704B1 Confers Male Sterility in Rapeseed

The cytochrome P450 (CYP450) monooxygenase superfamily, which is involved in the biosynthesis pathways of many primary and secondary metabolites, plays prominent roles in plant growth and development. However, systemic information about CYP450s in Brassica napus (BnCYP450) was previously undiscovered and their biological significance are far from understood. Members of clan 86 CYP450s, such as CYP704Bs, are essential for the formation of pollen exine in plant male reproduction, and the targeted mutagenesis of CYP704B genes has been used to create new male sterile lines in many crops. In the present study, a total of 687 BnCYP450 genes were identified in Brassica napus cultivar "Zhongshuang 11" (ZS11), which has nearly 2.8-fold as many CYP450 members as in Arabidopsis thaliana. It is rationally estimated since Brassica napus is a tetraploid oil plant with a larger genome compared with Arabidopsis thaliana. The BnCYP450 genes were divided into 47 subfamilies and clustered into nine clans. Phylogenetic relationship analysis reveals that CYP86 clan consists of four subfamilies and 109 BnCYP450s. Members of CYP86 clan genes display specific expression profiles in different tissues and in response to ABA and abiotic stresses. Two BnCYP450s within the CYP704 subfamily from CYP86 clan, BnCYP704B1a and BnCYP704B1b, display high similarity to MS26 (Male Sterility 26, also known as CYP704B1). These two BnCYP704B1 genes were specifically expressed in young buds. We then simultaneously knocked-out these two BnCYP704B1 genes through a clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) genome engineering system. The edited plants displayed a pollenless, sterile phenotype in mature anthers, suggesting that we successfully reproduced genic male sterility (GMS, also known as nuclear male sterility) lines in Brassica napus. This study provides a systemic view of BnCYP450s and offers a strategy to facilitate the commercial utility of the CRISPR/Cas9 system for the rapid generation of GMS in rapeseed via knocking-out GMS controlling genes.

Comprehensive Insight into Tapetum-Mediated Pollen Development in Arabidopsis thaliana

In flowering plants, pollen development is a key process that is essential for sexual reproduction and seed set. Molecular and genetic studies indicate that pollen development is coordinatedly regulated by both gametophytic and sporophytic factors. Tapetum, the somatic cell layer adjacent to the developing male meiocytes, plays an essential role during pollen development. In the early anther development stage, the tapetal cells secrete nutrients, proteins, lipids, and enzymes for microsporocytes and microspore development, while initiating programmed cell death to provide critical materials for pollen wall formation in the late stage. Therefore, disrupting tapetum specification, development, or function usually leads to serious defects in pollen development. In this review, we aim to summarize the current understanding of tapetum-mediated pollen development and illuminate the underlying molecular mechanism in Arabidopsis thaliana.

Duplicate mutations of GhCYP450 lead to the production of ms5m6 male sterile line in cotton

The duplicated male sterile genes ms₅m₆ in cotton were map-based cloned and validated by the virus-induced gene silencing assays. Duplicate mutations of the GhCYP450 gene encoding a cytochrome P450 protein are responsible for the male sterility in cotton. The utilization of male sterility in cotton plays a vital role in improving yield and fiber quality. A complete male sterile line (ms₅ms₆) has been extensively used to develop hybrid cotton worldwide. Using Zhongkang-A (ZK-A) developed by transferring Bt and ms₅ms₆ genes into the commercial cultivar Zhongmiansuo 12, the duplicate genes were map-based cloned and confirmed via the virus-induced gene silencing (VIGS) assays. The duplicate mutations of GhCYP450 genes encoding a cytochrome P450 protein were responsible for producing male sterility in ms₅ms₆ in cotton. Sequence alignment showed that GhCYP450-Dt in ZK-A differed in two critical aspects from the fertile wild-type TM-1: GhCYP450-Dt has three amino acid (D98E, E168K, G198R) changes in the coding region and a 7-bp (GGAAAAA) insertion in the promoter domain; GhCYP450-At appears to be premature termination of GhCYP450 translation. Further morphological observation and cytological examination of GhCYP450-silenced plants induced by VIGS exhibited shorter filaments and no mature pollen grains. These results indicate that GhCYP450 is essential for pollen exine formation and pollen development for male fertility. Investigating the mechanisms of ms₅ms₆ male sterility will deepen our understanding of the development and utilization of heterosis.

BrACOS5 mutations induced male sterility via impeding pollen exine formation in Chinese cabbage (Brassica rapa L. ssp. pekinensis)

BrACOS5 mutations led to male sterility of Chinese cabbage verified in three allelic male-sterile mutants. Chinese cabbage (Brassica rapa L. ssp. pekinensis) is one of the major vegetable crops in East Asia, and the utilization of male-sterile line is an important measure for its hybrid seed production. Herein, we isolated three allelic male-sterile mutants, msm1-1, msm1-2 and msm1-3, from an ethyl methane sulfonate (EMS) mutagenized population of Chinese cabbage double-haploid (DH) line 'FT', whose microspores were completely aborted with severely absent exine, and tapetums were abnormally developed. Genetic analyses indicated that the three male-sterile mutants belonged to allelic mutation and were triggered by the same recessive nuclear gene. MutMap-based gene mapping and kompetitive allele-specific PCR (KASP) analysis demonstrated that three different single-nucleotide polymorphisms (SNPs) of BraA09g012710.3C were responsible for the male sterility of msm1-1/2/3, respectively. BraA09g012710.3C is orthologous of Arabidopsis thaliana ACOS5 (AT1G62940), encoding an acyl-CoA synthetase in sporopollenin biosynthesis, and specifically expressed in anther, so we named BraA09g012710.3C as BrACOS5. BrACOS5 localizes to the endoplasmic reticulum (ER). Mutations of BrACOS5 resulted in decreased enzyme activities and altered fatty acid contents in msm1 anthers. As well as the transcript accumulations of putative orthologs involved in sporopollenin biosynthesis were significantly down-regulated excluding BrPKSA. These results provide strong evidence for the integral role of BrACOS5 in conserved sporopollenin biosynthesis pathway and also contribute to uncovering exine development pattern and underlying male sterility mechanism in Chinese cabbage.

Comparative transcriptome analysis provides insight into the important pathways and key genes related to the pollen abortion in the thermo-sensitive genic male sterile line 373S in Brassica napus L

The thermo-sensitive genic male sterility (TGMS) system plays a key role in the production of two-line hybrids in rapeseed (Brassica napus). To uncover key cellular events and genetic regulation associated with TGMS, a combined study using cytological methods and RNA-sequencing analysis was conducted for the rapeseed TGMS line 373S. Cytological studies showed that microspore cytoplasm of 373S plants was condensed, the microspore nucleus was degraded at an early stage, the exine was irregular, and the tapetum developed abnormally, eventually leading to male sterility. RNA-sequencing analysis identified 430 differentially expressed genes (298 upregulated and 132 downregulated) between the fertile and sterile samples. Gene ontology analysis demonstrated that the most highly represented biological processes included sporopollenin biosynthetic process, pollen exine formation, and extracellular matrix assembly. Kyoto encyclopedia of genes and genomes analysis indicated that the enriched pathways included amino acid metabolism, carbohydrate metabolism, and lipid metabolism. Moreover, 26 transcript factors were identified, which may be associated with abnormal tapetum degeneration and exine formation. Subsequently, 19 key genes were selected, which are considered to regulate pollen development and even participate in pollen exine formation. Our results will provide important insight into the molecular mechanisms underlying TGMS in rapeseed.

Transcription factor TaGAMYB from wheat (Triticum aestivum L.) regulates flowering time and fertility in transgenic Arabidopsis thaliana

The loss of TaGAMYB function in Arabidopsis results in abnormal pollen development and leads to decreased fertility. This process may be regulated by microRNAs, which suppress the expression of fatty acid pathway genes. Development of the anthers and pollen is significantly influenced by the transcription factor GAMYB. However, our knowledge of GAMYB in wheat is limited. Here, under fertility and sterility conditions, we identified the distinct transcripts TaGAMYB-d and TaGAMYB-g in thermosensitive genic sterile wheat YanZhan 4110S and confirmed their functions. TaGAMYB-g overexpression decreased the pollen vigor and germination rates, thereby reducing fertility. TaGAMYB-d overexpression lines exhibited early flowering. Due to aberrant pollen germination, the mutant homologous TaGAMYB genes in Arabidopsis thaliana also resulted in lower fertility. Our findings indicate that TaGAMYB controls the fertility and flowering time in transgenic Arabidopsis thaliana.

The genome of Areca catechu provides insights into sex determination of monoecious plants

The areca palm (Areca catechu) has a monoecious spadix, with male flowers on the apical side and females on the basal side. Here, we applied multiomics analysis to investigate sex determination and floral organ development in areca palms. We generated a chromosome-level reference genome of A. catechu with 16 pseudochromosomes, composed of 2.73 Gb and encoding 31 406 genes. Data from RNA-seq and ATAC-seq (assay for transposase accessible chromatin sequencing) suggested that jasmonic acid (JA) synthesis and signal transduction-related genes were differentially expressed between female and male flowers via epigenetic modifications. JA concentration in female flowers was c. 10 times than that in males on the same inflorescence, while JA concentration in hermaphroditic flowers of abnormal inflorescences was about twice that in male flowers of normal inflorescences. JA promotes the development of female flower organs by decreasing the expression of B-function genes, including AGL16, AP3, PIb and PIc. There is also a region on pseudochromosome 15 harboring sex-related genes, including CYP703, LOG, GPAT, AMS and BiP. Among them, CYP703, AMS and BiP were specifically expressed in male flowers.

The ZmMYB84-ZmPKSB regulatory module controls male fertility through modulating anther cuticle-pollen exine trade-off in maize anthers

Anther cuticle and pollen exine are two crucial lipid layers that ensure normal pollen development and pollen-stigma interaction for successful fertilization and seed production in plants. Their formation processes share certain common pathways of lipid biosynthesis and transport across four anther wall layers. However, molecular mechanism underlying a trade-off of lipid-metabolic products to promote the proper formation of the two lipid layers remains elusive. Here, we identified and characterized a maize male-sterility mutant pksb, which displayed denser anther cuticle but thinner pollen exine as well as delayed tapetal degeneration compared with its wild type. Based on map-based cloning and CRISPR/Cas9 mutagenesis, we found that the causal gene (ZmPKSB) of pksb mutant encoded an endoplasmic reticulum (ER)-localized polyketide synthase (PKS) with catalytic activities to malonyl-CoA and midchain-fatty acyl-CoA to generate triketide and tetraketide α-pyrone. A conserved catalytic triad (C171, H320 and N353) was essential for its enzymatic activity. ZmPKSB was specifically expressed in maize anthers from stages S8b to S9-10 with its peak at S9 and was directly activated by a transcription factor ZmMYB84. Moreover, loss function of ZmMYB84 resulted in denser anther cuticle but thinner pollen exine similar to the pksb mutant. The ZmMYB84-ZmPKSB regulatory module controlled a trade-off between anther cuticle and pollen exine formation by altering expression of a series of genes related to biosynthesis and transport of sporopollenin, cutin and wax. These findings provide new insights into the fine-tuning regulation of lipid-metabolic balance to precisely promote anther cuticle and pollen exine formation in plants.

The transcription factor TaGAMYB modulates tapetum and pollen development of TGMS wheat YanZhan 4110S via the gibberellin signaling

Male reproductive development in higher plants experienced a series of complex biological processes, which can be regulated by Gibberellins (GA). The transcriptional factor GAMYB is a crucial component of GA signaling in anther development. However, the mechanism of GAMYB in wheat male reproduction is less understood. Here, we found that the thermo-sensitive genic male sterilitywheat line YanZhan 4110S displayed delayed tapetum programmed cell death and pollen abortive under the hot temperature stress. Combined with RNA-Sequencing data analysis, TaGAMYB associated with fertility conversion was isolated, which was located in the nucleus and highly expressed in fertility anthers. The silencing of TaGAMYB in wheat displayed fertility decline, defects in tapetum, pollen and exine formation, where the abortion characteristics were the same as YanZhan 4110S. In addition, either hot temperature or GA3 treatment in YanZhan 4110S caused the downregulation of TaGAMYB at binucleate stage and trinucleate stage, as well as fertility decrease. Further, the transcription factor TaWRKY2 significantly changed under GA3-treatment and directly interacted with the TaGAMYB promoter by W-box cis-element. Therefore, we suggested that TaGAMYB may be essential for anther development and male fertility, and GA3 activates TaGAMYB by TaWRKY2 to regulate fertility in wheat.

Characterization of Glossy Spike Mutants and Identification of Candidate Genes Regulating Cuticular Wax Synthesis in Barley (Hordeum vulgare L.)

Cuticular waxes comprise the hydrophobic layer that protects crops against nonstomatal water loss and biotic and abiotic stresses. Expanding on our current knowledge of the genes that are involved in cuticular wax biosynthesis and regulation plays an important role in dissecting the processes of cuticular wax metabolism. In this study, we identified the Cer-GN1 barley (Hordeum vulgare L.) mutant that is generated by ethyl methanesulfonate mutagenesis with a glossy spike phenotype that is controlled by a single recessive nuclear gene. A physiological analysis showed that the total cuticular wax loads of Cer-GN1 were one-third that of the progenitor wild-type (WT), and its water loss rate was significantly accelerated (p < 0.05). In addition, Cer-GN1 was defective in the glume’s cuticle according to the toluidine blue dye test, and it was deficient in the tubule-shaped crystals which were observed on the glume surfaces by scanning electron microscopy. Using metabolomics and transcriptomics, we investigated the impacts of cuticular wax composition and waxy regulatory genes on the loss of the glaucous wax in the spikes of Cer-GN1. Among the differential metabolites, we found that 16-hydroxyhexadecanoic acid, which is one of the predominant C16 and C18 fatty acid-derived cutin monomers, was significantly downregulated in Cer-GN1 when it was compared to that of WT. We identified two novel genes that are located on chromosome 4H and are downregulated in Cer-GN1 (HvMSTRG.29184 and HvMSTRG.29185) that encode long-chain fatty acid omega-monooxygenase CYP704B1, which regulates the conversion of C16 palmitic acid to 16-hydroxyhexadecanoic acid. A quantitative real-time PCR revealed that the expression levels of HvMSTRG.29184 and HvMSTRG.29185 were downregulated at 1, 4, 8, 12, and 16 days after the heading stage in Cer-GN1 when it was compared to those of WT. These results suggested that HvMSTRG.29184 and HvMSTRG.29185 have CYP704B1 activity, which could regulate the conversion of C16 palmitic acid to 16-hydroxyhexadecanoic acid in barley. Their downregulation in Cer-GN1 reduced the synthesis of the cuticular wax components and ultimately caused the loss of the glaucous wax in the spikes. It is necessary to verify whether HvMSTRG.29184 and HvMSTRG.29185 truly encode a CYP704B1 that regulates the conversion of C16 palmitic acid to 16-hydroxyhexadecanoic acid in barley.

Cytochrome P450 mono-oxygenase CYP703A2 plays a central role in sporopollenin formation and ms5ms6 fertility in cotton

The double-recessive genic male-sterile (ms) line ms5 ms6 has been used to develop cotton (Gossypium hirsutum) hybrids for many years, but its molecular-genetic basis has remained unclear. Here, we identified the Ms5 and Ms6 loci through map-based cloning and confirmed their function in male sterility through CRISPR/Cas9 gene editing. Ms5 and Ms6 are highly expressed in stages 7-9 anthers and encode the cytochrome P450 mono-oxygenases CYP703A2-A and CYP703A2-D. The ms5 mutant carries a single-nucleotide C-to-T nonsense mutation leading to premature chain termination at amino acid 312 (GhCYP703A2-A^312aa ), and ms6 carries three nonsynonymous substitutions (D98E, E168K, and G198R) and a synonymous mutation (L11L). Enzyme assays showed that GhCYP703A2 proteins hydroxylate fatty acids, and the ms5 (GhCYP703A2-A^312aa ) and ms6 (GhCYP703A2-D^{D98E,E168K,G198R} ) mutant proteins have decreased enzyme activities. Biochemical and lipidomic analyses showed that in ms5 ms6 plants, C12-C18 free fatty acid and phospholipid levels are significantly elevated in stages 7-9 anthers, while stages 8-10 anthers lack sporopollenin fluorescence around the pollen, causing microspore degradation and male sterility. Overall, our characterization uncovered functions of GhCYP703A2 in sporopollenin formation and fertility, providing guidance for creating male-sterile lines to facilitate hybrid cotton production and therefore exploit heterosis for improvement of cotton.

Identification and Characterization of Circular RNAs Involved in the Flower Development and Senescence of Rhododendron delavayi Franch

Floral development and senescence are a crucial determinant for economic and ornamental value. CircRNAs play an essential role in regulating plant growth and development; however, there is no systematic identification of circRNAs during the lifespan of flowers. This study aims to explore the expression profile and functional role of circRNAs in the full flowering stages of Rhododendron delavayi Franch. We carried out transcriptome sequencing of the six stages of Rhododendron delavayi Franch flowers to identify the circular RNA expression profile. In addition, using bioinformatics methods, we explored the functions of circRNAs, including analysis of the circRNA-miRNA-mRNA network, short time-series expression miner (STEM), and so on. We identified 146 circRNAs, of which 79 were differentially expressed from the budding to fading stages. Furthermore, using STEM analysis, one of the 42 circRNA expression model profiles was significantly upregulated during the senescence stage, including 16 circRNAs. Additionally, 7 circRNA-miRNA-mRNA networks were constructed with 10 differentially expressed circRNAs, in which some target mRNA may regulate the development and senescence of the Rhododendron flowers. Finally, by analyzing the correlation between circRNAs and mRNA, combined with existing reports, we proposed that circRNAs play a regulatory role during flower development and senescence by mediating the jasmonate signaling pathway. Overall, these results provide new clues to the potential mechanism of circRNAs acting as novel post-transcriptional regulators in the development and senescence process of flowers.

Sporopollenin-inspired design and synthesis of robust polymeric materials

Sporopollenin is a mechanically robust and chemically inert biopolymer that constitutes the outer protective exine layer of plant spores and pollen grains. Recent investigation of the molecular structure of pine sporopollenin revealed unique monomeric units and inter-unit linkages distinct from other previously known biopolymers, which could be harnessed for new material design. Herein, we report the bioinspired synthesis of a series of sporopollenin analogues. This exercise confirms large portions of our previously proposed pine sporopollenin structural model, while the measured chemical, thermal, and mechanical properties of the synthetic sporopollenins constitute favorable attributes of a new kind of robust material. This study explores a new design framework of robust materials inspired by natural sporopollenins, and provides insights and reagents for future elucidation and engineering of sporopollenin biosynthesis in plants.

Cytosolic glucose-6-phosphate dehydrogenases play a pivotal role in Arabidopsis seed development

Embryo development is essential for seed yield and post-germination growth. Glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme in oxidative pentose phosphate pathway (OPPP), is widely involved in plant development and stress tolerance by providing nicotinamide adenine dinucleotide phosphate (NADPH). In this study, the double mutant (g6pd5/6), overexpression line (G6PD5/6^OE) and complementation line (g6pd5/6^Comp) of cytosolic glucose-6-phosphate dehydrogenases (Cyt-G6PD) were used to investigate Cyt-G6PD roles in embryo development of Arabidopsis. The results showed that the germination rate of g6pd5/6 seeds was delayed in comparison with that of Col-0; moreover, 11.5% of g6pd5/6 seeds did not germinate. The dysfunction of Cyt-G6PD resulted in decreased fresh weight and primary root length of g6pd5/6 seedlings. The height and silique length of g6pd5/6 plants were also decreased. Moreover, the abortion rate of siliques and seeds of g6pd5/6 plants were increased compared with those of Col-0, G6PD5/6^OE and g6pd5/6^Comp lines. However, the dysfunction of Cyt-G6PD did not affect pollen activity; but in g6pd5/6, the embryo development was partially delayed or inhibited. The contents of fatty acids and storage proteins, two main storage materials in Arabidopsis seeds, were decreased in g6pd5/6 seeds. Exogenous application of fatty acids (C18:2; C18:3) alleviated the delayed germination of g6pd5/6 seeds. RT-qPCR results further demonstrated that the early embryo development genes were down-regulated in g6pd5/6. Taken together, Cyt-G6PD plays a pivotal role in plant seed development by regulating the transcriptions of early embryo development genes and the accumulation of storage materials (especially fatty acids).

SWOLLEN TAPETUM AND STERILITY 1 is required for tapetum degeneration and pollen wall formation in rice

The pollen wall is important for protecting the male gametophyte and for fertilization. The lipid components of the pollen wall are mainly synthesized and transported from the sporophytic tapetum. Although several factors related to lipid biosynthesis have been characterized, the molecular mechanisms underlying lipid biosynthesis during pollen development in rice (Oryza sativa L.) remain elusive. Here, we showed that mutation in the SWOLLEN TAPETUM AND STERILITY 1 (STS1) gene causes delayed tapetum degradation and aborted pollen wall formation in rice. STS1 encodes an endoplasmic reticulum (ER)-localized protein that contains domain of unknown function (DUF) 726 and exhibits lipase activity. Lipidomic and transcriptomic analyses showed that STS1 is involved in anther lipid homeostasis. Moreover, STS1 interacts with Polyketide Synthase 2 (OsPKS2) and Acyl-CoA Synthetase 12 (OsACOS12), two enzymes crucial in lipidic sporopollenin biosynthesis in pollen wall formation, suggesting a potentially lipidic metabolon for sporopollenin biosynthesis in rice. Collectively, our results indicate that STS1 is an important factor for lipid biosynthesis in reproduction, providing a target for the artificial control of male fertility in hybrid rice breeding and insight into the function of DUF726-containing protein in plants.

Transcriptome analysis of flower bud identified genes associated with pistil abortions between long branches and spur twigs in apricots (Prunus armeniaca L.)

Pistil abortions of flower buds occur frequently in many apricot cultivars, especially in long branches. However, the molecular mechanism underlying pistil abortion in apricots remains unclear. To better understand the molecular mechanism of pistil abortions between long branches and spur twigs, paraffin sections and high-throughput sequencing technology were employed to analyze the expression patterns of genes associated with pistil abortions during later flower bud development stage in ‘Shajinhong’ apricot. The result of stage III (separation of bud scales) was the critical stage of pistil abortion in apricots. A total of 163 differentially expressed genes were identified as candidate genes related to pistil abortion in long branches. These genes are implicated in programmed cell death, hormone signaling, cell wall degeneration, and the carbohydrate metabolism pathway. The results showed that the up-regulation of gene expression of Xyloglucan endotransglucosylase/hydrolase and β-glucosidase in flower buds might be the direct cause of cell wall breakdown and pistil necrosis in long branches. We hypothesize that there is a molecular relationship between pistil abortion before blooming and cellulose degradation, and then carbohydrate transport in the case of carbon deficiency in long branches. Our work provides new insights into cellulose degradation in abortion pistils and valuable information on flower development in apricots, and also provides a useful reference for cultivation regulation in apricot or other fruit crops.

Identification of long non-coding RNAs and microRNAs involved in anther development in the tropical Camellia oleifera

BACKGROUND

Explored the molecular science of anther development is important for improving productivity and overall yield of crops. Although the role of regulatory RNAs, including long non-coding RNAs (lncRNAs) and microRNAs (miRNAs), in regulating anther development has been established, their identities and functions in Camellia oleifera, an important industrial crop, have yet not been clearly explored. Here, we report the identification and characterization of genes, lncRNAs and miRNAs during three stages of the tropical C. oleifera anther development by single-molecule real-time sequencing, RNA sequencing and small RNA sequencing, respectively.

RESULTS

These stages, viz. the pollen mother cells stage, tetrad stage and uninucleate pollen stage, were identified by analyzing paraffin sections of floral buds during rapid expansion periods. A total of 18,393 transcripts, 414 putative lncRNAs and 372 miRNAs were identified, of which 5,324 genes, 115 lncRNAs, and 44 miRNAs were differentially accumulated across three developmental stages. Of these, 44 and 92 genes were predicted be regulated by 37 and 30 differentially accumulated lncRNAs and miRNAs, respectively. Additionally, 42 differentially accumulated lncRNAs were predicted as targets of 27 miRNAs. Gene ontology enrichment indicated that potential target genes of lncRNAs were enriched in photosystem II, regulation of autophagy and carbohydrate phosphatase activity, which are essential for anther development. Functional annotation of genes targeted by miRNAs indicated that they are relevant to transcription and metabolic processes that play important roles in microspore development. An interaction network was built with 2 lncRNAs, 6 miRNAs and 10 mRNAs. Among these, miR396 and miR156 family were up-regulated, while their targets, genes (GROWTH REGULATING FACTORS and SQUAMOSA PROMOTER BINDING PROTEIN-LIKE genes) and lncRNAs, were down-regulated. Further, the trans-regulated targets of these lncRNAs, like wall-associated kinase2 and phosphomannose isomerase1, are involved in pollen wall formation during anther development.

CONCLUSIONS

This study unravels lncRNAs, miRNAs and miRNA-lncRNA-mRNA networks involved in development of anthers of the tropical C. oleifera lays a theoretical foundation for further elucidation of regulatory roles of lncRNAs and miRNAs in anther development.

An evolutionary look into the history of lentil reveals unexpected diversity

The characterization and preservation of genetic variation in crops is critical to meeting the challenges of breeding in the face of changing climates and markets. In recent years, the use of single nucleotide polymorphisms (SNPs) has become routine, allowing us to understand the population structure, find divergent lines for crosses, and illuminate the origin of crops. However, the focus on SNPs overlooks other forms of variation, such as copy number variation (CNVs). Lentil is the third most important cold-season legume and was domesticated in the Fertile Crescent. We genotyped 324 accessions that represent its global diversity, and using both SNPs and CNVs, we dissected the population structure and genetic variation, and identified candidate genes. Eight clusters were detected, most of them located in or near the Fertile Crescent, even though different clusters were present in distinct regions. The cluster from South Asia was particularly differentiated and presented low diversity, contrasting with the clusters from the Mediterranean and the northern temperate. Accessions from North America were mainly assigned to one cluster and were highly diverse, reflecting the efforts of breeding programs to integrate variation. Thirty-three genes were identified as candidates under selection and among their functions were sporopollenin synthesis in pollen, a component of chlorophyll B reductase that partially determines the antenna size, and two genes related to the import system of chloroplasts. Eleven percent of all lentil genes and 21% of lentil disease resistance genes were affected by CNVs. The gene categories overrepresented in these genes were "enzymes," "Cell Wall Organization," and "external stimuli response." All the genes found in the latter were associated with pathogen response. CNVs provided information about population structure and might have played a role in adaptation. The incorporation of CNVs in diversity studies is needed for a broader understanding of how they evolve and contribute to domestication.

The Transcription Factors TaTDRL and TaMYB103 Synergistically Activate the Expression of TAA1a in Wheat, Which Positively Regulates the Development of Microspore in Arabidopsis

Pollen fertility plays an important role in the application of heterosis in wheat (Triticum aestivum L.). However, the key genes and mechanisms underlying pollen abortion in K-type male sterility remain unclear. TAA1a is an essential gene for pollen development in wheat. Here, we explored the mechanism involved in its transcriptional regulation during pollen development, focusing on a 1315-bp promoter region. Several cis-acting elements were identified in the TAA1a promoter, including binding motifs for Arabidopsis thaliana AtAMS and AtMYB103 (CANNTG and CCAACC, respectively). Evolutionary analysis indicated that TaTDRL and TaMYB103 were the T. aestivum homologs of AtAMS and AtMYB103, respectively, and encoded nucleus-localized transcription factors containing 557 and 352 amino acids, respectively. TaTDRL and TaMYB103 were specifically expressed in wheat anthers, and their expression levels were highest in the early uninucleate stage; this expression pattern was consistent with that of TAA1a. Meanwhile, we found that TaTDRL and TaMYB03 directly interacted, as evidenced by yeast two-hybrid and bimolecular fluorescence complementation assays, while yeast one-hybrid and dual-luciferase assays revealed that both TaTDRL and TaMYB103 could bind the TAA1a promoter and synergistically increase its transcriptional activity. Furthermore, TaTDRL-EAR and TaMYB103-EAR transgenic Arabidopsis plants displayed abnormal microspore morphology, reduced pollen viability, and lowered seed setting rates. Additionally, the expression of AtMS2, a TAA1a homolog, was significantly lower in the two repressor lines than in the corresponding overexpression lines or WT plants. In summary, we identified a potential transcriptional regulatory mechanism associated with wheat pollen development.

Transcriptome Profiling Identifies Candidate Genes Contributing to Male and Female Gamete Development in Synthetic Brassica Allohexaploids

Polyploidy plays a crucial role in plant evolution and speciation. The development of male and female gametes is essential to the reproductive capacity of polyploids, but their gene expression pattern has not been fully explored in newly established polyploids. The present study aimed to reveal a detailed atlas of gene expression for gamete development in newly synthetic Brassica allohexaploids that are not naturally existing species. Comparative transcriptome profiling between developing anthers (staged from meiosis to mature pollen) and ovules (staged from meiosis to mature embryo sac) was performed using RNA-Seq analysis. A total of 8676, 9775 and 4553 upregulated differentially expressed genes (DEGs) were identified for the development of both gametes, for male-only, and for female-only gamete development, respectively, in the synthetic Brassica allohexaploids. By combining gene ontology (GO) biological process analysis and data from the published literature, we identified 37 candidate genes for DNA double-strand break formation, synapsis and the crossover of homologous recombination during male and female meiosis and 51 candidate genes for tapetum development, sporopollenin biosynthesis and pollen wall development in male gamete development. Furthermore, 23 candidate genes for mitotic progression, nuclear positioning and cell specification and development were enriched in female gamete development. This study lays a good foundation for revealing the molecular regulation of genes related to male and female gamete development in Brassica allohexaploids and provides more resourceful genetic information on the reproductive biology of Brassica polyploid breeding.

Proteomic and Phosphoproteomic Analyses Reveal a Complex Network Regulating Pollen Abortion and Potential Candidate Proteins in TCMS Wheat

Thermosensitive sterile lines are natural materials for exploring the effects of anther development on male fertility. To study the possible molecular mechanisms regulating protein activity during the induction of male sterility, proteomic and phosphoproteomic analyses with tandem mass tags (TMTs) were used to study the binucleate anther of the thermosensitive sterile wheat line YS3038. A total of 9072 proteins, including 5019 phosphoproteins, were identified. Enrichment analyses of differentially abundant proteins (DAPs) and phosphoproteins (DAPPs) in metabolic pathways showed that both were mainly related to energy metabolism. Soluble sugar and ATP content were significantly decreased, free fatty acid content was significantly increased, and ROS was abnormally accumulated in male sterile YS3038-A. In addition, 233 kinase-substrate pairs involved in potential phosphorylation control networks were predicted to regulate fertility. Candidate proteins were identified, and a quantitative real-time polymerase chain reaction (qRT-PCR) analysis was used to validate the TMT results. TaPDCD5 is likely to be involved in fertility conversion of YS3038 by barley stripe mosaic virus-induced gene silencing (BSMV-VIGS). Our data provide new insights into the mechanism of TCMS, which has value for identifying potential candidate proteins associated with the formation or abortion of pollen and promotion of wheat heterosis utilization.

Degradation of de-esterified pctin/homogalacturonan by the polygalacturonase GhNSP is necessary for pollen exine formation and male fertility in cotton

The pollen wall exine provides a protective layer for the male gametophyte and is largely composed of sporopollenin, which comprises fatty acid derivatives and phenolics. However, the biochemical nature of the external exine is poorly understood. Here, we show that the male sterile line 1355A of cotton mutated in NO SPINE POLLEN (GhNSP) leads to defective exine formation. The GhNSP locus was identified through map-based cloning and confirmed by genetic analysis (co-segregation test and allele prediction using the CRISPR/Cas9 system). In situ hybridization showed that GhNSP is highly expressed in tapetum. GhNSP encodes a polygalacturonase protein homologous to AtQRT3, which suggests a function for polygalacturonase in pollen exine formation. These results indicate that GhNSP is functionally different from AtQRT3, the latter has the function of microspore separation. Biochemical analysis showed that the percentage of de-esterified pectin was significantly increased in the 1355A anthers at developmental stage 8. Furthermore, immunofluorescence studies using antibodies to the de-esterified and esterified homogalacturonan (JIM5 and JIM7) showed that the Ghnsp mutant exhibits abundant of de-esterified homogalacturonan in the tapetum and exine, coupled with defective exine formation. The characterization of GhNSP provides new understanding of the role of polygalacturonase and de-esterified homogalacturonan in pollen exine formation.