CsAP3: A Cucumber Homolog to Arabidopsis APETALA3 with Novel Characteristics

Revisit and explore the ethylene-independent mechanism of sex expression in cucumber (Cucumis sativus)

KEY MESSAGE

This review provides a thorough and comprehensive perspective on the topic of cucumber sexual expression. Specifically, insights into sex expression mediated by pathways other than ethylene are highlighted. Cucumber (Cucumis sativus L.) is a common and important commercial crop that is cultivated and consumed worldwide. Additionally, this species is commonly used as a model for investigating plant sex expression. Cucumbers exhibit a variety of floral arrangements, comprising male, female, and hermaphroditic (bisexual) flowers. Generally, cucumber plants that produce female flowers are typically preferred due to their significant impact on the overall output. Various environmental conditions, such as temperature, light quality, and photoperiod, have been also shown to influence the sex expression in this species. Multiple lines of evidence indicate that ethylene and its biosynthesis genes are crucial in regulating cucumber sex expression. Gibberellins, another well-known phytohormone, can similarly influence cucumber sex expression via an ethylene-independent route. Further studies employing the next-generation sequencing technology also visualized a deeper slice of the molecular mechanism such as the role of the cell cycle program in the cucumber sex expression. This review aims to provide an overview of the sex expression of cucumber including its underlying molecular mechanism and regulatory aspects based on recent investigations.

Genic male and female sterility in vegetable crops

Vegetable crops are greatly appreciated for their beneficial nutritional and health components. Hybrid seeds are widely used in vegetable crops for advantages such as high yield and improved resistance, which require the participation of male (stamen) and female (pistil) reproductive organs. Male- or female-sterile plants are commonly used for production of hybrid seeds or seedless fruits in vegetables. In this review we will focus on the types of genic male sterility and factors affecting female fertility, summarize typical gene function and research progress related to reproductive organ identity and sporophyte and gametophyte development in vegetable crops [mainly tomato (Solanum lycopersicum) and cucumber (Cucumis sativus)], and discuss the research trends and application perspectives of the sterile trait in vegetable breeding and hybrid production, in order to provide a reference for fertility-related germplasm innovation.

The bHLH1-DTX35/DFR module regulates pollen fertility by promoting flavonoid biosynthesis in Capsicum annuum L

High pollen fertility can ensure the yield and efficiency of breeding work, but factors that affect the fertility of pepper pollen have not been studied extensively. In this work, we screened the reduced pollen fertility 1 (rpf1) mutant of Capsicum annuum with reduced pollen fertility and yellow anthers from an EMS (ethyl methanesulfonate)-mutagenized pepper population. Through construction of an F ₂ population followed by BSA (bulked segregant analysis) mapping and KASP genotyping, we identified CabHLH1 as a candidate gene for control of this trait. A G → A mutation at a splice acceptor site in CabHLH1 causes a frameshift mutation in the mutant, and the translated protein is terminated prematurely. Previous studies on CabHLH1 have focused on the regulation of flavonoid synthesis. Here, we found that CabHLH1 also has an important effect on pollen fertility. Pollen vigor, anther flavonoid content, and seed number were lower in CabHLH1-silenced pepper plants, whereas anther H₂O₂ and MDA (malondialdehyde) contents were higher. RNA-seq analyses showed that expression of the flavonoid synthesis genes DFR, ANS, and RT was significantly reduced in anthers of CabHLH1-silenced plants and rpf1 plants, as was the expression of DTX35, a gene related to pollen fertility and flavonoid transport. Yeast one-hybrid and dual-luciferase reporter assays showed that CabHLH1 can directly bind to the promoters of DTX35 and DFR and activate their expression. These results indicate that CabHLH1 regulates reactive oxygen species homeostasis by promoting the synthesis of anther flavonoids and acts as a positive regulator of pepper pollen fertility.

Gain-of-function of the 1-aminocyclopropane-1-carboxylate synthase gene ACS1G induces female flower development in cucumber gynoecy

Unisexual flowers provide a useful system for studying plant sex determination. In cucumber (Cucumis sativus L.), three major Mendelian loci control unisexual flower development, Female (F), androecious [a; 1-aminocyclopropane-1-carboxylate {ACC} synthase 11, acs11], and Monoecious (M; ACS2), referred to here as the Female, Androecious, Monoecious (FAM) model, in combination with two genes, gynoecious (g, the WIP family C2H2 zinc finger transcription factor gene WIP1) and the ethylene biosynthetic gene ACC oxidase 2 (ACO2). The F locus, conferring gynoecy and the potential for increasing fruit yield, is defined by a 30.2-kb tandem duplication containing three genes. However, the gene that determines the Female phenotype, and its mechanism, remains unknown. Here, we created a set of mutants and revealed that ACS1G is responsible for gynoecy conferred by the F locus. The duplication resulted in ACS1G acquiring a new promoter and expression pattern; in plants carrying the F locus duplication, ACS1G is expressed early in floral bud development, where it functions with ACO2 to generate an ethylene burst. The resulting ethylene represses WIP1 and activates ACS2 to initiate gynoecy. This early ACS1G expression bypasses the need for ACS11 to produce ethylene, thereby establishing a dominant pathway for female floral development. Based on these findings, we propose a model for how these ethylene biosynthesis genes cooperate to control unisexual flower development in cucumber.

Transcriptomic analysis reveals ethylene signal transduction genes involved in pistil development of pumpkin

Development of female flowers is an important process that directly affects the yield of Cucubits. Little information is available on the sex determination and development of female flowers in pumpkin, a typical monoecious plant. In the present study, we used aborted and normal pistils of pumpkin for RNA-Seq analysis and determined the differentially expressed genes (DEGs) to gain insights into the molecular mechanism underlying pistil development in pumpkin. A total of 3,817 DEGs were identified, among which 1,341 were upregulated and 2,476 were downregulated. The results of transcriptome analysis were confirmed by real-time quantitative RT-PCR. KEGG enrichment analysis showed that the DEGs were significantly enriched in plant hormone signal transduction and phenylpropanoid biosynthesis pathway. Eighty-four DEGs were enriched in the plant hormone signal transduction pathway, which accounted for 12.54% of the significant DEGs, and most of them were annotated as predicted ethylene responsive or insensitive transcription factor genes. Furthermore, the expression levels of four ethylene signal transduction genes in different flower structures (female calyx, pistil, male calyx, stamen, leaf, and ovary) were investigated. The ethyleneresponsive DNA binding factor, ERDBF3, and ethylene responsive transcription factor, ERTF10, showed the highest expression in pistils and the lowest expression in stamens, and their expression levels were 78- and 162-times more than that in stamens, respectively. These results suggest that plant hormone signal transduction genes, especially ethylene signal transduction genes, play an important role in the development of pistils in pumpkin. Our study provides a theoretical basis for further understanding of the mechanism of regulation of ethylene signal transduction genes in pistil development and sex determination in pumpkin.

A Bridge to the World

Zhi-Hong Xu is a plant physiologist who studied botany at Peking University (1959–1965). He joined the Shanghai Institute of Plant Physiology (SIPP), Chinese Academy of Sciences (CAS), as a graduate student in 1965. He recalls what has happened for the institute, during the Cultural Revolution, and he witnessed the spring of science eventually coming to China. Xu was a visiting scholar at the John Innes Institute and in the Department of Botany at Nottingham University in the United Kingdom (1979–1981). He became deputy director of SIPP in 1983 and director in 1991; he also chaired the State Key Laboratory of Plant Molecular Genetics SIPP (1988–1996). He worked as a visiting scientist in the Institute of Molecular and Cell Biology, National University of Singapore, for three months each year (1989–1992). He served as vice president of CAS (1992–2002) and as president of Peking University (1999–2008). Over these periods he was heavily involved in the design and implementation of major scientific projects in life sciences and agriculture in China. He is an academician of CAS and member of the Academy of Sciences for the Developing World. His scientific contributions mainly cover plant tissue culture, hormone mechanism in development, as well as plant developmental response to environment. Xu, as a scientist and leader who has made an impact in the community, called up a lot of excellent young scientists returning to China. His efforts have promoted the fast development of China's plant and agricultural sciences.

The MADS-Box Gene CsSHP Participates in Fruit Maturation and Floral Organ Development in Cucumber

Cucumber is an important vegetable crop bearing fleshy pepo fruit harvested immature. Fruits left unpicked in time during summer production, as well as unfavorable environmental conditions during post-harvest shelf, will cause cucumber fruits to turn yellow and ripen, and thus impair the market value. Identification of maturity-related genes is of great agricultural and economic importance for cucumber production. Here, we isolated and characterized a MADS-box gene, Cucumis sativus SHATTERPROOF (CsSHP) in cucumber. Expression analysis indicated that CsSHP was specifically enriched in reproductive organs including stamens and carpels. Ectopic expression of CsSHP was unable to rescue the indehiscence silique phenotype of shp1 shp2 mutant plant in Arabidopsis. Instead, overexpression of CsSHP resulted in early flowering, precocious phenotypes, and capelloid organs in wild-type Arabidopsis. Biochemical analysis indicated that CsSHP directly interacted with cucumber SEPALLATA (SEP) proteins. CsSHP expression increased significantly during the yellowing stage of cucumber ripening, and was induced by exogenous application of abscisic acid (ABA). Therefore, CsSHP may participate in fruit maturation through the ABA pathway and floral organ specification via interaction with CsSEPs to form protein complex in cucumber.

Genome-wide identification and expression profile of the MADS-box gene family in Erigeron breviscapus

The MADS-box gene family encodes transcription factors with many biological functions that extensively regulate plant growth, development and reproduction. Erigeron breviscapus is a medicinal herb used widely in traditional Chinese medicine, and is believed to improve blood circulation and ameliorate platelet coagulation. In order to gain a detailed understanding of how transcription factor expression may regulate the growth of this potentially important medicinal plant, a genome-wide analysis of the MADS-box gene family of E. breviscapus is needed. In the present study, 44 MADS-box genes were identified in E. breviscapus and categorized into five subgroups (MIKC, Mα, Mβ, Mγ and Mδ) according to their phylogenetic relationships with the Arabidopsis MADS-box genes. Additionally, the functional domain, subcellular location and motif compositions of the E. breviscapus MADS-box gene products were characterized. The expression levels for each of the E. breviscapus MADS-box (EbMADS) genes were analyzed in flower, leaf, stem and root organs, and showed that the majority of EbMADS genes were expressed in flowers. Meanwhile, some MADS genes were found to express high levels in leaf, stem and root, indicating that the MADS-box genes are involved in various aspects of the physiological and developmental processes of the E. breviscapus. The results from gene expression analysis under different pollination treatments revealed that the MADS-box genes were highly expressed after non-pollinated treatment. To the best of our knowledge, this study describes the first genome-wide analysis of the E. breviscapus MADS-box gene family, and the results provide valuable information for understanding of the classification, cloning and putative functions of the MADS-box family.

BpAP1 directly regulates BpDEF to promote male inflorescence formation in Betula platyphylla × B. pendula

Flowering is a crucial process for plants that is under complex genetic control. AP1 acts as an organizer and a switch for the transition from vegetative to reproductive growth. In our previous study, we found that overexpression of BpAP1 significantly promoted the formation of male inflorescence in birch (Betula platyphlla × B. pendula). In this study, we aimed at investigating the molecular regulatory mechanism of BpAP1 during the process of male inflorescence formation in birch. We found that overexpression of BpAP1 affected the expression of many flowering-related genes, and had significant effect on B class MADS-box genes. A BpAP1-mediated gene regulatory network was constructed and B class gene BpDEF was finally predicted as a key target gene of BpAP1. Chromatin immunoprecipitation results indicated that BpAP1 could directly regulate BpDEF during the process of male inflorescence formation. Yeast one-hybrid assays and its validation in tobacco results suggested that BpAP1 regulated BpDEF via binding to a consensus DNA sequence known as CArG box. Gene function analysis of BpDEF indicated that BpDEF may function in sex-determination, and in particular specify the identity of male inflorescence in birch. Our results provide valuable clues for our understanding of the molecular mechanism of BpAP1 during the process of male inflorescence formation in birch.

Integrative Analysis of Transcriptomic and Methylomic Data in Photoperiod-Dependent Regulation of Cucumber Sex Expression

The cucumber (Cucumis sativus) is characterized by its diversity and seasonal plasticity in sexual type. A long day length condition significantly decreased the cucumber female flower ratio by 17.7-52.9%, and the effect of photoperiod treatment is more significant under low temperature than under high temperature. Transcriptome analysis indicates that the photoperiod treatment preferentially significantly influenced flower development processes, particularly MADS-box genes in shoot apices. The long-day treatment resulted in predominantly transposable element (TE)- and gene-associated CHH-types of DNA methylation changes. Nevertheless, there was significant enrichment of CG- and CHG-types of DNA methylation changes nearing transcription start sites (TSSs)/transcription end sites (TESs) and gene bodies, respectively. Predominantly negative association between differentially methylated regions (DMRs) and differentially expressed genes (DEGs) were observed which implied epiregulation of DEGs. Two MADS-box genes that were significantly downregulated by long photoperiod showed significant hypermethylation in promoter regions that is essentially TE-rich. This study indicates MADS-box genes which are partially regulated by promoter methylation state may mediate photoperiod-dependent regulation of cucumber sex expression.

Temperature and photoperiod changes affect cucumber sex expression by different epigenetic regulations

BACKGROUND

Cucumbers (Cucumis sativus) are known for their plasticity in sex expression. DNA methylation status determines gene activity but is susceptible to environmental condition changes. Thus, DNA methylation-based epigenetic regulation may at least partially account for the instability of cucumber sex expression. Do temperature and photoperiod that are the two most important environmental factors have equal effect on cucumber sex expression by similar epigenetic regulation mechanism? To answer this question, we did a two-factor experiment of temperature and photoperiod and generated methylome and transcriptome data from cucumber shoot apices.

RESULTS

The seasonal change in the femaleness of a cucumber core germplasm collection was investigated over five consecutive years. As a result, 71.3% of the 359 cucumber accessions significantly decreased their femaleness in early autumn when compared with spring. High temperature and long-day photoperiod treatments, which mimic early autumn conditions, are both unfavorable for female flower formation, and temperature is the predominant factor. High temperatures and long-day treatments both predominantly resulted in hypermethylation compared to demethylation, and temperature effect was decisive. The targeted cytosines shared in high-temperature and long-day photoperiod treatment showed the same change in DNA methylation level. Moreover, differentially expressed TEs (DETs) and the predicted epiregulation sites were clustered across chromosomes, and importantly, these sites were reproducible among different treatments. Essentially, the photoperiod treatment preferentially and significantly influenced flower development processes, while temperature treatment produced stronger responses from phytohormone-pathway-related genes. Cucumber AGAMOUS was likely epicontrolled exclusively by photoperiod while CAULIFLOWER A and CsACO₃ were likely epicontrolled by both photoperiod and temperature.

CONCLUSIONS

Seasonal change of sex expression is a germplasm-wide phenomenon in cucumbers. High temperature and long-day photoperiod might have the same effect on the methylome via the same mechanism of gene-TE interaction but resulted in different epicontrol sites that account for different mechanisms between temperature- and photoperiod-dependent sex expression changes.

Ethylene responsive factor ERF110 mediates ethylene-regulated transcription of a sex determination-related orthologous gene in two Cucumis species

In plants, unisexual flowers derived from developmental sex determination form separate stamens and pistils that facilitate cross pollination. In cucumber and melon, ethylene plays a key role in sex determination. Six sex determination-related genes have been identified in ethylene biosynthesis in these Cucumis species. The interactions among these genes are thought to involve ethylene signaling; however, the underlying mechanism of regulation remains unknown. In this study, hormone treatment and qPCR assays were used to confirm expression of these sex determination-related genes in cucumber and melon is ethylene sensitive. RNA-Seq analysis subsequently helped identify the ethylene responsive factor (ERF) gene, CsERF110, related to ethylene signaling and sex determination. CsERF110 and its melon ortholog, CmERF110, shared a conserved AP2/ERF domain and showed ethylene-sensitive expression. Yeast one-hybrid and ChIP-PCR assays further indicated that CsERF110 bound to at least two sites in the promoter fragment of CsACS11, while transient transformation analysis showed that CsERF110 and CmERF110 enhance CsACS11 and CmACS11 promoter activity, respectively. Taken together, these findings suggest that CsERF110 and CmERF110 respond to ethylene signaling, mediating ethylene-regulated transcription of CsACS11 and CmACS11 in cucumber and melon, respectively. Furthermore, the mechanism involved in its regulation is thought to be conserved in these two Cucumis species.

CsLFY is required for shoot meristem maintenance via interaction with WUSCHEL in cucumber (Cucumis sativus)

Cucumber (Cucumis sativus) is an agronomically important vegetable with indeterminant growth habit, in which leaves are produced from the shoot apical meristem (SAM), and unisexual flowers are generated from the leaf axils. LEAFY (LFY) and its homologs have been shown to play important roles in promoting flower development and branching. The LFY homolog gene CsLFY was cloned in cucumber. Molecular biology, developmental biology and biochemical tools were combined to explore the biological function of the LFY homologous gene CsLFY in cucumber. CsLFY was expressed in the SAM, floral meristem and floral organ primordia. Ectopic expression of CsLFY rescued the phenotype of the lfy-5 mutant in Arabidopsis. Knockdown of CsLFY by RNA interference (RNAi) led to defective shoot development and premature discontinuance of leaf initiation in cucumber. Transcription of CsWUS and putative CsLFY target genes including CsAP3 and CUM1 were significantly reduced in the CsLFY-RNAi lines. Further biochemical analyses indicated that CsLFY physically interacts with CsWUS in cucumber. These data suggested that CsLFY has a novel function in regulating shoot meristem maintenance through interaction with CsWUS, and promotes flower development via activation of CsAP3 and CUM1 in cucumber.

Isolation and Functional Characterization of a Floral Repressor, BcMAF1, From Pak-choi (Brassica rapa ssp. Chinensis)

MADS-box genes form a large gene family in plants and are involved in multiple biological processes, such as flowering. However, the regulation mechanism of MADS-box genes in flowering remains unresolved, especially under short-term cold conditions. In the present study, we isolated BcMAF1, a Pak-choi (Brassica rapa ssp. Chinensis) MADS AFFECTING FLOWERING (MAF), as a floral repressor and functionally characterized BcMAF1 in Arabidopsis and Pak-choi. Subcellular localization and sequence analysis indicated that BcMAF1 was a nuclear protein and contained a conserved MADS-box domain. Expression analysis revealed that BcMAF1 had higher expression levels in leaves, stems, and petals, and could be induced by short-term cold conditions in Pak-choi. Overexpressing BcMAF1 in Arabidopsis showed that BcMAF1 had a negative function in regulating flowering, which was further confirmed by silencing endogenous BcMAF1 in Pak-choi. In addition, qPCR results showed that AtAP3 expression was reduced and AtMAF2 expression was induced in BcMAF1-overexpressing Arabidopsis. Meanwhile, BcAP3 transcript was up-regulated and BcMAF2 transcript was down-regulated in BcMAF1-silencing Pak-choi. Yeast one-hybrid and dual luciferase transient assays showed that BcMAF1 could bind to the promoters of BcAP3 and BcMAF2. These results indicated that BcAP3 and BcMAF2 might be the targets of BcMAF1. Taken together, our results suggested that BcMAF1 could negatively regulate flowering by directly activating BcMAF2 and repressing BcAP3.

Transcription Factor OsTGA10 Is a Target of the MADS Protein OsMADS8 and Is Required for Tapetum Development

This study aimed at elucidating regulatory components behind floral organ identity determination and tissue development. It remains unclear how organ identity proteins facilitate development of organ primordia into tissues with a determined identity, even though it has long been accepted that floral organ identity is genetically determined by interaction of identity genes according to the ABC model. Using the chromatin immunoprecipitation sequencing technique, we identified OsTGA10, encoding a bZIP transcription factor, as a target of the MADS box protein OsMADS8, which is annotated as an E-class organ identity protein. We characterized the function of OsTGA10 using genetic and molecular analyses. OsTGA10 was preferentially expressed during stamen development, and mutation of OsTGA10 resulted in male sterility. OsTGA10 was required for tapetum development and functioned by interacting with known tapetum genes. In addition, in ostga10 stamens, the hallmark cell wall thickening of the endothecium was defective. Our findings suggest that OsTGA10 plays a mediator role between organ identity determination and tapetum development in rice stamen development, between tapetum development and microspore development, and between various regulatory components required for tapetum development. Furthermore, the defective endothecium in ostga10 implies that cell wall thickening of endothecium is dependent on tapetum development.

Differential Gene Expression Caused by the F and M Loci Provides Insight Into Ethylene-Mediated Female Flower Differentiation in Cucumber

In cucumber (Cucumis sativus L.), the differentiation and development of female flowers are important processes that directly affect the fruit yield and quality. Sex differentiation is mainly controlled by three ethylene synthase genes, F (CsACS1G), M (CsACS2), and A (CsACS11). Thus, ethylene plays a key role in the sex differentiation in cucumber. The "one-hormone hypothesis" posits that F and M regulate the ethylene levels and initiate female flower development in cucumber. Nonetheless, the precise molecular mechanism of this process remains elusive. To investigate the mechanism by which F and M regulate the sex phenotype, three cucumber near-isogenic lines, namely H34 (FFmmAA, hermaphroditic), G12 (FFMMAA, gynoecious), and M12 (ffMMAA, monoecious), with different F and M loci were generated. The transcriptomic analysis of the apical shoots revealed that the expression of the B-class floral homeotic genes, CsPI (Csa4G358770) and CsAP3 (Csa3G865440), was immensely suppressed in G12 (100% female flowers) but highly expressed in M12 (∼90% male flowers). In contrast, CAG2 (Csa1G467100), which is an AG-like C-class floral homeotic gene, was specifically highly expressed in G12. Thus, the initiation of female flowers is likely to be caused by the downregulation of B-class and upregulation of C-class genes by ethylene production in the floral primordium. Additionally, CsERF31, which was highly expressed in G12, showed temporal and spatial expression patterns similar to those of M and responded to the ethylene-related chemical treatments. The biochemical experiments further demonstrated that CsERF31 could directly bind the promoter of M and promote its expression. Thus, CsERF31 responded to the ethylene signal derived from F and mediated the positive feedback regulation of ethylene by activating M expression, which offers an extended "one-hormone hypothesis" of sex differentiation in cucumber.