The loss-of-function GLABROUS 3 mutation in cucumber is due to LTR-retrotransposon insertion in a class IV HD-ZIP transcription factor gene CsGL3 that is epistatic over CsGL1

Identification and functional analysis of an LTR retrotransposon insertion in CsPHYB associated with early senescence in cucumber (Cucumis sativus L.)

Early senescence in plants significantly affects photosynthetic efficiency, crop yield, and overall plant vigor. In this study, we identified a spontaneous cucumber mutant, NW079, exhibiting premature leaf yellowing, reduced chlorophyll content, and impaired photosynthetic performance. To uncover the genetic basis of this phenotype, we generated F₂ mapping populations and employed bulked segregant analysis and fine mapping. These efforts led to the identification of a 5.5-kb long terminal repeat (LTR) retrotransposon insertion within the first exon of CsPHYB, a gene encoding phytochrome B. This insertion disrupted normal splicing and gave rise to two aberrant transcript variants: one containing a 261-bp LTR-derived sequence with premature stop codons, and the other harboring a 1,914-bp deletion due to exon skipping. Both variants are predicted to produce truncated, nonfunctional proteins. Functional analyses revealed that CsPHYB deficiency resulted in heightened sensitivity to varying light qualities and intensities, leading to pronounced leaf yellowing and reduced leaf area. RNA sequencing revealed widespread transcriptional reprogramming in NW079, with 580 differentially expressed genes (DEGs) implicated in heme metabolism, tetrapyrrole binding, and chloroplast development. These transcriptional disruptions were closely linked to the observed structural and functional abnormalities in chloroplasts. This study provides a molecular framework for understanding the early senescence in cucumber, offering valuable insights for breeding strategies aimed at improving crop resilience and productivity. Keymessage An LTR retrotransposon insertion in the first exon of CsPhyB disrupts its expression and splicing, leading to early leaf senescence in cucumber. This finding provides novel insights into the role of CsPHYB in chloroplast development and light signaling, offering valuable molecular markers and a target gene for cucumber breeding programs focused on enhancing yield and stress resilience.

Micromorphology and Molecular Insights Into Glandular Trichomes in Two Different Thymes: Glandular Trichomes Formation Process and the Function of the Main Regulator TqHD1

Thyme is widely distributed in the worldwild. In China, there are 15 species, 2 varieties and 1 variant. Thymus quinquecostatus which contains abundant bioactive terpenoids is an important wild medicinal and aromatic plant in Chinese native thymes. Thymus vulgaris 'Elsbeth' comes from Europe and is known for its medicinal properties. The terpenoids exist in the glandular trichomes (GTs), a special epidemal structure. In Lamiaceae, glandular trichomes usually include peltate glandular trichomes (PGTs) and capitate glandular trichomes (CGTs). In previous study, we had analysed the molecular mechanisms of GTs but the formation process was not revealed. In this study, we observed the formation of PGTs and CGTs in thyme. The PGT underwent the complex process, including the basal, stalk, and head cells, there were 8-12 head cells. The CGT also had three cells, but its head cell only had one cell. Meanwhile, molecular biology research was carried out and we identified 68 HD-ZIP proteins and selected several key genes related to the formation of GTs according to the expression levels. Then, we cloned an HD-ZIP IV transcription factor TqHD1 from T. quinquecostatus and characterised it. TqHD1 not only can promote the formation of GTs but also can lead to the changes of volatile components and some relative genes levels. These findings complete the study of cell micromorphology of thyme and lay the foundation for characterisation of factors in epidermis-related functions in thyme.

SlHDZIV3 and SlHDZIV9 Are Two Positive Regulators in the Formation of Tomato Trichomes

Trichomes serve as the first line of defense for plants against biotic and abiotic stress. The HD-ZIP IV family plays conserved roles in regulating the development of trichomes in different species, but its specific regulatory mechanism in tomato is not fully understood. In this study, we demonstrate that SlHDZIV3 and SlHDZIV9 are positive regulators of trichome development in tomato, and they function in a partially redundant yet distinct manner. SlHDZIV3 primarily affects the formation of long trichomes on tomato leaves and early formed leaf petiole, while SlHDZIV9 influence the formation of long trichomes across a broader range of tissues, including the fruit trichomes. The CRISPR/Cas9 mutants of SlHDZIV9 exhibit a reduction in fruit trichomes, leading to an extended postharvest shelf-life of tomato fruit. In addition, both SlHDZIV3 and SlHDZIV9 are essential for jasmonic acid-mediated regulation of long trichomes formation. This study enhances our understanding of trichome initiation and development in tomato and provides new strategies for improving postharvest preservation of tomato fruits.

The CsTM alters multicellular trichome morphology and enhances resistance against aphid by interacting with CsTIP1;1 in cucumber

The multicellular trichomes of cucumber (Cucumis sativus L.) serve as the primary defense barrier against external factors, whose impact extends beyond plant growth and development to include commercial characteristics of fruits. The aphid (Aphis gossypii Glover) is one of prominent pests in cucumber cultivation. However, the relationship between physical properties of trichomes and the aphid resistance at molecular level remains largely unexplored. Here, a spontaneous mutant trichome morphology (tm) was characterized by increased susceptibility towards aphid. Further observations showed the tm exhibited a higher and narrower trichome base, which was significantly distinguishable from that in wild-type (WT). We conducted map-based cloning and identified the candidate, CsTM, encoding a C-lectin receptor-like kinase. The knockout mutant demonstrated the role of CsTM in trichome morphogenesis. The presence of SNP does not regulate the relative expression of CsTM, but diminishes the CsTM abundance of membrane proteins in tm. Interestingly, CsTM was found to interact with CsTIP1;1, which encodes an aquaporin with extensive reports in plant resistance and growth development. The subsequent aphid resistance experiments revealed that both CsTM and CsTIP1;1 regulated the development of trichomes and conferred resistance against aphid by affecting cytoplasmic H2O2 contents. Transcriptome analysis revealed a significant enrichment of genes associated with pathogenesis, calcium binding and cellulose synthase. Overall, our study elucidates an unidentified mechanism that CsTM-CsTIP1;1 alters multicellular trichome morphology and enhances resistance against aphid, thus providing a wholly new perspective for trichome morphogenesis in cucumber.

CsMYB36-mediated ROS homeostasis modulates the switch from cell division to differentiation in cucumber glandular trichome

Glandular trichomes (GTs) synthesize, store, and secrete diverse specialized metabolites that protect plants against biotic and abiotic stress. The bloom is deposited on the GTs and is perceptible on the surface of the cucumber fruit. Our previous investigation revealed the absence of bloom on the fruit surface in the loss-of-function CsMYB36 plants. GTs are formed through a series of cell differentiation events that support compound production. However, the mechanisms governing these events remain unclear. Here, we found GT cells initiate excessive periclinal divisions and fail to differentiate into functional GT cells in the absence of CsMYB36 based on the establishment of a detailed developmental process of GT in cucumber. We further found that CsMYB36 and CsGL1 form a positive feedback loop to regulate the cell differentiation of GT. DNA affinity purification (DAP)-seq, combined with RNA-seq data demonstrated that CsMYB36/CsGL1 can regulate the expression of phenylalanine synthesis-related genes, including peroxidase 53 (CsPRX53) which is a reactive oxygen species (ROS)-scavenging enzyme. H2O2 effectively inhibited GT cell division in Csmyb36 mutant plants. Collectively, our findings demonstrate that CsMYB36 combined with CsGL1 balances cell division and differentiation in the GT by mediating ROS homeostasis, thus affecting bloom production in cucumbers.

Mutations in a Leucine-Rich Repeat Receptor-Like Kinase gene result in male sterility and reduction in the number and size of fruit warts in cucumber (Cucumis sativus L.)

KEY MESSAGE

Mutations in the CsEMS1 gene result in male sterility and reduced wart number and density. Male sterility and fruit wart formation are two significant agronomic characteristics in cucumber (Cucumis sativus), yet knowledge of our underlying genetics is limited. In this study, we identified an EMS-induced male sterility and few small warts mutant (msfsw). Histological observations revealed defects the absence of tapetum, meiotic aberration and impaired microspore formation in the anthers of the mutant. The mutant also exhibits a reduction in both the size and number of fruit spines and fruit tubercules. Genetic analysis revealed that a single recessive gene is responsible for the mutant phenotypes. BSA-Seq and fine genetic mapping mapped the msfsw locus to a 63.7 kb region with four predicted genes. Multiple lines of evidence support CsEMS1(CsaV3_3G016940) as the candidate for the mutant allele which encodes an LRR receptor-like kinase, and a non-synonymous SNP inside the exon of CsEMS1 is the causal polymorphisms for the mutant phenotypes. This function of CsEMS1 in determination of pollen fertility was confirmed with generation and characterization of multiple knockout mutations with CRISPR/Cas9 based gene editing. In the wild-type (WT) plants, CsEMS1 was highly expressed in male flowers. In the mutant, the expression level of CsEMS1, several tapetum identity-related genes, and trichome-related genes were all significantly reduced as compared with the wild-type. Protein-protein interaction assays revealed physical interactions between CsEMS1 and CsTPD1. Quantitation of endogenous phytohormones revealed a reduction in the ethylene precursor ACC in CsEMS1 knockout lines. This work identified an important role of CsEMS1 in anther and pollen development as well as fruit spine/wart development in cucumber.

A gradient of the HD-Zip regulator Woolly regulates multicellular trichome morphogenesis in tomato

Homeodomain (HD) proteins regulate embryogenesis in animals such as the fruit fly (Drosophila melanogaster), often in a concentration-dependent manner. HD-leucine zipper (Zip) IV family genes are unique to plants and often function in the L1 epidermal cell layer. However, our understanding of the roles of HD-Zip IV family genes in plant morphogenesis is limited. In this study, we investigated the morphogenesis of tomato (Solanum lycopersicum) multicellular trichomes, a type of micro-organ in plants. We found that a gradient of the HD-Zip IV regulator Woolly (Wo) coordinates spatially polarized cell division and cell expansion in multicellular trichomes. Moreover, we identified a TEOSINTE BRANCHED1, CYCLOIDEA, and PROLIFERATING CELL NUCLEAR ANTIGEN BINDING FACTOR (TCP) transcription factor-encoding gene, SlBRANCHED2a (SlBRC2a), as a key downstream target of Wo that regulates the transition from cell division to cell expansion. High levels of Wo promote cell division in apical trichome cells, whereas in basal trichome cells, Wo mediates a negative feedback loop with SlBRC2a that forces basal cells to enter endoreduplication. The restricted high and low activities of Wo pattern the morphogenesis of tomato multicellular trichomes. These findings provide insights into the functions of HD-Zip IV genes during plant morphogenesis.

ELONGATED HYPOTCOTYL5 and SPINE BASE SIZE1 together mediate light-regulated spine expansion in cucumber

Plant trichome development is influenced by diverse developmental and environmental signals, but the molecular mechanisms involved are not well understood in most plant species. Fruit spines (trichomes) are an important trait in cucumber (Cucumis sativus L.), as they affect both fruit smoothness and commercial quality. Spine Base Size1 (CsSBS1) has been identified as essential for regulating fruit spine size in cucumber. Here, we discovered that CsSBS1 controls a season-dependent phenotype of spine base size in wild-type plants. Decreased light intensity led to reduced expression of CsSBS1 and smaller spine base size in wild-type plants, but not in the mutants with CsSBS1 deletion. Additionally, knockout of CsSBS1 resulted in smaller fruit spine base size and eliminated the light-induced expansion of spines. Overexpression of CsSBS1 increased spine base size and rescued the decrease in spine base size under low light conditions. Further analysis revealed that ELONGATED HYPOTCOTYL5 (HY5), a major transcription factor involved in light signaling pathways, directly binds to the promoter of CsSBS1 and activates its expression. Knockout of CsHY5 led to smaller fruit spine base size and abolished the light-induced expansion of spines. Taken together, our study findings have clarified a CsHY5-CsSBS1 regulatory module that mediates light-regulated spine expansion in cucumber. This finding offers a strategy for cucumber breeders to develop fruit with stable appearance quality under changing light conditions.

Genetic basis of nectar guide trichome variation between bumblebee- and self-pollinated monkeyflowers (Mimulus): role of the MIXTA-like gene GUIDELESS

Nectar guide trichomes play crucial ecological roles in bee-pollinated flowers, as they serve as footholds and guides for foraging bees to access the floral rewards. However, the genetic basis of natural variation in nectar guide trichomes among species remains poorly understood. In this study, we performed genetic analysis of nectar guide trichome variation between two closely related monkeyflower (Mimulus) species, the bumblebee-pollinated Mimulus lewisii and self-pollinated M. parishii. We demonstrate that a MIXTA-like R2R3-MYB gene, GUIDELESS, is a major contributor to the nectar guide trichome length variation between the two species. The short-haired M. parishii carries a recessive allele due to non-synonymous substitutions in a highly conserved motif among MIXTA-like MYB proteins. Furthermore, our results suggest that besides GUIDELESS, additional loci encoding repressors of trichome elongation also contribute to the transition from bumblebee-pollination to selfing. Taken together, these results suggest that during a pollination syndrome switch, changes in seemingly complex traits such as nectar guide trichomes could have a relatively simple genetic basis, involving just a few genes of large effects.

Lipid-Related Domestication Accounts for the Extreme Cold Sensitivity of Semiwild and Tropic Xishuangbanna Cucumber (Cucumis sativus L. var. xishuangbannanesis)

Xishuangbanna (XIS) cucumber (Cucumis sativus L. var. xishuangbannanesis) is a semiwild variety originating from low latitude tropic areas, and therefore shows extreme cold sensitivity and heat tolerance. Here, we mapped the quantitative trait loci (QTLs) that control the cold sensitivity and heat tolerance of XIS cucumber seedlings. Using bulked segregant analysis (BSA), we identified three QTLs (HTT1.1, HTT3.1, and HTT3.2, with a total length of 11.98 Mb) for heat tolerance and two QTLs (LTT6.1 and LTT6.2, with a total length of 8.74 Mb) for cold sensitivity. The QTL LTT6.1 was then narrowed down to a length of 641 kb by using kompetitive allele-specific PCR (KASP) markers. Based on structural variants (SVs) and single-nucleotide polymorphisms (SNPs), we found the LTT6.1 is covered by a high divergent region including a 50 kb deletion in the XIS49 genome, which affects the gene structure of lipase abhydrolase domain containing 6 (ABHD6, Csa_6G032560). Accordingly, there is a very big difference in lipid composition, but not in other osmoprotectants like free amino acids and fatty acids, between XIS49 and cultivated cucumber CL. Moreover, we calculated the composite likelihood ratio (CLR) and identified selective sweeps from 115 resequencing data, and found that lipid- and fatty-acid-related processes are major aspects in the domestication of the XIS group cucumber. LTT6.1 is a particularly special region positioned nearby lipid-related selective sweeps. These studies above suggested that the lipid-related domestication of XIS cucumbers should account for their extreme cold sensitivity.

Beyond skin-deep: targeting the plant surface for crop improvement

The above-ground plant surface is a well-adapted tissue layer that acts as an interface between the plant and its surrounding environment. As such, its primary role is to protect against desiccation and maintain the gaseous exchange required for photosynthesis. Further, this surface layer provides a barrier against pathogens and herbivory, while attracting pollinators and agents of seed dispersal. In the context of agriculture, the plant surface is strongly linked to post-harvest crop quality and yield. The epidermal layer contains several unique cell types adapted for these functions, while the non-lignified above-ground plant organs are covered by a hydrophobic cuticular membrane. This review aims to provide an overview of the latest understanding of the molecular mechanisms underlying crop cuticle and epidermal cell formation, with focus placed on genetic elements contributing towards quality, yield, drought tolerance, herbivory defence, pathogen resistance, pollinator attraction, and sterility, while highlighting the inter-relatedness of plant surface development and traits. Potential crop improvement strategies utilizing this knowledge are outlined in the context of the recent development of new breeding techniques.

The COPII subunit CsSEC23 mediates fruit glossiness in cucumber

To improve our understanding of the mechanism underlying cucumber glossiness regulation, a novel cucumber mutant with a glossy peel (Csgp) was identified. MutMap, genotyping, and gene editing results demonstrated that CsSEC23, which is the core component of COPII vesicles, mediates the glossiness of cucumber fruit peel. CsSEC23 is functionally conserved and located in the Golgi and endoplasmic reticulum. CsSEC23 could interact with CsSEC31, but this interaction was absent in the Csgp mutant, which decreased the efficiency of COPII vesicle transportation. Genes related to wax and cutin transport were upregulated in the Csgp mutant, and the cuticle structure of the Csgp-mutant peel became thinner. Moreover, the wax and cutin contents were also changed due to CsSEC23 mutation. Taken together, the results obtained from this study revealed that CsSEC23 mediates cucumber glossiness, and this mediating might be affected by COPII vesicle transportation.

Advances in understanding epigenetic regulation of plant trichome development: a comprehensive review

Plant growth and development are controlled by a complex gene regulatory network, which is currently a focal point of research. It has been established that epigenetic factors play a crucial role in plant growth. Trichomes, specialized appendages that arise from epidermal cells, are of great significance in plant growth and development. As a model system for studying plant development, trichomes possess both commercial and research value. Epigenetic regulation has only recently been implicated in the development of trichomes in a limited number of studies, and microRNA-mediated post-transcriptional regulation appears to dominate in this context. In light of this, we have conducted a review that explores the interplay between epigenetic regulations and the formation of plant trichomes, building upon existing knowledge of hormones and transcription factors in trichome development. Through this review, we aim to deepen our understanding of the regulatory mechanisms underlying trichome formation and shed light on future avenues of research in the field of epigenetics as it pertains to epidermal hair growth.

QTL Mapping for Ovary- and Fruit-Related Traits in Cucumis sativus-C. hystrix Introgression Line IL52

IL52 is a valuable introgression line obtained from interspecific hybridization between cultivated cucumber (Cucumis sativus L., 2n = 14) and the wild relative species C. hystrix Chakr. (2n = 24). IL52 exhibits high resistance to a number of diseases, including downy mildew, powdery mildew, and angular leaf spot. However, the ovary- and fruit-related traits of IL52 have not been thoroughly investigated. Here, we conducted quantitative trait loci (QTL) mapping for 11 traits related to ovary size, fruit size, and flowering time using a previously developed 155 F_7:8 RIL population derived from a cross between CCMC and IL52. In total, 27 QTL associated with the 11 traits were detected, distributed on seven chromosomes. These QTL explained 3.61% to 43.98% of the phenotypic variance. Notably, we identified a major-effect QTL (qOHN4.1) on chromosome 4 associated with the ovary hypanthium neck width and further delimited it into a 114-kb candidate region harboring 13 candidate genes. Furthermore, the QTL qOHN4.1 is co-localized with the QTL detected for ovary length, mature fruit length, and fruit neck length, all residing within the consensus QTL FS4.1, suggesting a plausible pleiotropic effect.

Novel players in organogenesis and flavonoid biosynthesis in cucumber glandular trichomes

Glandular trichomes (GTs) are outgrowths of plant epidermal cells that secrete and store specialized secondary metabolites that protect plants against biotic and abiotic stresses and have economic importance for human use. While extensive work has been done to understand the molecular mechanisms of trichome organogenesis in Arabidopsis (Arabidopsis thaliana), which forms unicellular, non-glandular trichomes (NGTs), little is known about the mechanisms of GT development or regulation of secondary metabolites in plants with multicellular GTs. Here, we identified and functionally characterized genes associated with GT organogenesis and secondary metabolism in GTs of cucumber (Cucumis sativus). We developed a method for effective separation and isolation of cucumber GTs and NGTs. Transcriptomic and metabolomic analyses showed that flavonoid accumulation in cucumber GTs is positively associated with increased expression of related biosynthesis genes. We identified 67 GT development-related genes, the functions of 7 of which were validated by virus-induced gene silencing. We further validated the role of cucumber ECERIFERUM1 (CsCER1) in GT organogenesis by overexpression and RNA interference transgenic approaches. We further show that the transcription factor TINY BRANCHED HAIR (CsTBH) serves as a central regulator of flavonoid biosynthesis in cucumber glandular trichomes. Work from this study provides insight into the development of secondary metabolite biosynthesis in multi-cellular glandular trichomes.

Natural variation in CRABS CLAW contributes to fruit length divergence in cucumber

Fruit length is a key domestication trait that affects crop yield and appearance. Cucumber (Cucumis sativus) fruits vary from 5 to 60 cm in length. Despite the identification of several regulators and multiple quantitative trait loci (QTLs) underlying fruit length, the natural variation, and molecular mechanisms underlying differences in fruit length are poorly understood. Through map-based cloning, we identified a nonsynonymous polymorphism (G to A) in CRABS CLAW (CsCRC) as underlying the major-effect fruit size/shape QTL FS5.2 in cucumber. The short-fruit allele CsCRCA is a rare allele that has only been found in round-fruited semi-wild Xishuangbanna cucumbers. A near-isogenic line (NIL) homozygous for CsCRCA exhibited a 34∼39% reduction in fruit length. Introducing CsCRCG into this NIL rescued the short-fruit phenotype, and knockdown of CsCRCG resulted in shorter fruit and smaller cells. In natural cucumber populations, CsCRCG expression was positively correlated with fruit length. Further, CsCRCG, but not CsCRCA, targets the downstream auxin-responsive protein gene CsARP1 to regulate its expression. Knockout of CsARP1 produced shorter fruit with smaller cells. Hence, our work suggests that CsCRCG positively regulates fruit elongation through transcriptional activation of CsARP1 and thus enhances cell expansion. Using different CsCRC alleles provides a strategy to manipulate fruit length in cucumber breeding.

Morphological and Genetic Diversity of Cucumber (Cucumis sativus L.) Fruit Development

Cucumber (Cucumis sativus L.) fruits, which are eaten at an immature stage of development, can vary extensively in morphological features such as size, shape, waxiness, spines, warts, and flesh thickness. Different types of cucumbers that vary in these morphological traits are preferred throughout the world. Numerous studies in recent years have added greatly to our understanding of cucumber fruit development and have identified a variety of genetic factors leading to extensive diversity. Candidate genes influencing floral organ establishment, cell division and cell cycle regulation, hormone biosynthesis and response, sugar transport, trichome development, and cutin, wax, and pigment biosynthesis have all been identified as factors influencing cucumber fruit morphology. The identified genes demonstrate complex interplay between structural genes, transcription factors, and hormone signaling. Identification of genetic factors controlling these traits will facilitate breeding for desired characteristics to increase productivity, improve shipping, handling, and storage traits, and enhance consumer-desired qualities. The following review examines our current understanding of developmental and genetic factors driving diversity of cucumber fruit morphology.

Phenotypic Characterization and Fine Mapping of a Major-Effect Fruit Shape QTL FS5.2 in Cucumber, Cucumis sativus L., with Near-Isogenic Line-Derived Segregating Populations

Cucumber (Cucumis sativus L.) fruit size/shape (FS) is an important yield and quality trait that is quantitatively inherited. Many quantitative trait loci (QTLs) for fruit size/shape have been identified, but very few have been fine-mapped or cloned. In this study, through marker-assisted foreground and background selections, we developed near-isogenic lines (NILs) for a major-effect fruit size/shape QTL FS5.2 in cucumber. Morphological and microscopic characterization of NILs suggests that the allele of fs5.2 from the semi-wild Xishuangbanna (XIS) cucumber (C. s. var. xishuangbannesis) reduces fruit elongation but promotes radial growth resulting in shorter but wider fruit, which seems to be due to reduced cell length, but increased cellular layers. Consistent with this, the NIL carrying the homozygous XIS allele (fs5.2) had lower auxin/IAA contents in both the ovary and the developing fruit. Fine genetic mapping with NIL-derived segregating populations placed FS5.2 into a 95.5 kb region with 15 predicted genes, and a homolog of the Arabidopsis CRABS CLAW (CsCRC) appeared to be the most possible candidate for FS5.2. Transcriptome profiling of NIL fruits at anthesis identified differentially expressed genes enriched in the auxin biosynthesis and signaling pathways, as well as genes involved in cell cycle, division, and cell wall processes. We conclude that the major-effect QTL FS5.2 controls cucumber fruit size/shape through regulating auxin-mediated cell division and expansion for the lateral and longitudinal fruit growth, respectively. The gibberellic acid (GA) signaling pathway also plays a role in FS5.2-mediated fruit elongation.

Molecular Mechanisms of Plant Trichome Development

Plant trichomes, protrusions formed from specialized aboveground epidermal cells, provide protection against various biotic and abiotic stresses. Trichomes can be unicellular, bicellular or multicellular, with multiple branches or no branches at all. Unicellular trichomes are generally not secretory, whereas multicellular trichomes include both secretory and non-secretory hairs. The secretory trichomes release secondary metabolites such as artemisinin, which is valuable as an antimalarial agent. Cotton trichomes, also known as cotton fibers, are an important natural product for the textile industry. In recent years, much progress has been made in unraveling the molecular mechanisms of trichome formation in Arabidopsis thaliana, Gossypium hirsutum, Oryza sativa, Cucumis sativus, Solanum lycopersicum, Nicotiana tabacum, and Artemisia annua. Here, we review current knowledge of the molecular mechanisms underlying fate determination and initiation, elongation, and maturation of unicellular, bicellular and multicellular trichomes in several representative plants. We emphasize the regulatory roles of plant hormones, transcription factors, the cell cycle and epigenetic modifications in different stages of trichome development. Finally, we identify the obstacles and key points for future research on plant trichome development, and speculated the development relationship between the salt glands of halophytes and the trichomes of non-halophytes, which provides a reference for future studying the development of plant epidermal cells.

Identification of the yield traits related haplotype combinations of transcription factor genes TaHDZ34 in common wheat

A predominant objective in wheat breeding is improving yield-related traits. The homeodomain-leucine zipper (HD-Zip) transcription factor plays a significant role in plant growth and development. In this study, we cloned all homeologs of TaHDZ34, which is a member of the HD-Zip class IV transcription factor family in wheat (Triticum aestivum L.). Sequence polymorphism analysis showed that TaHDZ-A34, TaHDZ-B34, and TaHDZ-D34 formed five, six, and six haplotypes, respectively, and the genes were divided into two main haplotype groups. We also developed functional molecular markers. The TaHDZ34 genes were divided into eight main haplotype combinations. Association analysis and distinct population validation preliminarily indicated that TaHDZ34 genes modulate grain number per spike, effective spikelet number per spike, thousand kernel weight, and flag leaf area per plant in wheat. Hap-ABD was the most effective haplotype combination of TaHDZ34. Subcellular localization showed that TaHDZ-A34 was localized to the nucleus. The interacting proteins of TaHDZ-A34 were involved in protein synthesis/degradation, energy production and transportation, and photosynthesis. Geographic distribution and frequencies of TaHDZ34 haplotype combinations suggested that Hap-Abd and Hap-AbD were preferentially selected in Chinese wheat breeding programs. The high-yield-related haplotype combination Hap-ABD provided beneficial genetic resources for the marker-assisted selection of new wheat cultivars.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01298-5.

Transcriptomic and functional analysis provides molecular insights into multicellular trichome development

Trichomes, the hair-like structures located on aerial parts of most vascular plants, are associated with a wide array of biological processes and affect the economic value of certain species. The processes involved in unicellular trichome formation have been well-studied in Arabidopsis (Arabidopsis thaliana). However, our understanding of the morphological changes and the underlying molecular processes involved in multicellular trichome development is limited. Here, we studied the dynamic developmental processes involved in glandular and nonglandular multicellular trichome formation in cucumber (Cucumis sativus L.) and divided these processes into five sequential stages. To gain insights into the underlying mechanisms of multicellular trichome formation, we performed a time-course transcriptome analysis using RNA-sequencing analysis. A total of 711 multicellular trichome-related genes were screened and a model for multicellular trichome formation was developed. The transcriptome and co-expression datasets were validated by reverse transcription-quantitative PCR and in situ hybridization. In addition, virus-induced gene silencing analysis revealed that CsHOMEOBOX3 (CsHOX3) and CsbHLH1 are involved in nonglandular trichome elongation and glandular trichome formation, respectively, which corresponds with the transcriptome data. This study presents a transcriptome atlas that provides insights into the molecular processes involved in multicellular trichome formation in cucumber and can be an important resource for future functional studies.

NS encodes an auxin transporter that regulates the 'numerous spines' trait in cucumber (Cucumis sativus) fruit

Fruit spine is an important agronomic trait in cucumber and the "numerous spines (ns)" cucumber varieties are popular in Europe and West Asia. Although the classical genetic locus of ns was reported more than two decades ago, the NS gene has not been cloned yet. In this study, nine genetic loci for the different densities of fruit spines were identified by a genome-wide association study. Among the nine loci, fsdG2.1 was closely associated with the classical genetic locus ns, which harbors a candidate gene Csa2G264590. Overexpression of Csa2G264590 resulted in lower fruit spine density, and the knockout mutant generated by CRISPR/Cas9 displayed an increased spine density, demonstrating that the Csa2G264590 gene is NS. NS is specifically expressed in the fruit peel and spine. Genetic analysis showed that NS regulates fruit spine development independently of the tuberculate gene, Tu, which regulates spine development on tubercules; the cucumber glabrous mutants csgl1 and csgl3 are epistatic to ns. Furthermore, we found that auxin levels in the fruit peel and spine were significantly lower in the knockout mutant ns-cr. Moreover, RNA-sequencing showed that the plant hormone signal transduction pathway was enriched. Notably, most of the auxin responsive Aux/IAA family genes were downregulated in ns-cr. Haplotype analysis showed that the non-functional haplotype of NS exists exclusively in the Eurasian cucumber backgrounds. Taken together, the cloning of NS gene provides new insights into the regulatory network of fruit spine development.

Comparative Transcriptome Analysis of Hard and Tender Fruit Spines of Cucumber to Identify Genes Involved in the Morphological Development of Fruit Spines

The spines of cucumber fruit not only have important commercial value but are also a classical tissue to study cell division and differentiation modes of multicellular trichomes. It has been reported that CsTs (C-type Lectin receptor-like kinase) can influence the development of fruit spines. In this study, we took a pair of cucumber materials defined as hard (Ts, wild type) and tender spines (ts, mutant) and defined the developmental process of fruit spines as consisting of four stages (stage I to stage IV) by continuously observing by microscope and SEM. Comparisons of transcriptome profiles at different development stages of wild-type spines showed that 803 and 722 genes were upregulated in the stalk (stage II and stage III) and base (stage IV) development stages of fruit spines, respectively. The function analysis of DEGs showed that genes related to auxin polar transport and HD-ZIP transcription factor are significantly upregulated during the development of the stalk. bHLH transcription factors and cytoskeleton-related genes were significantly upregulated during the development of the base. In addition, stage III is the key point for the difference between wild-type and mutant spines. We detected 628 DEGs between wild type and mutant at stage III. These DEGs are mainly involved in the calcium signaling of the cytoskeleton and auxin polar transport. Coincidentally, we found that CsVTI11, a factor involved in auxin signal transmission, can interact with CsTs in vivo, but this interaction does not occur between CsVTI11 and Csts, further suggesting that CsTs may regulate the development of fruit spines by influencing cell polarity. These results provide useful tools to study the molecular networks associated with cucumber fruit spine development and elucidate the biological pathways that C-type Lectin receptor-like kinase plays in regulating the development of fruit spines.

A novel HD-Zip I/C2H2-ZFP/WD-repeat complex regulates the size of spine base in cucumber

Fruit spine is an important trait in cucumber, affecting not only commercial quality, but also fruit smoothness, transportation and storage. Spine size is determined by a multi-cellular base. However, the molecular mechanism underlying the regulation of cucumber spine base remains largely unknown. Here, we report map-based cloning and characterization of a spine base size 1 (SBS1) gene, encoding a C2H2 zinc-finger transcription factor. Near-isogenic lines of cucumber were used to map, identify and quantify cucumber spine base size 1 (CsSBS1). Yeast-hybrid, bimolecular fluorescence complementation (BiFC), co-immunoprecipitation (Co-IP) and RNA-sequencing assays were used to explore the molecular mechanism of CsSBS1 in regulating spine base size development. CsSBS1 was specifically expressed in cucumber ovaries with particularly high expression in fruit spines. Overexpression of CsSBS1 resulted in large fruit spine base, while RNA-interference silencing of CsSBS1 inhibited the expansion of fruit spine base. Sequence analysis of natural cucumber accessions revealed that CsSBS1 was lost in small spine base accessions, resulting from a 4895 bp fragment deletion in CsSBS1 locus. CsSBS1 can form a trimeric complex with two positive regulators CsTTG1 and CsGL1 to regulate spine base development through ethylene signaling. A novel regulator network is proposed that the CsGL1/CsSBS1/CsTTG1 complex plays a significant role in regulating spine base formation and size, which offers a strategy for cucumber breeders to develop smooth fruit.

Graph-based pan-genome reveals structural and sequence variations related to agronomic traits and domestication in cucumber

Structural variants (SVs) represent a major source of genetic diversity and are related to numerous agronomic traits and evolutionary events; however, their comprehensive identification and characterization in cucumber (Cucumis sativus L.) have been hindered by the lack of a high-quality pan-genome. Here, we report a graph-based cucumber pan-genome by analyzing twelve chromosome-scale genome assemblies. Genotyping of seven large chromosomal rearrangements based on the pan-genome provides useful information for use of wild accessions in breeding and genetic studies. A total of ~4.3 million genetic variants including 56,214 SVs are identified leveraging the chromosome-level assemblies. The pan-genome graph integrating both variant information and reference genome sequences aids the identification of SVs associated with agronomic traits, including warty fruits, flowering times and root growth, and enhances the understanding of cucumber trait evolution. The graph-based cucumber pan-genome and the identified genetic variants provide rich resources for future biological research and genomics-assisted breeding.

Increasing studies have suggested that single reference genome is insufficient to capture all variations in the genome. Here, the authors report a graph-based cucumber pan-genome by analyzing 12 chromosome-scale assemblies and reveal variations associated with agronomic traits and domestication.

HECATE2 acts with GLABROUS3 and Tu to boost cytokinin biosynthesis and regulate cucumber fruit wart formation

Warty fruit in cucumber (Cucumis sativus L.) is an important quality trait that greatly affects fruit appearance and market value. The cucumber wart consists of fruit trichomes (spines) and underlying tubercules, in which the existence of spines is prerequisite for tubercule formation. Although several regulators have been reported to mediate spine or tubercule formation, the direct link between spine and tubercule development remains unknown. Here, we found that the basic Helix-Loop-Helix (bHLH) gene HECATE2 (CsHEC2) was highly expressed in cucumber fruit peels including spines and tubercules. Knockout of CsHEC2 by the CRISPR/Cas9 system resulted in reduced wart density and decreased cytokinin (CTK) accumulation in the fruit peel, whereas overexpression of CsHEC2 led to elevated wart density and CTK level. CsHEC2 is directly bound to the promoter of the CTK hydroxylase-like1 gene (CsCHL1) that catalyzes CTK biosynthesis, and activated CsCHL1 expression. Moreover, CsHEC2 physically interacted with GLABROUS3 (CsGL3, a key spine regulator) and Tuberculate fruit (CsTu, a core tubercule formation factor), and such interactions further enhanced CsHEC2-mediated CsCHL1 expression. These data suggested that CsHEC2 promotes wart formation by acting as an important cofactor for CsGL3 and CsTu to directly stimulate CTK biosynthesis in cucumber. Thus, CsHEC2 can serve as a valuable target for molecular breeding of cucumber varieties with different wart density requirements.

Advances in the Regulation of Epidermal Cell Development by C2H2 Zinc Finger Proteins in Plants

Plant epidermal cells, such as trichomes, root hairs, salt glands, and stomata, play pivotal roles in the growth, development, and environmental adaptation of terrestrial plants. Cell fate determination, differentiation, and the formation of epidermal structures represent basic developmental processes in multicellular organisms. Increasing evidence indicates that C2H2 zinc finger proteins play important roles in regulating the development of epidermal structures in plants and plant adaptation to unfavorable environments. Here, we systematically summarize the molecular mechanism underlying the roles of C2H2 zinc finger proteins in controlling epidermal cell formation in plants, with an emphasis on trichomes, root hairs, and salt glands and their roles in plant adaptation to environmental stress. In addition, we discuss the possible roles of homologous C2H2 zinc finger proteins in trichome development in non-halophytes and salt gland development in halophytes based on bioinformatic analysis. This review provides a foundation for further study of epidermal cell development and abiotic stress responses in plants.

A SNP Mutation in Homeodomain-DDT (HD-DDT) Transcription Factor Results in Multiple Trichomes (mt) in Cucumber (Cucumis sativus L.)

Trichome is a natural physical barrier protecting plants against environmental stresses, natural infestations, ultraviolet rays and pathogenicity. Trichome also helps plants in maintaining appropriate water content by reducing transpiration rate. The molecular mechanism regulating unicellular trichome development in Arabidopsis has been extensively elucidated, but the molecular mechanism regulating multicellular trichome development remains unclear. In this study, we identified a multiple trichomes (mt) mutant from a cucumber EMS (Ethylmethylsulfone) mutagenesis population. Genetic analysis indicated that an incomplete dominant gene controls the mt trait. Using a combination of map-based cloning and BSA-seq (Bulked Segregant Analysis -Sequencing), we identified the candidate gene, CsaV3_6G050410, responsible for the mt mutation. Sequence alignment revealed one base substitution in gene CsaV3_6G050410, resulting in an amino acid substitution. The deduced amino acid sequence of CsaV3_6G050410 encodes a HD-DDT (homeodomain-DDT) transcriptional regulatory protein containing a conserved homeobox domain and a DDT domain. Gene expression analysis revealed that the expression level of CsaV3_6G050410 in the mt mutant was similar to that in the WT (wild type). Transcriptome analysis indicated that the mt gene may regulate the development of the epidermis by influencing plant hormone signaling pathways or participating in several transcription factor pathways. The results of this study are fundamental for a better understanding of the function of the HD-DDT transcription factor in the trichome development of cucumber.

An HD-ZIP-MYB complex regulates glandular secretory trichome initiation in Artemisia annua

Plant glandular secretory trichomes (GSTs) produce various specialized metabolites. Increasing GST density represents a strategy to enhance the yield of these chemicals; however, the gene regulatory network that controls GST initiation remains unclear. In a previous study of Artemisia annua L., we found that a HD-ZIP IV transcription factor, AaHD1, promotes GST initiation by directly regulating AaGSW2. Here, we identified two AaHD1-interacting transcription factors, namely AaMIXTA-like 2 (AaMYB16) and AaMYB5. Through the generation and characterization of transgenic plants, we found that AaMYB16 is a positive regulator of GST initiation, whereas AaMYB5 has the opposite effect. Notably, neither of them regulates GST formation independently. Rather, they act competitively, by interacting and modulating AaHD1 promoter binding activity. Additionally, the phytohormone jasmonic acid (JA) was shown to be associated with the AaHD1-AaMYB16/AaMYB5 regulatory network through transcriptional regulation via a JASMONATE-ZIM DOMAIN (JAZ) protein repressor. These results bring new insights into the mechanism of GST initiation through regulatory complexes, which appear to have similar functions in a range of vascular plant taxa.

Glandular trichomes: new focus on horticultural crops

Plant glandular trichomes (GTs) are epidermal outgrowths with the capacity to biosynthesize and secrete specialized metabolites, that are of great scientific and practical significance. Our understanding of the developmental process of GTs is limited, and no single plant species serves as a unique model. Here, we review the genetic mechanisms of GT initiation and development and provide a summary of the biosynthetic pathways of GT-specialized metabolites in nonmodel plant species, especially horticultural crops. We discuss the morphology and classification of GT types. Moreover, we highlight technological advancements in methods employed for investigating GTs. Understanding the molecular basis of GT development and specialized metabolites not only offers useful avenues for research in plant breeding that will lead to the improved production of desirable metabolites, but also provides insights for plant epidermal development research.

Gene expression underlying floral epidermal specialization in Aristolochia fimbriata (Aristolochiaceae)

BACKGROUND AND AIMS

The epidermis constitutes the outermost tissue of the plant body. Although it plays major structural, physiological and ecological roles in embryophytes, the molecular mechanisms controlling epidermal cell fate, differentiation and trichome development have been scarcely studied across angiosperms, and remain almost unexplored in floral organs.

METHODS

In this study, we assess the spatio-temporal expression patterns of GL2, GL3, TTG1, TRY, MYB5, MYB6, HDG2, MYB106-like, WIN1 and RAV1-like homologues in the magnoliid Aristolochia fimbriata (Aristolochiaceae) by using comparative RNA-sequencing and in situ hybridization assays.

KEY RESULTS

Genes involved in Aristolochia fimbriata trichome development vary depending on the organ where they are formed. Stem, leaf and pedicel trichomes recruit most of the transcription factors (TFs) described above. Conversely, floral trichomes only use a small subset of genes including AfimGL2, AfimRAV1-like, AfimWIN1, AfimMYB106-like and AfimHDG2. The remaining TFs, AfimTTG1, AfimGL3, AfimTRY, AfimMYB5 and AfimMYB6, are restricted to the abaxial (outer) and the adaxial (inner) pavement epidermal cells.

CONCLUSIONS

We re-evaluate the core genetic network shaping trichome fate in flowers of an early-divergent angiosperm lineage and show a morphologically diverse output with a simpler genetic mechanism in place when compared to the models Arabidopsis thaliana and Cucumis sativus. In turn, our results strongly suggest that the canonical trichome gene expression appears to be more conserved in vegetative than in floral tissues across angiosperms.

Comparative ultrastructure of trichomes on various organs of Rosa roxburghii

Chestnut rose, R. roxburghii Tratt. (Rosaceae) (RR) is an important crop in China due to its nutritional and medicinal values. RR frequently produces trichomes on the surfaces of a diverse range of organs, however a genetic component exists to the control of trichome development, with some cultivars having significantly fewer trichomes to others. Certain varieties have fruits that are thickly covered with macroscopic trichomes, which is an undesirable trait for fruit processing and consumption. However, smooth-fruit cultivars exist, such as R. roxburghii Tratt. f. esetosa Ku (RRE). Despite their economic importance, the anatomical features of trichomes have not been explored in detail for these two chestnut rose germplasms. Here, we investigate the ultrastructure of trichomes distributed on the stem, sepal, and fruit of RR and RRE using transmission electron microscopy (TEM). The internal structure of stem prickle trichomes in RR and RRE was oval in shape and did not contain nucleoli or other organelles. The cell walls of stem prickles in RR are thick and the intercellular spaces occupied with liquid, whereas the cells wall of stem prickles in RRE are thin and have air-filled intercellular spaces. The cells of sepal acicular trichomes in RR and glandular trichomes (GTs) of sepals in RRE had similar vacuole sizes, cytoplasm content, intercellular spaces, and arrangement of plastids within cells. However, there were osmiophilic granules present in the GTs of RRE. The flagelliform trichomes in the sepals of the two germplasms are composed of oval or rod-shaped cells. Although the flagelliform trichomes in the sepals of the two germplasms had a similar internal structure, and both contained starch grains and plastids with visible thylakoid membranes, the flagelliform trichomes in the sepals of RR had a thinner cell wall and a higher proportion of cytoplasm which was more evenly distributed across the cell. There were granules that stained heavily with osmium tetroxide which occurred infrequently in the flagelliform trichomes of sepals in RRE but were not observed in RR. On the acicular trichomes of fruit in RR, the flagelliform trichomes and the GTs of fruit in RRE shared similar cell morphology, arrangement and vacuole size as well as intercellular space. Both the fruit flagelliform trichomes and GTs in RRE contain granules which stain heavily with osmium tetroxide, and the GTs contain plastids and starch grains. These differences in trichome cell ultrastructure may be related to developmental processes or biological functions of the trichomes. These results also suggest that the two chestnut rose germplasms are good candidates for further study of trichome ontogeny in the genus and subsequent breeding of the smooth organ trait in this species.

TINY BRANCHED HAIR functions in multicellular trichome development through an ethylene pathway in Cucumis sativus L

The fruit trichomes of Cucurbitaceae are widely desired in many Asian countries and have been a key determinant of cucumber (Cucumis sativus L.) cultivar selection for commercial production and breeding. However, our understanding of the initiation and development of cucumber trichomes is still limited. Here, we found that the cucumber TINY BRANCHED HAIR (TBH) gene is preferentially expressed in multicellular trichomes. Overexpression of CsTBH in tbh mutants restored the trichome phenotype and increased the percentage of female flowers, whereas silencing of CsTBH in wild-type plants resulted in stunted trichomes with a lower rate of female flowers. Furthermore, we provide evidence that CsTBH can directly bind to the promoters of cucumber 1-Aminocyclopropane-1-Carboxylate Synthase (CsACS) genes and regulate their expression, which affects multicellular trichome development, ethylene accumulation, and sex expression. Two cucumber acs mutants with different trichome morphology and sex morphs compared with their near-isogenic line further support our findings. Collectively, our study provides new information on the molecular mechanism of CsTBH in regulating multicellular trichome development and sex expression through an ethylene pathway.

A SNP of HD-ZIP I transcription factor leads to distortion of trichome morphology in cucumber (Cucumis sativus L.)

BACKGROUND

Trichomes are excellent model systems for the analysis of cell differentiation and play essential roles in plant protection. From cucumber inbred line 'WD1', we identified an EMS-induced trichome abnormally developing mutant, nps, which exhibited smaller, denser and no pyramid-shaped head trichomes.

RESULTS

Using F₂ and BC₁ populations constructed from a cross between nps and '9930', the genetic analysis showed that the nps trait is controlled by a single recessive nuclear gene. We identified CsNps by map-based cloning with 576 individuals of the F₂ population generated from the cross of nps and inbred line '9930'. The CsNps was located at a 13.4-kb genomic region on chromosome 3, which region contains three predicted genes. Sequence analysis showed that only one single nucleotide mutation (C → T) between 9930 and nps was found in the second exon of Csa3G748220, a plant-specific class I HD-Zip gene. The result of allelism test also indicated that nps is a novel allelic mutant of Mict (Micro-trichome). Thus, nps was renamed mict-L130F. By comparing the transcriptome of mict-L130F vs WD1 and 06-2 (mict) vs 06-1 (wildtype, near-isogenic line of 06-2), several potential target genes that may be related to trichome development were identified.

CONCLUSIONS

Our results demonstrate that Mict-L130F is involved in the morphogenesis of trichomes. Map-based cloning of the Mict-L130F gene could promote the study of trichome development in cucumber.

Improved Brassica oleracea JZS assembly reveals significant changing of LTR-RT dynamics in different morphotypes

Brassica oleracea is an important vegetable crop that has provided ancestor genomes of the two most important Brassica oil crops, Brassica napus and Brassica carinata. The current B. oleracea reference genome (JZS, also named 02-12) displays problems of large mis-assemblies, low sequence continuity, and low assembly integrity, thus limiting genomic analysis. We reported an updated assembly of the B. oleracea reference genome (JZS v2) obtained through single-molecule sequencing and chromosome conformation capture technologies. We assembled an additional 83.16 Mb of genomic sequences, and the updated genome features a contig N50 size of 2.37 Mb, representing an ~ 88-fold improvement. We detected a new round of long terminal repeat retrotransposon (LTR-RT) burst in the new assembly. Comparative analysis with the reported genome sequences of two other genomes of B. oleracea (TO1000 and HDEM) identified extensive gene order and gene structural variation. In addition, we found that the genome-specific amplification of Gypsy-like LTR-RTs occurred around 0-1 million years ago (MYA). In particular, the athila, tat, and Del families were extensively amplified in JZS around 0-1 MYA. Moreover, we identified that the syntenic genes were modified due to the insertion of genome-specific LTR-RTs. These results indicated that the genome-specific LTR-RT dynamics were associated with genome diversification in B. oleracea.

The MIXTA-LIKE transcription factor CsMYB6 regulates fruit spine and tubercule formation in cucumber

Cucumber fruit wart composed of tubercule and spine (trichome on fruit) is not only an important fruit quality trait in cucumber production, but also a well-studied model for plant cell-fate determination. The development of spine is closely related to the initiation and formation of tubercule. The spine differentiation regulator CsGL1 has been proved to be epistatic to the tubercule initiation factor CsTu, which is the only connection to be identified between spine and tubercule formations. Our previous studies found that the MIXTA-LIKE transcription factor CsMYB6 can suppress fruit spine initiation, which is independent of CsGL1. How the formation of spine and tubercule is regulated at the molecular level by CsMYB6 remains poorly understood. In this study, we characterized cucumber 35S:CsMYB6 transgenic plants, which displayed an obvious reduction in the number and size of fruit spines and tubecules. Molecular analyses showed that CsMYB6 directly interacted with the key spine formation factor CsTTG1 in regulating the formation of fruit spine, and CsTu in regulating the initiation of fruit tubercule, respectively. Based on these evidences, a novel regulatory network is proposed by which CsMYB6/CsTTG1 and CsMYB6/CsTu complexes play an important role in regulating epidermal development, including spine formation and tubercule initiation in cucumber.

The HD-ZIP IV transcription factor Tril regulates fruit spine density through gene dosage effects in cucumber

Trichomes and fruit spines are important traits that directly affect the appearance quality and commercial value of cucumber (Cucumis sativus). Tril (Trichome-less), encodes a HD-Zip IV transcription factor that plays a crucial role in the initiation of trichomes and fruit spines, but little is known about the details of the regulatory mechanisms involved. In this study, analysis of tissue expression patterns indicated that Tril is expressed and functions in the early stages of organ initiation and development. Expression of Tril under the control of its own promoter (the TrilPro::Tril-3*flag fragment) could partly rescue the mutant phenotypes of tril, csgl3 (cucumber glabrous 3, an allelic mutant of tril), and fs1 (few spines 1, a fragment substitution in the Tril promoter region), providing further evidence that Tril is responsible for the initiation of trichomes and fruit spines. In lines with dense spine, fs1-type lines, and transgenic lines of different backgrounds containing the TrilPro::Tril-3*flag foreign fragment, spine density increased in conjunction with increases in Tril expression, indicating that Tril has a gene dosage effect on fruit spine density in cucumber. Numerous Spines (NS) is a negative regulatory factor of fruit spine density. Characterization of the molecular and genetic interaction between Tril and NS/ns demonstrated that Tril functions upstream of NS with respect to spine initiation. Overall, our results reveal a novel regulatory mechanism governing the effect of Tril on fruit spine development, and provide a reference for future work on breeding for physical quality in cucumber.

Genome wide identification, characterization and expression analysis of HD-ZIP gene family in Cucumis sativus L. under biotic and various abiotic stresses

Information retrieved from genomic assembly may provide important clues and various molecular aspects in plants. Our research identified 40 CsHDZ genes in the Cucumber genome database. Subsequently; we performed the conserved motif and domain analysis of CsHDZ proteins. The phylogeny of the CsHDZ proteins further divides into 4 subfamilies (HD-ZIP I, HD-ZIP II, HD-ZIP III, and HD-ZIP IV) based on the structural similarities and functional diversities. The GO (Gene ontology) analysis of CsHDZ proteins showed that they are responsive to environmental stimuli and involved in numerous growth and developmental processes. The qRT-PCR analysis of 11 CsHDZ genes showed that they are expressed in all the tested tissues of Cucumis sativus. The differential expression pattern of CsHDZ genes unfolded their possible involvement in responding to various abiotic stresses and powdery mildew stress. It has been found that the CsHDZ22 localized in the nucleus which possibly participates in the regulatory mechanisms of various biological and cellular processes. In the light of above-mentioned outcomes, it has been deducted that CsHDZ genes in the Cucumis sativus genome play an important role in mediating the resistance to various abiotic stresses and powdery mildew stress as well as provide significant clues for functional studies.

Gynoecy instability in cucumber (Cucumis sativus L.) is due to unequal crossover at the copy number variation-dependent Femaleness (F) locus

Cucumber, Cucumis sativus is an important vegetable crop, and gynoecy has played a critical role in yield increase of hybrid cucumber production. Cucumber has a unique genetic system for gynoecious sex expression, which is determined by the copy number variation (CNV)-based, dominant, and dosage-dependent femaleness (F) locus. However, this gynoecy expression system seems unstable since monecious plants could often be found in F-dependent gynoecious cucumber inbreds. We hypothesized that gynoecy instability (gynoecy loss) may be due to unequal crossing over (UCO) during meiosis among repeat units of the CNV. In this study, using high throughput genome resequencing, fiber-FISH and genomic qPCR analyses, we first confirmed and refined the structure of the F locus, which was a CNV of a 30.2-kb tandem repeat. Gynoecious plants contained three genes: CsACS1, CsACS1G, and CsMYB, of which CsACS1G is a duplication of CsACS1 but with a recombinant distal promoter that may contribute to gynoecy sex expression. In two large populations from self-pollinated gynoecious inbred lines, 'gynoecy loss' mutants were identified with similar mutation rates (~0.12%). We show that these monecious mutants have lost CsACS1G. In addition, we identified gynoecious lines in natural populations that carry two copies of CSACS1G. We proposed a model to explain gynoecy instability in F-dependent cucumbers, which is caused by UCO among CSACS1/G units during meiosis. The findings present a convincing case that the phenotypic variation of an economically important trait is associated with the dynamic changes of copy numbers at the F locus. This work also has important implications in cucumber breeding.

Transcriptome profiling reveals genes involved in spine development during CsTTG1-regulated pathway in cucumber (Cucumis sativus L.)

The cucumber (Cucumis sativus L.), a type of fleshy fruit, is covered with spines (multicellular trichomes), which have a crucial impact on the economic value of the crop. Previous studies have found that CsTTG1 plays important roles in the initiation and further differentiation of cucumber spines, but how spine formation is regulated at the molecular level by CsTTG1 remains poorly understood. In this study, we characterized a cucumber 35S:CsTTG1 transgenic T2 line, OE-2, which bears relatively large and long spines compared with the small and short spines of the wild type (WT). Phenotypic measurements and histological analyses revealed that this phenotypic change was attributed to significant increases in cell number and size. Comparison of ovary epidermis transcriptomes between OE-2 and WT by DGE (Digital Gene Expression) analysis identified 1241 differentially expressed genes, among which 712 genes were dramatically upregulated and 529 downregulated in the ovary epidermis of OE-2. XTH23 and Cyclin family genes were significantly activated in OE-2, and transcription factors (TFs) were found to participate in spine size regulation in OE-2. Further analyses confirmed that GA was implicated in the regulation of fruit spine development in cucumber. Thus, our study provides a foundation for dissecting the molecular regulatory networks of fruit spine control in cucumber.

Functional characterization of a HD-ZIP IV transcription factor NtHDG2 in regulating flavonols biosynthesis in Nicotiana tabacum

The HD-ZIP Ⅳ transcription factors have been identified and functional characterized in many plant species. However, no tobacco HD-ZIP IV gene has been isolated, and it is not yet known whether HD-ZIP IV genes are involved in controlling flavonols accumulation in plants. Here, we cloned a HD ZIP gene named NtHDG2 from Nicotiana tabacum, which belongs to the class IV of HD-ZIP family, and the NtHDG2-GFP fusion protein is localized to the nucleus. We further observed that the flavonols contents in the NtHDG2 overexpression leaves increase to 1.9-4.5 folds of that in WT plants, but in the NtHDG2-RNAi plants the flavonols contents reduce to 20.9%-52.7% of that in WT plants. The transcriptions of one regulatory gene NtMYB12, and three structural genes (NtPAL, NtF3'H, NtF3GT), contributing to flavonols biosynthesis, were significantly induced by NtHDG2. However, the transcription level of NtNAC002, a flavonols biosynthesis repressor, was also significantly up-regulated in NtHDG2-overexpression lines, but significantly down-regulated in the RNAi lines, indicating that HDG2 regulates the synthesis of flavonols as a complex regulatory network. Moreover, ectopic expression of NtHDG2 gene promoted the transcription of several AP2/ERF genes, including NtERF1-5, NtERF109, NtDREB1, and NtCIPK11, which participate in regulating root development and resistance to abiotic stresses. Our findings reveal the new function of HD-ZIP IV transcription factors in flavonoids biosynthesis, and indicate that HD-ZIP IV members may play an important role in plant resistance to abiotic stress. The NtHDG2 gene provides a promising target for genetically manipulating to increase the amounts of flavonols in tobacco leaves.

Molecularly tagged genes and quantitative trait loci in cucumber with recommendations for QTL nomenclature

Cucumber, Cucumis sativus L. (2n = 2x = 14), is an important vegetable crop worldwide. It was the first specialty crop with a publicly available draft genome. Its relatively small, diploid genome, short life cycle, and self-compatible mating system offers advantages for genetic studies. In recent years, significant progress has been made in molecular mapping, and identification of genes and QTL responsible for key phenotypic traits, but a systematic review of the work is lacking. Here, we conducted an extensive literature review on mutants, genes and QTL that have been molecularly mapped or characterized in cucumber. We documented 81 simply inherited trait genes or major-effect QTL that have been cloned or fine mapped. For each gene, detailed information was compiled including chromosome locations, allelic variants and associated polymorphisms, predicted functions, and diagnostic markers that could be used for marker-assisted selection in cucumber breeding. We also documented 322 QTL for 42 quantitative traits, including 109 for disease resistances against seven pathogens. By alignment of these QTL on the latest version of cucumber draft genomes, consensus QTL across multiple studies were inferred, which provided insights into heritable correlations among different traits. Through collaborative efforts among public and private cucumber researchers, we identified 130 quantitative traits and developed a set of recommendations for QTL nomenclature in cucumber. This is the first attempt to systematically summarize, analyze and inventory cucumber mutants, cloned or mapped genes and QTL, which should be a useful resource for the cucurbit research community.

Updates on molecular mechanisms in the development of branched trichome in Arabidopsis and nonbranched in cotton

Trichomes are specialized epidermal cells and a vital plant organ that protect plants from various harms and provide valuable resources for plant development and use. Some key genes related to trichomes have been identified in the model plant Arabidopsis thaliana through glabrous mutants and gene cloning, and the hub MYB-bHLH-WD40, consisting of several factors including GLABRA1 (GL1), GL3, TRANSPARENT TESTA GLABRA1 (TTG1), and ENHANCER OF GLABRA3 (EGL3), has been established. Subsequently, some upstream transcription factors, phytohormones and epigenetic modification factors have also been studied in depth. In cotton, a very important fibre and oil crop globally, in addition to the key MYB-like factors, more important regulators and potential molecular mechanisms (e.g. epigenetic modifiers, distinct metabolic pathways) are being exploited during different fibre developmental stages. This occurs due to increased cotton research, resulting in the discovery of more complex regulation mechanisms from the allotetraploid genome of cotton. In addition, some conservative as well as specific mediators are involved in trichome development in other species. This study summarizes molecular mechanisms in trichome development and provides a detailed comparison of the similarities and differences between Arabidopsis and cotton, analyses the possible reasons for the discrepancy in identification of regulators, and raises future questions and foci for understanding trichome development in more detail.

Classification of fruit trichomes in cucumber and effects of plant hormones on type II fruit trichome development

Cucumber fruit trichomes could be classified into eight types; all of them are multicellular with complex and different developmental processes as compared with unicellular trichomes in other plants. The fruit trichomes or fruit spines of cucumber, Cucumis sativus L., are highly specialized structures originating from epidermal cells with diverse morphology, which grow perpendicular to the fruit surface. To understand the underlying molecular mechanisms of fruit trichome development, in this study, we conducted morphological characterization and classification of cucumber fruit trichomes and their developmental processes. We examined the fruit trichomes among 200 cucumber varieties, which could be classified into eight morphologically distinct types (I-VIII). Investigation of the organogenesis of the eight types of trichomes revealed two main developmental patterns. The development of glandular trichomes had multiple stages including initiation and expansion of the trichome precursor cell protuberating out of the epidermal surface, followed by periclinal bipartition to two cells (top and bottom) which later formed the head region and the stalk, respectively, through subsequent cell divisions. The non-glandular trichome development started with the expansion of the precursor cell perpendicularly to the epidermal plane followed by cell periclinal division to form a stalk comprising of some rectangle cells and a pointed apex cell. The base cell then started anticlinal bipartition to two cells, which then underwent many cell divisions to form a multicellular spherical structure. In addition, phytohormones as environmental cues were closely related to trichome development. We found that GA and BAP were capable of increasing trichome number per fruit with distinct effects under different concentrations.

Molecular basis of cucumber fruit domestication

Cucumber (Cucumis sativus L.) is an economically important vegetable crop that is cultivated worldwide. Compared to the wild ancestor bearing small, bitter and seedy fruit, domesticated cucumbers exhibit significant variation in fruit appearance, size and flavor. Understanding the molecular basis of domestication related traits can provide insights into fruit evolution and make crop breeding more efficient. Here we review recent advances in relating to the genetic basis of fruit morphological traits (femaleness, fruit spine, wart, size, color and carpel development) and organoleptic features (bitterness) during cucumber domestication.

Cucumber CsTRY Negatively Regulates Anthocyanin Biosynthesis and Trichome Formation When Expressed in Tobacco

The development of trichomes (spines) on cucumber fruits is an important agronomic trait. It has been reported that two MYB family members, CsMYB6 (Csa3G824850) and CsTRY (Csa5G139610) act as negative regulators of trichome or fruit spine initiation. To further study the functions of these two genes, we overexpressed them in tobacco, and found that the flowers and seed coats of transformants overexpressing CsTRY displayed an unexpected defect in pigmentation that was not observed in plants overexpressing CsMYB6. Moreover, the expression of key genes in the flavonoid synthesis pathway was repressed in CsTRY overexpressing plants, which resulted in the decrease of several important flavonoid secondary metabolites. In addition, CsTRY could interact with the AN1 homologous gene CsAN1 (Csa7G044190) in cucumber, which further confirmed that CsTRY not only regulates the development of fruit spines, but also functions in the synthesis of flavonoids, acting as the repressor of anthocyanin synthesis.

Novel loci fsd6.1 and Csgl3 regulate ultra-high fruit spine density in cucumber

Quantitative Trait Loci (QTL) analysis of multiple populations in multiple environments revealed that the fsd6.2 locus, which includes the candidate gene Csgl3, controls high fruit spine density in natural cucumbers. GWAS identified a novel locus fsd6.1, which regulates ultra-high fruit spine density in combination with Csgl3, and evolved during cucumber domestication. Fruit spine density, a domestication trait, largely influences the commercial value of cucumbers. However, the molecular basis of fruit spine density in cucumber remains unclear. In this study, four populations were derived from five materials, which included three with low fruit spine density, one with high fruit spine density, and one with ultra-high fruit spine density. Fruit spine densities were measured in 15 environments over a span of 6 years. The distributions were bimodal suggesting that fruit spine density is controlled by a major-effect QTL. QTL analysis determined that the same major-effect QTL, fsd6.2, is present in four populations. Fine mapping indicated that Csgl3 is the candidate gene at the fsd6.2 locus. Phylogenetic and geographical distribution analyses revealed that Csgl3 originated from China, which has the highest genetic diversity for fruit spine density. One novel minor-effect QTL, fsd6.1, was detected in the HR and HP populations derived from the cross between 65G and 02245. In addition, GWAS identified a novel locus that colocates with fsd6.1. Inspection of a candidate region of about 18 kb in size using pairwise LD correlations, combined with genetic diversity and phylogenetic analysis of fsd6.1 in natural populations, indicated that Csa6G421750 is the candidate gene responsible for ultra-high fruit spine density in cucumber. This study provides new insights into the origin of fruit spine density and the evolution of high/ultra-high fruit spine density during cucumber domestication.

A CsTu-TS1 regulatory module promotes fruit tubercule formation in cucumber

The fruit epidermal features such as the size of tubercules are important fruit quality traits for cucumber production. But the mechanisms underlying tubercule formation remain elusive. Here, tubercule size locus CsTS1 was identified by map-based cloning and was found to encode an oleosin protein. Allelic variation was identified in the promoter region of CsTS1, resulting in low expression of CsTS1 in all 22 different small-warty or nonwarty cucumber lines. High CsTS1 expression levels were closely correlated with increased fruit tubercule size among 44 different cucumber lines. Transgenic complementation and RNAi-mediated gene silencing of CsTS1 in transgenic cucumber plants demonstrated that CsTS1 positively regulates the development of tubercules. CsTS1 is highly expressed in the peel at fruit tubercule forming and enlargement stage. Auxin content and expression of three auxin signalling pathway genes were altered in the 35S:CsTS1 and CsTS1-RNAi fruit tubercules, a result that was supported by comparing the cell size of the control and transgenic fruit tubercules. CsTu, a C₂ H₂ zinc finger domain transcription factor that regulates tubercule initiation, binds directly to the CsTS1 promoter and promotes its expression. Taken together, our results reveal a novel mechanism in which the CsTu-TS1 complex promotes fruit tubercule formation in cucumber.