Micro-trichome as a class I homeodomain-leucine zipper gene regulates multicellular trichome development in Cucumis sativus

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.

CsTs, a C-type lectin receptor-like kinase, regulates the development trichome development and cuticle metabolism in cucumber (Cucumis sativus)

Cucumber (Cucumis sativus) fruit spines are a classic material for researching the development of multicellular trichomes. Some key genes that influence trichome development have been confirmed to be associated with cuticle biosynthesis and secondary metabolism. However, the biological mechanisms underlying trichome development, cuticle biosynthesis, and secondary metabolism in cucumber remain poorly understood. CsTs, a C-type lectin receptor-like kinase gene, reportedly causes a tender trichome phenotype in cucumber when it mutates. In this study, the role of CsTs in cucumber fruit spines morphogenesis was confirmed using gene editing technology. Sectioning and cell wall component detection were used to analyse the main reason of tender fruit spines in the ts mutant. Subsequently, transcriptome data and a series of molecular biology experiments were used to further investigate the relationship between CsTs and cytoskeletal homeostasis in cucumber. CsTs overexpression partially compensated for the abnormal trichome phenotype of an Arabidopsis homolog mutant. Genetic hybridization and metabolic analysis indicated that CsTs and CsMict can affect trichome development and cuticle biosynthesis in the same pathway. Our findings provide important background information for further researching on the molecular mechanism underlying cucumber trichome development and contribute to understanding the biological function of C-type lectin receptor-like kinases.

Relevance and regulation of alternative splicing in plant secondary metabolism: current understanding and future directions

The secondary metabolism of plants is an essential life process enabling organisms to navigate various stages of plant development and cope with ever-changing environmental stresses. Secondary metabolites, abundantly found in nature, possess significant medicinal value. Among the regulatory mechanisms governing these metabolic processes, alternative splicing stands out as a widely observed post-transcriptional mechanism present in multicellular organisms. It facilitates the generation of multiple mRNA transcripts from a single gene by selecting different splicing sites. Selective splicing events in plants are widely induced by various signals, including external environmental stress and hormone signals. These events ultimately regulate the secondary metabolic processes and the accumulation of essential secondary metabolites in plants by influencing the synthesis of primary metabolites, hormone metabolism, biomass accumulation, and capillary density. Simultaneously, alternative splicing plays a crucial role in enhancing protein diversity and the abundance of the transcriptome. This paper provides a summary of the factors inducing alternative splicing events in plants and systematically describes the progress in regulating alternative splicing with respect to different secondary metabolites, including terpenoid, phenolic compounds, and nitrogen-containing compounds. Such elucidation offers critical foundational insights for understanding the role of alternative splicing in regulating plant metabolism and presents novel avenues and perspectives for bioengineering.

The mechanism underlying asymmetric bending of lateral petals in Delphinium (Ranunculaceae)

During the process of flower opening, most petals move downward in the direction of the pedicel (i.e., epinastic movement). In most Delphinium flowers, however, their two lateral petals display a very peculiar movement, the mirrored helical rotation, which requires the twist of the petal stalk. However, in some lineages, their lateral petals also exhibit asymmetric bending that increases the degree of mirrored helical rotation, facilitating the formation of a 3D final shape. Notably, petal asymmetric bending is a novel trait that has not been noticed yet, so its morphological nature, developmental process, and molecular mechanisms remain largely unknown. Here, by using D. anthriscifolium as a model, we determined that petal asymmetric bending was caused by the localized expansion of cell width, accompanied by the specialized array of cell wall nano-structure, on the adaxial epidermis. Digital gene analyses, gene expression, and functional studies revealed that a class I homeodomain-leucine zipper family transcription factor gene, DeanLATE MERISTEM IDENTITY1 (DeanLMI1), contributes to petal asymmetric bending; knockdown of it led to the formation of explanate 2D petals. Specifically, DeanLMI1 promotes cell expansion in width and influences the arrangement of cell wall nano-structure on the localized adaxial epidermis. These results not only provide a comprehensive portrait of petal asymmetric bending for the first time but also shed some new insights into the mechanisms of flower opening and helical movement in plants.

Comparative morphoanatomy and transcriptomic analyses reveal key factors controlling floral trichome development in Aristolochia (Aristolochiaceae)

Trichomes are specialized epidermal cells in aerial plant parts. Trichome development proceeds in three stages, determination of cell fate, specification, and morphogenesis. Most genes responsible for these processes have been identified in the unicellular branched leaf trichomes from the model Arabidopsis thaliana. Less is known about the molecular basis of multicellular trichome formation across flowering plants, especially those formed in floral organs of early diverging angiosperms. Here, we aim to identify the genetic regulatory network (GRN) underlying multicellular trichome development in the kettle-shaped trap flowers of Aristolochia (Aristolochiaceae). We selected two taxa for comparison, A. fimbriata, with trichomes inside the perianth, which play critical roles in pollination, and A. macrophylla, lacking specialized trichomes in the perianth. A detailed morphoanatomical characterization of floral epidermis is presented for the two species. We compared transcriptomic profiling at two different developmental stages in the different perianth portions (limb, tube, and utricle) of the two species. Moreover, we present a comprehensive expression map for positive regulators and repressors of trichome development, as well as cell cycle regulators. Our data point to extensive modifications in gene composition, expression, and putative roles in all functional categories when compared with model species. We also record novel differentially expressed genes (DEGs) linked to epidermis patterning and trichome development. We thus propose the first hypothetical genetic regulatory network (GRN) underlying floral multicellular trichome development in Aristolochia, and pinpoint key factors responsible for the presence and specialization of floral trichomes in phylogenetically distant species of the genus.

Evolutionary Consequences of Functional and Regulatory Divergence of HD-Zip I Transcription Factors as a Source of Diversity in Protein Interaction Networks in Plants

The HD superfamily has been studied in detail for several decades. The plant-specific HD-Zip I subfamily attracts the most attention because of its involvement in plant development and stress responses. In this review, we provide a comprehensive insight into the evolutionary events responsible for the functional redundancy and diversification of the HD-Zip I genes in regulating various biological processes. We summarized the evolutionary history of the HD-Zip family, highlighting the important role of WGDs in its expansion and divergence of retained duplicates in the genome. To determine the relationship between the evolutionary origin and functional conservation of HD-Zip I in different species, we performed a phylogenetic analysis, compared their expression profiles in different tissues and under stress and traced the role of orthologs and paralogs in regulating developmental processes. We found that HD-Zip I from different species have similar gene structures with a highly conserved HD and Zip, bind to the same DNA sequences and are involved in similar biological processes. However, they exhibit a functional diversity, which is manifested in altered expression patterns. Some of them are involved in the regulation of species-specific leaf morphology and phenotypes. Here, we discuss the role of changes in functional domains involved in DNA binding and protein interaction of HD-Zip I and in cis-regulated regions of its target genes in promoting adaptive innovations through the formation of de novo regulatory systems. Understanding the role of the HD-Zip I subfamily in organism-environment interactions remains a challenge for evolutionary developmental biology (evo-devo).

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 and stability analysis of trichome density in zucchini (Cucurbita pepo L.)

Trichomes provide an excellent model for studying cell differentiation and proliferation. The aboveground tissues of plants with long dense trichomes (LDTs) can cause skin itching in people working in a zucchini field, in which management, pollination, and fruit harvesting are difficult. In this study, an F2 population was constructed with the LDT inbred line "16" and the sparse micro trichome (SMT) inbred line "63" for QTL analysis of type I and II trichome density. Two QTLs were identified on chromosomes 3 and 15 using the QTL-seq method. Additionally, 191 InDel markers were developed on 20 chromosomes, a genetic map was constructed for QTL mapping, and three QTLs were identified on chromosomes 3, 6, and 15. Two QTLs, CpTD3.1 and CpTD15.1, were identified in both QTL-seq and genetic map-based QTL analyses, and CpTD15.1 was the major-effect QTL. The stability of CpTD3.1 and CpTD15.1 was confirmed using data from F2 plants under different environmental conditions. The major-effect QTL CpTD15.1 was located between markers chr15-4991349 and chr15-5766791, with a physical distance of 775.44 kb, and explained 12.71%-29.37% of the phenotypic variation observed in the three environments. CpTD3.1 was located between markers chr3-218350 and chr3-2891236, in a region with a physical distance of 2,672.89 kb, and explained 5.00%-10.64% of the phenotypic variation observed in the three environments. The functional annotations of the genes within the CpTD15.1 region were predicted, and five genes encoding transcription factors regulating trichome development were selected. Cp4.1LG15g04400 encoded zinc finger protein (ZFP) and harbored nonsynonymous SNPs in the conserved ring finger domain between the two parental lines. There were significant differences in Cp4.1LG15g04400 expression between "16" and "63", and a similar pattern was found between germplasm resources of LDT lines and SMT lines. It was presumed that Cp4.1LG15g04400 might regulate trichome density in zucchini. These results lay a foundation for better understanding the density of multicellular nonglandular trichomes and the regulatory mechanism of trichome density in zucchini.

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.

Identification and Functional Characterization of CsMYCs in Cucumber Glandular Trichome Development

Glandular trichomes (GTs), specialized structures formed by the differentiation of plant epidermal cells, are known to play important roles in the resistance of plants to external biotic and abiotic stresses. These structures are capable of storing and secreting secondary metabolites, which often have important agricultural and medicinal values. In order to better understand the molecular developmental mechanisms of GTs, studies have been conducted in a variety of crops, including tomato (Solanum lycopersicum), sweetworm (Artemisia annua), and cotton (Gossypium hirsutum). The MYC transcription factor of the basic helix-loop-helix (bHLH) transcription factor family has been found to play an important role in GT development. In this study, a total of 13 cucumber MYC transcription factors were identified in the cucumber (Cucumis sativus L.) genome. After performing phylogenetic analyses and conserved motifs on the 13 CsMYCs in comparison to previously reported MYC transcription factors that regulate trichome development, seven candidate MYC transcription factors were selected. Through virus-induced gene silencing (VIGS), CsMYC2 is found to negatively regulate GT formation while CsMYC4, CsMYC5, CsMYC6, CsMYC7, and CsMYC8 are found to positively regulate GT formation. Furthermore, the two master effector genes, CsMYC2 and CsMYC7, are observed to have similar expression patterns indicating that they co-regulate the balance of GT development in an antagonistic way.

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.

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.

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.

Cucumber glossy fruit 1 (CsGLF1) encodes the zinc finger protein 6 that regulates fruit glossiness by enhancing cuticular wax biosynthesis

Cucumber glossiness is an important visual quality trait that affects consumer choice. Accumulating evidence suggests that glossy trait is associated with cuticular wax accumulation. However, the molecular genetic mechanism controlling cucumber glossiness remains largely unknown. Here, we report the map-based cloning and functional characterization of CsGLF1, a locus that determines the glossy trait in cucumber. CsGLF1 encodes a homolog of the Cys₂His₂-like fold group (C2H2) -type zinc finger protein 6 (ZFP6) and its deletion leads to glossier pericarp and decreased cuticular wax accumulation. Consistently, transcriptomic analysis demonstrated that a group of wax biosynthetic genes were downregulated when CsZFP6 was absent. Further, transient expression assay revealed that CsZFP6 acted as a transcription activator of cuticular wax biosynthetic genes. Taken together, our findings demonstrated a novel regulator of fruit glossiness, which will provide new insights into regulatory mechanism of fruit glossiness in cucumber.

Hairiness Gene Regulated Multicellular, Non-Glandular Trichome Formation in Pepper Species

Trichomes are unicellular or multicellular epidermal structures that play a defensive role against environmental stresses. Although unicellular trichomes have been extensively studied as a mechanistic model, the genes involved in multicellular trichome formation are not well understood. In this study, we first classified the trichome morphology structures in Capsicum species using 280 diverse peppers. We cloned a key gene (Hairiness) on chromosome 10, which mainly controlled the formation of multicellular non-glandular trichomes (types II, III, and V). Hairiness encodes a Cys2-His2 zinc-finger protein, and virus-induced gene silencing of the gene resulted in a hairless phenotype. Differential expression of Hairiness between the hairiness and hairless lines was due to variations in promoter sequences. Transgenic experiments verified the hypothesis that the promoter of Hairiness in the hairless line had extremely low activity causing a hairless phenotype. Hair controlled the formation of type I glandular trichomes in tomatoes, which was due to nucleotide differences. Taken together, our findings suggest that the regulation of multicellular trichome formation might have similar pathways, but the gene could perform slightly different functions in crops.

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.

Molecular research progress and improvement approach of fruit quality traits in cucumber

Recent molecular studies revealed new opportunities to improve cucumber fruit quality. However, the fruit color and spine traits molecular basis remain vague despite the vast sources of genetic diversity. Cucumber is agriculturally, economically and nutritionally important vegetable crop. China produces three-fourths of the world's total cucumber production. Cucumber fruit quality depends on a number of traits such as the fruit color (peel and flesh color), spine (density, size and color), fruit shape, fruit size, defects, texture, firmness, taste, maturity stage and nutritional composition. Fruit color and spine traits determine critical quality attributes and have been the interest of researchers at the molecular level. Evaluating the molecular mechanisms of fruit quality traits is important to improve production and quality of cucumber varieties. Genes and qualitative trait locus (QTL) that are responsible for cucumber fruit color and fruit spine have been identified. The purpose of this paper is to reveal the molecular research progress of fruit color and spines as key quality traits of cucumber. The markers and genes identified so far could help for marker-assisted selection of the fruit color and spine trait in cucumber breeding and its associated nutritional improvement. Based on the previous studies, peel color and spine density as examples, we proposed a comprehensive approach for cucumber fruit quality traits improvement. Moreover, the markers and genes can be useful to facilitate cloning-mediated genetic breeding in cucumber. However, in the era of climate change, increased human population and high-quality demand of consumers, studies on molecular mechanisms of cucumber fruit quality traits are limited.

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.

LATE MERISTEM IDENTITY1 regulates leaf margin development via the auxin transporter gene SMOOTH LEAF MARGIN1

Plant leaves have evolved into diverse shapes and LATE MERISTEM IDENTITY1 (LMI1) and its putative paralogous genes encode homeodomain leucine zipper transcription factors that are proposed evolutionary hotspots for the regulation of leaf development in plants. However, the LMI1-mediated regulatory mechanism underlying leaf shape formation is largely unknown. MtLMI1a and MtLMI1b are putative orthologs of LMI1 in the model legume barrelclover (Medicago truncatula). Here, we investigated the role of MtLMI1a and MtLMI1b in leaf margin morphogenesis by characterizing loss-of-function mutants. MtLMI1a and MtLMI1b are expressed along leaf margin in a near-complementary pattern, and they redundantly promote development of leaf margin serrations, as revealed by the relatively smooth leaf margin in their double mutants. Moreover, MtLMI1s directly activate expression of SMOOTH LEAF MARGIN1 (SLM1), which encodes an auxin efflux carrier, thereby regulating auxin distribution along the leaf margin. Further analysis indicates that MtLMI1s genetically interact with NO APICAL MERISTEM (MtNAM) and the ARGONAUTE7 (MtAGO7)-mediated trans-acting short interfering RNA3 (TAS3 ta-siRNA) pathway to develop the final leaf margin shape. The participation of MtLMI1s in auxin-dependent leaf margin formation is interesting in the context of functional conservation. Furthermore, the diverse expression patterns of LMI1s and their putative paralogs within key domains are important drivers for functional specialization, despite their functional equivalency among species.

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.

Study of micro-trichome (mict) reveals novel connections between transcriptional regulation of multicellular trichome development and specific metabolism in cucumber

Trichomes that cover the epidermis of aerial plant organs play multiple roles in plant protection. Compared with a unicellular trichome in model plants, the development mechanism of the multicellular trichome is largely unclear. Notably, variations in trichome development are often accompanied by defects in the biosynthesis of cuticle and secondary metabolites; however, major questions about the interactions between developmental differences in trichomes and defects in metabolic pathways remain unanswered. Here, we characterized the glabrous mutant mict/csgl1/cstbh via combined metabolomic and transcriptomic analyses to extend our limited knowledge regarding multicellular trichome development and metabolism in cucumber. Mict was found to be explicitly expressed within trichome cells. Transcriptomic analysis indicated that genes involved in flavonoid and cuticle metabolism are significantly downregulated in mict mutants. Further metabolomic analysis confirmed that flavonoids, lipids, and cuticle compositions are dramatically altered in mict mutants. Additional studies revealed that Mict regulates flavonoid, lipid, and cuticle biosynthesis by likely directly binding to downstream functional genes, such as CsTT4, CsFLS1, CsCER26, and CsMYB36. These findings suggest that specific metabolic pathways (e.g., flavonoids and cuticle components) are co-regulated by Mict and provide insights into transcriptional regulation mechanisms of multicellular trichome development and its specific metabolism in cucumber.

Identification of Quantitative Trait Loci Controlling the Development of Prickles in Eggplant by Genome Re-sequencing Analysis

Eggplant (Solanum melongena L.) is the third most important crop in the family of Solanaceae. Prickles are considered as the undesirable traits during the plantation of eggplant and the transportation of fruits. In this study, we constructed a high-quality genetic linkage Bin map derived from the re-sequencing analysis on a cross of a prickly wild landrace, 17C01, and a cultivated variety, 17C02. The major quantitative trait locus (QTL) controlling the development of prickles on the calyx (explained 30.42% of the phenotypic variation), named as qPC.12, was identified on a ~7 kb region on chromosome 12. A gene within qPC.12, which encodes a WUSCHEL-related homeobox-like protein, with higher expression levels in 17C01 calyx and 22-bp deletion in 17C02 was probably the functional gene for prickle formation. Results from this study would ultimately facilitate uncovering the molecular regulatory mechanisms underlying the development of a prickle in eggplant.

The HD-Zip IV transcription factor SlHDZIV8 controls multicellular trichome morphology by regulating the expression of Hairless-2

Trichomes are specialized epidermal appendages that serve as excellent models to study cell morphogenesis. Although the molecular mechanism underlying trichome morphogenesis in Arabidopsis has been well characterized, most of the regulators essential for multicellular trichome morphology remain unknown in tomato. In this study, we determined that the recessive hairless-2 (hl-2) mutation in tomato causes severe distortion of all trichome types, along with increased stem fragility. Using map-based cloning, we found that the hl-2 phenotype was associated with a 100 bp insertion in the coding region of Nck-associated protein 1, a component of the SCAR/WAVE complex. Direct protein-protein interaction was detected between Hl-2 and Hl (SRA1, specifically Rac1-associated protein) using yeast two-hybrid and co-immunoprecipitation assays, suggesting that these proteins may work together during trichome formation. In addition, knock-down of a HD-Zip IV transcription factor, HDZIPIV8, distorted trichomes similar to the hl-2 mutant. HDZIPIV8 regulates the expression of Hl-2 by binding to the L1-box in the Hl-2 promoter region, and is involved in organizing actin filaments. The brittleness of hl-2 stems was found to result from decreased cellulose content. Taken together, these findings suggest that the Hl-2 gene plays an important role in controlling multicellular trichome morphogenesis and mechanical properties of stems in tomato plants.

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.

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.

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.

High accumulation of tilianin in in-vitro cultures of Agastache mexicana and its potential vasorelaxant action

Agastache mexicana has gained importance during the last decade as a natural source of bioactive compounds, mainly due to the antidiabetic, antihyperlipidemic, and vasorelaxant effects derived from its flavonoids, particularly tilianin. The goal of this work was to evaluate the production of tilianin during the in-vitro process of morphogenesis leading to plant regeneration and to investigate the vasorelaxant activity of its methanolic extracts. The cultures were established from nodal segments and leaf explants, inoculated on Murashige and Skoog (MS) media supplemented with various concentrations of benzyl aminopurine (BAP) alone or in combination with 2,4-Dichlorophenoxyacetic acid (2,4-D). Callus inductions were obtained in all treatments from both types of explants, but the presence of auxin was essential. Maximal shoot multiplication and elongation was achieved with 0.1 mg/l 2,4-D and 1.0 mg/l BAP from nodal- segment explants. Shoots were rooted in 75% MS medium and the plantlets were transferred to a greenhouse with 33% average survival. Analysis of tilianin production in methanolic extracts from calli (0.15-2.01 ± 0.06 mg/g dry weight), shoots (4.45 ± 0.01 mg/g DW), and whole plants (9.77 ± 0.02 mg/g DW) derived from in-vitro cultured nodal segments reveals that tilianin accumulation is associated with high cell differentiation and morphogenetic response to the plant-growth regulators. All of the extracts showed strong vasorelaxant activity, as compared to those of wild plant extracts. These results indicate that plant-tissue cultures of A. mexicana possess vast potential as a source of tilianin and other bioactive compounds.

Pyramiding of nine transgenes in maize generates high-level resistance against necrotrophic maize pathogens

Key message Nine transgenes from different categories, viz. plant defense response genes and anti-apoptosis genes, played combined roles in maize to inhibit the necrotrophic pathogens Rhizoctonia solani and Bipolaris maydis. Maize sheath blight and southern corn leaf blight are major global threats to maize production. The management of these necrotrophic pathogens has encountered limited success due to the characteristics of their lifestyle. Here, we presented a transgenic pyramiding breeding strategy to achieve nine different resistance genes integrated in one transgenic maize line to combat different aspects of necrotrophic pathogens. These nine genes, selected from two different categories, plant defense response genes (Chi, Glu, Ace-AMP1, Tlp, Rs-AFP2, ZmPROPEP1 and Pti4), and anti-apoptosis genes (Iap and p35), were successfully transferred into maize and further implicated in resistance against the necrotrophic pathogens Rhizoctonia solani and Bipolaris maydis. Furthermore, the transgenic maize line 910, with high expression levels of the nine integrated genes, was selected from 49 lines. Under greenhouse and field trial conditions, line 910 showed significant resistance against maize sheath blight and southern corn leaf blight diseases. Higher-level resistance was obtained after the pyramiding of more resistance transgenes from different categories that function via different mechanisms. The present study provides a successful strategy for the management of necrotrophic pathogens.

Combined fine mapping, genetic diversity, and transcriptome profiling reveals that the auxin transporter gene ns plays an important role in cucumber fruit spine development

Map-based cloning was used to identify the ns gene, which was involved in the formation of cucumber numerous fruit spines together with other genes under regulation by plant hormone signal transduction. The cucumber (Cucumis sativus) fruit spine density has an important impact on the commercial value. However, little is known about the regulatory mechanism for the fruit spine formation. Here, we identified NUMEROUS SPINES (NS), which regulate fruit spine development by modulating the Auxin signaling pathway. We fine-mapped the ns using a 2513 F₂ population derived from NCG122 (numerous fruit spines line) and NCG121 (few fruit spines line), and showed that NS encoded auxin transporter-like protein 3. Genetic diversity analysis of the NS gene in natural populations revealed that one SNP and one InDel in the coding region of ns are co-segregated with the fruit spine density. The NS protein sequence was highly conserved among plants, but its regulation of fruit spine development in cucumber seems to be a novel function. Transcriptome profiling indicated that the plant hormone signal transduction-related genes were highly enriched in the up-regulated genes in NCG122 versus NCG121. Moreover, expression pattern analysis of the auxin signal pathway-related genes in NCG122 versus NCG121 showed that upstream genes of the pathway (like ns candidate gene Csa2M264590) are down-regulated, while the downstream genes are up-regulated. Quantitative reverse transcription PCR confirmed the differential expression during the fruit spine development. Therefore, reduced expression of ns may promote the fruit spine formation. Our findings provide a valuable framework for dissecting the regulatory mechanism for the fruit spine development.

A RAD-Based Genetic Map for Anchoring Scaffold Sequences and Identifying QTLs in Bitter Gourd (Momordica charantia)

Genetic mapping is a basic tool necessary for anchoring assembled scaffold sequences and for identifying QTLs controlling important traits. Though bitter gourd (Momordica charantia) is both consumed and used as a medicinal, research on its genomics and genetic mapping is severely limited. Here, we report the construction of a restriction site associated DNA (RAD)-based genetic map for bitter gourd using an F2 mapping population comprising 423 individuals derived from two cultivated inbred lines, the gynoecious line 'K44' and the monoecious line 'Dali-11.' This map comprised 1,009 SNP markers and spanned a total genetic distance of 2,203.95 cM across the 11 linkage groups. It anchored a total of 113 assembled scaffolds that covered about 251.32 Mb (85.48%) of the 294.01 Mb assembled genome. In addition, three horticulturally important traits including sex expression, fruit epidermal structure, and immature fruit color were evaluated using a combination of qualitative and quantitative data. As a result, we identified three QTL/gene loci responsible for these traits in three environments. The QTL/gene gy/fffn/ffn, controlling sex expression involved in gynoecy, first female flower node, and female flower number was detected in the reported region. Particularly, two QTLs/genes, Fwa/Wr and w, were found to be responsible for fruit epidermal structure and white immature fruit color, respectively. This RAD-based genetic map promotes the assembly of the bitter gourd genome and the identified genetic loci will accelerate the cloning of relevant genes in the future.

A CsMYB6-CsTRY module regulates fruit trichome initiation in cucumber

Fruit epidermal features such as the number and size of trichomes or spines are important fruit quality traits in cucumber production. Little is known about the molecular mechanisms underlying fruit spine formation in cucumber. Here, we report functional characterization of the cucumber CsMYB6 gene, which encodes a MIXTA-like MYB transcription factor that plays an important role in regulating fruit trichome development. Spatial-temporal expression analyses revealed high-level expression of CsMYB6 in the epidermis of cucumber ovaries during fruit spine initiation, which was similar to the expression of CsTRY, a homolog of the Arabidopsis TRY gene that also plays a key role in trichome development. Overexpression of CsMYB6 and CsTRY in cucumber and Arabidopsis revealed that CsMYB6 and CsTRY act as negative regulators of trichome initiation in both species, and that CsMYB6 acted upstream of CsTRY in this process. CsMYB6 was found to bind to the three MYB binding sites inside the promoter region of CsTRY, and protein-protein interaction assays suggested that CsTRY also directly interacted with CsMYB6 protein. The results also revealed conserved and divergent roles of CsMYB6 and its Arabidopsis homolog AtMYB106 in trichome development. Collectively, our results reveal a novel mechanism in which the CsMYB6-CsTRY complex negatively regulates fruit trichome formation in cucumber.

GLABROUS (CmGL) encodes a HD-ZIP IV transcription factor playing roles in multicellular trichome initiation in melon

Map-based cloning identified CmGL that encodes a HD-ZIP type IV transcription factor that controls multicellular trichome initiation in melon. Trichomes are small hairs covering the aerial parts of plants that originate from the epidermal cells, which can protect plants against the damage by insects and pathogens. The regulatory pathway of unicellular trichomes has been well studied in the model plant Arabidopsis. Little is known about the genetic control and regulation of trichome development in melon (Cucumis melo L.) which has multicellular trichomes. In this study, we identified a melon mutant, cmgl, which showed completely glabrous on all aerial organs. A bulked segregant analysis was conducted to identify polymorphic markers for linkage analysis in a population with 256 F₂ plants, which allowed to locate the cmgl locus in melon chromosome VIII. Next-generation sequencing-aided marker discovery and fine mapping in a large population with 1536 F₂ plants narrowed the candidate gene region to 12 kb that harbored only one candidate gene for cmgl, which encoded a class IV homeodomain-associated leucine zipper transcription factor. Four SNPs in the coding region of the CmGL gene were identified between the two parental lines; a single base substitution from C to A resulted in a premature termination codon and a truncated protein in the cmgl. The SNP was converted into a dCAPS marker, which showed co-segregation in the F₂ population and 564 melon accessions. Result of this study will be helpful for better understanding of genetic control of trichome development in melon and marker-assisted selection in developing new cultivars.

Transcriptomic changes in Cucurbita pepo fruit after cold storage: differential response between two cultivars contrasting in chilling sensitivity

Background

Zucchini fruit is susceptible to chilling injury (CI), but the response to low storage temperature is cultivar dependent. Previous reports about the response of zucchini fruit to chilling storage have been focused on the physiology and biochemistry of this process, with little information about the molecular mechanisms underlying it. In this work, we present a comprehensive analysis of transcriptomic changes that take place after cold storage in zucchini fruit of two commercial cultivars with contrasting response to chilling stress.

Results

RNA-Seq analysis was conducted in exocarp of fruit at harvest and after 14 days of storage at 4 and 20 °C. Differential expressed genes (DEGs) were obtained comparing fruit stored at 4 °C with their control at 20 °C, and then specific and common up and down-regulated DEGs of each cultivar were identified. Functional analysis of these DEGs identified similarities between the response of zucchini fruit to low temperature and other stresses, with an important number of GO terms related to biotic and abiotic stresses overrepresented in both cultivars. This study also revealed several molecular mechanisms that could be related to chilling tolerance, since they were up-regulated in cv. Natura (CI tolerant) or down-regulated in cv. Sinatra (CI sensitive). These mechanisms were mainly those related to carbohydrate and energy metabolism, transcription, signal transduction, and protein transport and degradation. Among DEGs belonging to these pathways, we selected candidate genes that could regulate or promote chilling tolerance in zucchini fruit including the transcription factors MYB76-like, ZAT10-like, DELLA protein GAIP, and AP2/ERF domain-containing protein.

Conclusions

This study provides a broader understanding of the important mechanisms and processes related to coping with low temperature stress in zucchini fruit and allowed the identification of some candidate genes that may be involved in the acquisition of chilling tolerance in this crop. These genes will be the basis of future studies aimed to identify markers involved in cold tolerance and aid in zucchini breeding programs.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4500-9) contains supplementary material, which is available to authorized users.

A fragment substitution in the promoter of CsHDZIV11/CsGL3 is responsible for fruit spine density in cucumber (Cucumis sativus L.)

The indel in the promoter of CsHDZIV11 co-segregates with fruit spine density and could be used for molecular breeding in cucumber. Fruit spine density is an important quality trait for marketing in cucumber (Cucumis sativus L.). However, the molecular basis of fruit spine density in cucumber remains unclear. In this study, we isolated a mutant, few spines 1 (fs1), from CNS2 (wild type, WT), a North China-type cucumber with a high density of fruit spines. Genetic analysis showed that fs1 was controlled by a single recessive Mendelian factor. Bulked segregant analysis combined with genome resequencing were used for mapping fs1 in the F2 population derived from a cross between the fs1 mutant and WT, and it was located on chromosome 6 through association analysis. To develop more polymorphic markers to locate fs1, another F2 population was constructed from the cross between fs1 and 'Chinese long' 9930. Then, fs1 was narrowed down to a 110.4-kb genomic region containing 25 annotated genes. A fragment substitution was identified in the promoter region of Csa6M514870 between fs1 and WT. This fragment in fs1 was also present in wild cucumber. Csa6M514870 encodes a PDF2-related protein, a homeodomain-leucine zipper IV transcription factor (CsHDZIV11/CsGL3) sharing high identity and similarity with proteins related to trichome formation or epidermal cell differentiation. Quantitative reverse-transcription PCR revealed a higher expression level of CsHDZIV11 in young fruits from fs1 compared to WT. A molecular marker based on this indel co-segregated with the spine density. This work provides a solid foundation not only for understanding the molecular mechanism of fruit spine density, but also for molecular breeding in cucumber.

Identification and mapping of Tril, a homeodomain-leucine zipper gene involved in multicellular trichome initiation in Cucumis sativus

KEY MESSAGE

Using map-based cloning of Tril gene, we identified a homeodomain-leucine zipper gene involved in the initiation of multicellular trichomes (including the spines of fruit) in cucumber.

ABSTRACT

Fruit spines are a special type of trichome that impacts the quality and appearance of cucumber (Cucumis sativus L.) fruit. Scanning electron microscopy revealed that the trichome-less (tril) mutant originating from European greenhouse cucumber has a completely glabrous phenotype on cotyledons, hypocotyls, young leaves, fruits, and fruit stalks. Genetic analysis revealed that tril was inherited as a recessive allele at a single locus. Using 1058 F2 individuals derived from a cross between cucumber tril mutant CGN19839 and the micro-trichome (mict) mutant 06-2, tril was mapped to chromosome 6, and narrowed down to a 37.4 kb genomic region which carries seven predicted genes. Genetic and molecular analyses revealed that gene Cucsa.045360 is a possible candidate gene for the differentiation of epidermal cells to trichomes. It is a member of the class IV homeodomain-leucine zipper (HD-Zip IV) family and encodes homeodomain and START domain, sharing 66.7% predicted amino acid sequence identity to PROTODERMAL FACTOR2 (PDF2) and 35.0% to GLABRA2 (GL2) of Arabidopsis. The homeobox domain had changed amino acid sequence because of an insertion in tril mutant. The results of genetic analysis and transcriptome profiling indicated that the Tril gene had an epistatic effect on the Mict gene in trichome development. Phenotypes of the tril mutant such as glabrous fruits and female flowers at every node could be used in developing new cultivars.

Trichome-Related Mutants Provide a New Perspective on Multicellular Trichome Initiation and Development in Cucumber (Cucumis sativus L)

Trichomes are specialized epidermal cells located in aerial parts of plants that function in plant defense against biotic and abiotic stresses. The simple unicellular trichomes of Arabidopsis serve as an excellent model to study the molecular mechanism of cell differentiation and pattern formation in plants. Loss-of-function mutations in Arabidopsis thaliana have suggested that the core genes GL1 (which encodes a MYB transcription factor) and TTG1 (which encodes a WD40 repeat-containing protein) are important for the initiation and spacing of leaf trichomes, while for normal trichome initiation, the genes GL3, and EGL3 (which encode a bHLH protein) are needed. However, the positive regulatory genes involved in multicellular trichrome development in cucumber remain unclear. This review focuses on the phenotype of mutants (csgl3, tril, tbh, mict, and csgl1) with disturbed trichomes in cucumber and then infers which gene(s) play key roles in trichome initiation and development in those mutants. Evidence indicates that MICT, TBH, and CsGL1 are allelic with alternative splicing. CsGL3 and TRIL are allelic and override the effect of TBH, MICT, and CsGL1 on the regulation of multicellular trichome development; and affect trichome initiation. CsGL3, TRIL, MICT, TBH, and CsGL1 encode HD-Zip proteins with different subfamilies. Genetic and molecular analyses have revealed that CsGL3, TRIL, MICT, TBH, and CsGL1 are responsible for the differentiation of epidermal cells and the development of trichomes. Based on current knowledge, a positive regulator pathway model for trichome development in cucumber was proposed and compared to a model in Arabidopsis. These data suggest that trichome development in cucumber may differ from that in Arabidopsis.

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

BACKGROUND

Trichomes, developed from the protodermal cells (the outermost cell layer of the embryo), are hair-like structures covering the aerial parts of plants. The genetic network regulating trichome development has been extensively studied and well understood in the model species Arabidopsis thaliana, which bears unicellular, non-glandular and branched trichomes. However, little is known about the genetic and molecular basis of organogenesis of multi-cellular trichomes in plant species like cucumber (Cucumis sativus L.), which are likely different from Arabidopsis.

RESULTS

We identified a new trichome mutant in cucumber which exhibited a completely glabrous phenotype on all aerial organs. Genetic analysis indicated that the glabrous phenotype was inherited as a single recessive gene, csgl3. Fine genetic mapping delimited the csgl3 locus into a 68.4 kb region with 12 predicted genes. Genetic analysis, sequence alignment and allelic variation survey in natural populations identified Csa6G514870 encoding a class IV homeodomain-associated leucine zipper (HD-ZIP) transcription factor as the only candidate for CsGL3, which was 5188 bp in length with 10 predicted exons. Gene expression analysis revealed the loss-of-function of CsGL3 in the mutant due to the insertion of a 5-kb long terminal repeat (LTR) retrotransposon in the 4th exon of CsGL3. Linkage analysis in a segregating population and gene expression analysis of the CsGL1 and CsGL3 genes in csgl1, csgl3, and csgl1 + 3 genetic backgrounds uncovered interactions between the two genes. Phylogenetic analysis among 28 class IV HD-ZIP protein sequences from five species placed cucumber CsGL3 into the same clade with 7 other members that play important roles in trichome initiation.

CONCLUSIONS

The new glabrous mutation in cucumber was controlled by a single recessive locus csgl3, which was phenotypically and genetically distinct from two previously reported glabrous mutants csgl1 and csgl2. The glabrous phenotype in csgl3 was due to insertion of an autonomous, active, class I transposable element in CsGL3, a class IV HD-ZIP transcription factor. CsGL3 was epistatic to CsGL1. CsGL3 seemed to play important roles in cucumber trichome initiation whereas CsGL1 may act downstream in the trichome development pathway(s). Findings from the present study provide new insights into genetic control of trichome development in cucumber.