A CsMYB6-CsTRY module regulates fruit trichome initiation in cucumber

Research on the function of CsMYB36 based on an effective hair root transformation system

The hairy root induction system was used to efficiently investigate gene expression and function in plant root. Cucumber is a significant vegetable crop worldwide, with shallow roots, few lateral roots, and weak root systems, resulting in low nutrient absorption and utilization efficiency. Identifying essential genes related to root development and nutrient absorption is an effective way to improve the growth and development of cucumbers. However, genetic mechanisms underlying cucumber root development have not been explored. Here, we report a novel, rapid, effective hairy root transformation system. Compared to the in vitro cotyledon transformation method, this method shortened the time needed to obtain transgenic roots by 13 days. Furthermore, we combined this root transformation method with CRISPR/Cas9 technology and validated our system by exploring the expression and function of CsMYB36, a pivotal gene associated with root development and nutrient uptake. The hairy root transformation system established in this study provides a powerful method for rapidly identifying essential genes related to root development in cucumber and other horticultural crop species. This advancement holds promise for expediting research on root biology and molecular breeding strategies, contributing to the broader understanding and improvements crop growth and development.

Exploring the influence of a single-nucleotide mutation in EIN4 on tomato fruit firmness diversity through fruit pericarp microstructure

Tomato (Solanum lycopersicum) stands as one of the most valuable vegetable crops globally, and fruit firmness significantly impacts storage and transportation. To identify genes governing tomato firmness, we scrutinized the firmness of 266 accessions from core collections. Our study pinpointed an ethylene receptor gene, SlEIN4, located on chromosome 4 through a genome-wide association study (GWAS) of fruit firmness in the 266 tomato core accessions. A single-nucleotide polymorphism (SNP) (A → G) of SlEIN4 distinguished lower (AA) and higher (GG) fruit firmness genotypes. Through experiments, we observed that overexpression of SlEIN4AA significantly delayed tomato fruit ripening and dramatically reduced fruit firmness at the red ripe stage compared with the control. Conversely, gene editing of SlEIN4AA with CRISPR/Cas9 notably accelerated fruit ripening and significantly increased fruit firmness at the red ripe stage compared with the control. Further investigations revealed that fruit firmness is associated with alterations in the microstructure of the fruit pericarp. Additionally, SlEIN4AA positively regulates pectinase activity. The transient transformation assay verified that the SNP (A → G) on SlEIN4 caused different genetic effects, as overexpression of SlEIN4GG increased fruit firmness. Moreover, SlEIN4 exerts a negative regulatory role in tomato ripening by impacting ethylene evolution through the abundant expression of ethylene pathway regulatory genes. This study presents the first evidence of the role of ethylene receptor genes in regulating fruit firmness. These significant findings will facilitate the effective utilization of firmness and ripening traits in tomato improvement, offering promising opportunities for enhancing tomato storage and transportation capabilities.

Characterization and functions of Myeloblastosis (MYB) transcription factors in cucurbit crops

Myeloblastosis (MYB) is one of the largest family of transcription factors (TFs) in plants. It plays a key role in plant life activities, such as metabolic regulation, stress resistant, as well as helpful for plant growth and development. In China, cucurbit is an important and nutrients rich vegetable crop, which have high medicinal and socio-economic values. In this review, we discussed the structure and characterization of MYB TFs and howdo regulate flower development, fruit maturity, fruit quality, and flavonoid biosynthesis. Furthermore, we highlight the effect and contribution of MYB TFs in the regulation of biotic and abiotic stress resistance. This comprehensivereview will provide a new reference for the more effective application of MYB TF in quality control, stress resistance research and molecular breeding of cucurbit crops.

High Tartronic Acid Content Germplasms Screening of Cucumber and Its Response to Exogenous Agents

Tartronic acid is known for its potential to inhibit sugar-to-lipid conversion in the human body, leading to weight loss and fat reduction. This compound is predominantly found in cucumbers and other cucurbit crops. Therefore, cultivating cucumbers with high tartronic acid content holds significant health implications. In this study, we assessed the tartronic acid content in 52 cucumber germplasms with favorable overall traits and identified 8 cucumber germplasms with elevated tartronic acid levels. Our investigation into factors influencing cucumber tartronic acid revealed a decrease in content with fruit development from the day of flowering. Furthermore, tartronic acid content was higher in early-harvested fruits compared to late-harvested ones, with the rear part of the fruit exhibiting significantly higher content than other parts. Foliar spraying of microbial agents increased tartronic acid content by 84.4%. This study provides valuable resources for breeding high tartronic acid cucumbers and offers practical insights for optimizing cucumber production practices.

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.

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.

TrichomeLess Regulator 3 is required for trichome initial and cuticle biosynthesis in Artemisia annua

Artemisinin is primarily synthesized and stored in the subepidermal space of the glandular trichomes of Artemisia annua. The augmentation of trichome density has been demonstrated to enhance artemisinin yield. However, existing literature lacks insights into the correlation between the stratum corneum and trichomes. This study aims to unravel the involvement of TrichomeLess Regulator 3 (TLR3), which encodes the transcription factor, in artemisinin biosynthesis and its potential association with the stratum corneum. TLR3 was identified as a candidate gene through transcriptome analysis. The role of TLR3 in trichome development and morphology was investigated using yeast two-hybrid, pull-down analysis, and RNA electrophoresis mobility assay. Our research revealed that TLR3 negatively regulates trichome development. It modulates the morphology of Arabidopsis thaliana trichomes by inhibiting branching and inducing the formation of abnormal trichomes in Artemisia annua. Overexpression of the TLR3 gene disrupts the arrangement of the stratum corneum and reduces artemisinin content. Simultaneously, TLR3 possesses the capacity to regulate stratum corneum development and trichome follicle morphology by interacting with TRICHOME AND ARTEMISININ REGULATOR 1, and CycTL. Consequently, our findings underscore the pivotal role of TLR3 in the development of glandular trichomes and stratum corneum biosynthesis, thereby influencing the morphology of Artemisia annua trichomes.

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.

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.

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.

Impact of Climate Change on Regulation of Genes Involved in Sex Determination and Fruit Production in Cucumber

Environmental changes, both natural and anthropogenic, mainly related to rising temperatures and water scarcity, are clearly visible around the world. Climate change is important for crop production and is a major issue for the growth and productivity of cucumbers. Processes such as sex determination, flower morphogenesis and fruit development in cucumbers are highly sensitive to various forms of stress induced by climatic changes. It is noteworthy that many factors, including genetic factors, transcription factors, phytohormones and miRNAs, are crucial in regulating these processes and are themselves affected by climate change. Changes in the expression and activity of these factors have been observed as a consequence of climatic conditions. This review focuses primarily on exploring the effects of climate change and abiotic stresses, such as increasing temperature and drought, on the processes of sex determination, reproduction, and fruit development in cucumbers at the molecular level. In addition, it highlights the existing research gaps that need to be addressed in order to improve our understanding of the complex interactions between climate change and cucumber physiology. This, in turn, may lead to strategies to mitigate the adverse effects and enhance cucumber productivity in a changing climate.

AaSEPALLATA1 integrates jasmonate and light-regulated glandular secretory trichome initiation in Artemisia annua

Glandular secretory trichomes (GSTs) can secrete and store a variety of specific metabolites. By increasing GST density, valuable metabolites can be enhanced in terms of productivity. However, the comprehensive and detailed regulatory network of GST initiation still needs further investigation. By screening a complementary DNA library derived from young leaves of Artemisia annua, we identified a MADS-box transcription factor, AaSEPALLATA1 (AaSEP1), that positively regulates GST initiation. Overexpression of AaSEP1 in A. annua substantially increased GST density and artemisinin content. The HOMEODOMAIN PROTEIN 1 (AaHD1)-AaMYB16 regulatory network regulates GST initiation via the jasmonate (JA) signaling pathway. In this study, AaSEP1 enhanced the function of AaHD1 activation on downstream GST initiation gene GLANDULAR TRICHOME-SPECIFIC WRKY 2 (AaGSW2) through interaction with AaMYB16. Moreover, AaSEP1 interacted with the JA ZIM-domain 8 (AaJAZ8) and served as an important factor in JA-mediated GST initiation. We also found that AaSEP1 interacted with CONSTITUTIVE PHOTOMORPHOGENIC 1 (AaCOP1), a major repressor of light signaling. In this study, we identified a MADS-box transcription factor that is induced by JA and light signaling and that promotes the initiation of GST in A. annua.

Regulation of capsule spine formation in castor

Castor (Ricinus communis L.) is a dicotyledonous oilseed crop that can have either spineless or spiny capsules. Spines are protuberant structures that differ from thorns or prickles. The developmental regulatory mechanisms governing spine formation in castor or other plants have remained largely unknown. Herein, using map-based cloning in 2 independent F2 populations, F2-LYY5/DL01 and F2-LYY9/DL01, we identified the RcMYB106 (myb domain protein 106) transcription factor as a key regulator of capsule spine development in castor. Haplotype analyses demonstrated that either a 4,353-bp deletion in the promoter or a single nucleotide polymorphism leading to a premature stop codon in the RcMYB106 gene could cause the spineless capsule phenotype in castor. Results of our experiments indicated that RcMYB106 might target the downstream gene RcWIN1 (WAX INDUCER1), which encodes an ethylene response factor known to be involved in trichome formation in Arabidopsis (Arabidopsis thaliana) to control capsule spine development in castor. This hypothesis, however, remains to be further tested. Nevertheless, our study reveals a potential molecular regulatory mechanism underlying the spine capsule trait in a nonmodel plant species.

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.

Comparative transcriptome analysis reveals hormone, transcriptional and epigenetic regulation involved in prickle formation in Zanthoxylum armatum

Zanthoxylum armatum is an evergreen plant with high economical and medicinal values. The presence of prickles on stems and leaves is undesirable for them make picking difficult. To date, little is known of prickle formation in Z. armatum. Herein, the morphological and molecular features of prickle initiation in prickless (WC) and three types of prickly Z. armatum were characterized. Compared to WC, the levels of cytokinin and auxin were increased, while GA and JA declined in prickly Z. armatum. Transcriptome analysis identified 6258 differentially expressed genes (DEGs)between prickless and prickly Z. armatum. Among them, several DEGs related to hormone metabolism and signaling, including LOG7, CKX3, AHK1, three DELLAs, six JAZs and TIR1, were candidate genes involved in prickle formation. Transcription factors associated with prickle formation was screened, including MYB6-1/MYB6-2, WER, GL3-2, SPL4/5, SOC1, and SCL32. Of them, MYB6-1 and WER might negatively regulate prickles initiation via interacting with GL3-2. Additionally, the histone acetylation and DNA methylation levels, the transcripts of histone acetyltransferase/deacetylase and DNA methyltransferases showed significant differences between prickless and prickly plants, indicating their involvements in prickle initiation. These findings illustrate the regulation of prickle formation might be mediated by phytohormones (especially cytokinin), transcription factors and epigenetic modifications in Z. armatum.

Melon yellow-green plant (Cmygp) encodes a Golden2-like transcription factor regulating chlorophyll synthesis and chloroplast development

A SNP mutation in CmYGP gene encoding Golden2-like transcription factor is responsible for melon yellow-green plant trait. Chlorophylls are essential and beneficial substances for both plant and human health. Identifying the regulatory network of chlorophyll is necessary to improve the nutritional quality of fruits. At least six etiolation genes have been identified in different melon varieties, but none of them have been cloned, and the molecular mechanisms underlying chlorophyll synthesis and chloroplast development in melon remain unclear. Here, the NSL73046, a yellow-green plant (Cmygp) mutant, enabled the map-based cloning of the first etiolation gene in melon. CmYGP encodes a Golden2-like transcription factor. Spatiotemporal expression analyses confirmed the high CmYGP expression in all green tissues, particularly in young leaves and fruit peels. Virus-induced gene silencing and the development of near-isogenic line by marker-assisted selection further confirmed that downregulation of CmYGP can reduce chloroplast number and chlorophyll content, thereby resulting in yellow-green leaves and fruits in melon, and overexpression of CmYGP in tomatoes also led to dark-green leaves and fruits. RNA-seq analysis revealed that CmYGP greatly affected the expression of key genes associated with chloroplast development. Taken together, these findings demonstrated that CmYGP regulate chlorophyll synthesis and chloroplast development thus affect fruit development in melon. This study also offers a new strategy to enhance fruit quality in melon.

Moving Away from 12:12; the Effect of Different Photoperiods on Biomass Yield and Cannabinoids in Medicinal Cannabis

The standard practice to initiate flowering in medicinal cannabis involves reducing the photoperiod from a long-day period to an equal duration cycle of 12 h light (12L)/12 h dark (12D). This method reflects the short-day flowering dependence of many cannabis varieties but may not be optimal for all. We sought to identify the effect of nine different flowering photoperiod treatments on the biomass yield and cannabinoid concentration of three medicinal cannabis varieties. The first, "Cannatonic", was a high cannabidiol (CBD)-accumulating line, whereas the other two, "Northern Lights" and "Hindu Kush", were high Δ9-tetrahydrocannabinol (THC) accumulators. The nine treatments tested, following 18 days under 18 h light/6 h dark following cloning and propagation included a standard 12L:12D period, a shortened period of 10L:14D, and a lengthened period of 14L:10D. The other six treatments started in one of the aforementioned and then 28 days later (mid-way through flowering) were switched to one of the other treatments, thus causing either an increase of 2 or 4 h, or a decrease of 2 or 4 h. Measured parameters included the timing of reproductive development; the dry weight flower yield; and the % dry weight of the main target cannabinoids, CBD and THC, from which the total g cannabinoid per plant was calculated. Flower biomass yields were highest for all lines when treatments started with 14L:10D; however, in the two THC lines, a static 14L:10D photoperiod caused a significant decline in THC concentration. Conversely, in Cannatonic, all treatments starting with 14L:10D led to a significant increase in the CBD concentration, which led to a 50-100% increase in total CBD yield. The results show that the assumption that a 12L:12D photoperiod is optimal for all lines is incorrect as, in some lines, yields can be greatly increased by a lengthened light period during flowering.

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.

PpTCP18 is upregulated by lncRNA5 and controls branch number in peach (Prunus persica) through positive feedback regulation of strigolactone biosynthesis

Branch number is an important agronomic trait in peach (Prunus persica) trees because plant architecture affects fruit yield and quality. Although breeders can select varieties with different tree architecture, the biological mechanisms underlying architecture remain largely unclear. In this study, a pillar peach ('Zhaoshouhong') and a standard peach ('Okubo') were compared. 'Zhaoshouhong' was found to have significantly fewer secondary branches than 'Okubo'. Treatment with the synthetic strigolactone (SL) GR24 decreased branch number. Transcriptome analysis indicated that PpTCP18 (a homologous gene of Arabidopsis thaliana BRC1) expression was negatively correlated with strigolactone synthesis gene expression, indicating that PpTCP18 may play an important role in peach branching. Yeast one-hybrid, electrophoretic mobility shift, dual-luciferase assays and PpTCP18-knockdown in peach leaf buds indicated that PpTCP18 could increase expression of PpLBO1, PpMAX1, and PpMAX4. Furthermore, transgenic Arabidopsis plants overexpressing PpTCP18 clearly exhibited reduced primary rosette-leaf branches. Moreover, lncRNA sequencing and transient expression analysis revealed that lncRNA5 targeted PpTCP18, significantly increasing PpTCP18 expression. These results provide insights into the mRNA and lncRNA network in the peach SL signaling pathway and indicate that PpTCP18, a transcription factor downstream of SL signaling, is involved in positive feedback regulation of SL biosynthesis. This role of PpTCP18 may represent a novel mechanism in peach branching regulation. Our study improves current understanding of the mechanisms underlying peach branching and provides theoretical support for genetic improvement of peach tree architecture.

Differential environmental and genomic architectures shape the natural diversity for trichome patterning and morphology in different Arabidopsis organs

Despite the adaptive and taxonomic relevance of the natural diversity for trichome patterning and morphology, the molecular and evolutionary mechanisms underlying these traits remain mostly unknown, particularly in organs other than leaves. In this study, we address the ecological, genetic and molecular bases of the natural variation for trichome patterning and branching in multiple organs of Arabidopsis (Arabidopsis thaliana). To this end, we characterized a collection of 191 accessions and carried out environmental and genome-wide association (GWA) analyses. Trichome amount in different organs correlated negatively with precipitation in distinct seasons, thus suggesting a precise fit between trichome patterning and climate throughout the Arabidopsis life cycle. In addition, GWA analyses showed small overlapping between the genes associated with different organs, indicating partly independent genetic bases for vegetative and reproductive phases. These analyses identified a complex locus on chromosome 2, where two adjacent MYB genes (ETC2 and TCL1) displayed differential effects on trichome patterning in several organs. Furthermore, analyses of transgenic lines carrying different natural alleles demonstrated that TCL1 accounts for the variation for trichome patterning in all organs, and for stem trichome branching. By contrast, two other MYB genes (TRY and GL1), mainly showed effects on trichome patterning or branching, respectively.

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.

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.

Cucurbitaceae genome evolution, gene function and molecular breeding

The Cucurbitaceae is one of the most genetically diverse plant families in the world. Many of them are important vegetables or medicinal plants and are widely distributed worldwide. The rapid development of sequencing technologies and bioinformatic algorithms has enabled the generation of genome sequences of numerous important Cucurbitaceae species. This has greatly facilitated research on gene identification, genome evolution, genetic variation and molecular breeding of cucurbit crops. So far, genome sequences of 18 different cucurbit species belonging to tribes Benincaseae, Cucurbiteae, Sicyoeae, Momordiceae and Siraitieae have been deciphered. This review summarizes the genome sequence information, evolutionary relationship, and functional genes associated with important agronomic traits (e.g., fruit quality). The progress of molecular breeding in cucurbit crops and prospects for future applications of Cucurbitaceae genome information are also discussed.

Hair interacts with SlZFP8-like to regulate the initiation and elongation of trichomes by modulating SlZFP6 expression in tomato

Trichomes are specialized glandular or non-glandular structures that provide physical or chemical protection against insect and pathogen attack. Trichomes in Arabidopsis have been extensively studied as typical non-glandular structures. By contrast, the molecular mechanism underlying glandular trichome formation and elongation remains largely unknown. We previously demonstrated that Hair is essential for the formation of type I and type VI trichomes. Here, we found that overexpression of Hair increased the density and length of tomato trichomes. Biochemical assays revealed that Hair physically interacts with its close homolog SlZFP8-like (SlZFP8L), and SlZFP8L also directly interacts with Woolly. SlZFP8L-overexpressing plants showed increased trichome density and length. We further found that the expression of SlZFP6, which encodes a C2H2 zinc finger protein, is positively regulated by Hair. Using chromatin immunoprecipitation, yeast one-hybrid, and dual-luciferase assays we identified that SlZFP6 is a direct target of Hair. Similar to Hair and SlZFP8L, the overexpression of SlZFP6 also increased the density and length of tomato trichomes. Taken together, our results suggest that Hair interacts with SlZFP8-like to regulate the initiation and elongation of trichomes by modulating SlZFP6 expression in tomato.

AcERF1B and AcERF073 Positively Regulate Indole-3-acetic Acid Degradation by Activating AcGH3.1 Transcription during Postharvest Kiwifruit Ripening

Ethylene can accelerate the postharvest ripening process of kiwifruit, while indole-3-acetic acid (IAA) delays it. However, the molecular mechanism by which ethylene regulates IAA degradation is unclear. Here, we found that ethephon promotes the degradation of free IAA in kiwifruit. Furthermore, ethylene can promote the expression of AcGH3.1 and enhance its promoter activity. Two ethylene response factors (ERFs), AcERF1B and AcERF073, were obtained using an AcGH3.1 promoter as bait for a yeast one-hybrid screening library. Both AcERF1B and AcERF073 bind to the AcGH3.1 promoter to activate it. Also, AcERF1B/073 enhanced AcGH3.1 expression, decreased the free IAA content, and increased the IAA-Asp content in kiwifruit. In addition, we found that the AcERF1B and AcERF073 proteins directly interact, and this interaction enhanced their binding to the AcGH3.1 promoter. In summary, our results suggest that AcERF1B and AcERF073 positively regulate IAA degradation by activating AcGH3.1 transcription, which accelerated postharvest kiwifruit ripening.

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.

Capsicum chinense MYB Transcription Factor Genes: Identification, Expression Analysis, and Their Conservation and Diversification With Other Solanaceae Genomes

Myeloblastosis (MYB) genes are important transcriptional regulators of plant growth, development, and secondary metabolic biosynthesis pathways, such as capsaicinoid biosynthesis in Capsicum. Although MYB genes have been identified in Capsicum annuum, no comprehensive study has been conducted on other Capsicum species. We identified a total of 251 and 240 MYB encoding genes in Capsicum chinense MYBs (CcMYBs) and Capsicum baccatum MYBs (CbMYBs). The observation of twenty tandem and 41 segmental duplication events indicated expansion of the MYB gene family in the C. chinense genome. Five CcMYB genes, i.e., CcMYB101, CcMYB46, CcMYB6, CcPHR8, and CcRVE5, and two CaMYBs, i.e., CaMYB3 and CaHHO1, were found within the previously reported capsaicinoid biosynthesis quantitative trait loci. Based on phylogenetic analysis with tomato MYB proteins, the Capsicum MYBs were classified into 24 subgroups supported by conserved amino acid motifs and gene structures. Also, a total of 241 CcMYBs were homologous with 225 C. annuum, 213 C. baccatum, 125 potato, 79 tomato, and 23 Arabidopsis MYBs. Synteny analysis showed that all 251 CcMYBs were collinear with C. annuum, C. baccatum, tomato, potato, and Arabidopsis MYBs spanning over 717 conserved syntenic segments. Using transcriptome data from three fruit developmental stages, a total of 54 CcMYBs and 81 CaMYBs showed significant differential expression patterns. Furthermore, the expression of 24 CcMYBs from the transcriptome data was validated by quantitative real-time (qRT) PCR analysis. Eight out of the 24 CcMYBs validated by the qRT-PCR were highly expressed in fiery hot C. chinense than in the lowly pungent C. annuum. Furthermore, the co-expression analysis revealed several MYB genes clustered with genes from the capsaicinoid, anthocyanin, phenylpropanoid, carotenoid, and flavonoids biosynthesis pathways, and related to determining fruit shape and size. The homology modeling of 126 R2R3 CcMYBs showed high similarity with that of the Arabidopsis R2R3 MYB domain template, suggesting their potential functional similarity at the proteome level. Furthermore, we have identified simple sequence repeat (SSR) motifs in the CcMYB genes, which could be used in Capsicum breeding programs. The functional roles of the identified CcMYBs could be studied further so that they can be manipulated for Capsicum trait improvement.

Transcriptome profiling reveals key genes in regulation of the tepal trichome development in Lilium pumilum D.C

A number of potential genes and pathways involved in tepal trichome development were identified in a natural lily mutant by transcriptome analysis and were confirmed with trichome and trichomeless species. Trichome is a specialized structure found on the surface of the plant with an important function in survival against abiotic and biotic stress. It is also an important economic trait in crop breeding. Extensive research has investigated the foliar trichome in model plants (Arabidopsis and tomato). However, the developmental mechanism of tepal trichome remains elusive. Lilium pumilum is an edible ornamental bulb and a good breeding parent possessing cold and salt-alkali resistance. Here, we found a natural mutant of Lilium pumilum grown on a highland whose tepals are covered by trichomes. Our data indicate that trichomes of the mutant are multicellular and branchless. Notably, stomata are also developed on the tepal of the mutant as well, suggesting there may be a correlation between trichome and stomata regulation. Furthermore, we isolated 27 differentially expressed genes (DEGs) by comparing the transcriptome profiling between the natural mutant and the wild type. These 27 genes belong to 4 groups: epidermal cell cycle and division, trichome morphogenesis, stress response, and transcription factors. Quantitative real-time PCR in Lilium pumilum (natural mutant and the wild type) and other lily species (Lilium leichtlinii var. maximowiczii/trichome; Lilium davidii var. willmottiae/, trichomeless) confirmed the validation of RNA-seq data and identified several trichome-related genes.

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.

Genome-wide identification of the tea plant bHLH transcription factor family and discovery of candidate regulators of trichome formation

Leaf trichomes play vital roles in plant resistance and the quality of tea. Basic helix-loop-helix (bHLH) transcription factors (TFs) play an important role in regulating plant development and growth. In this study, a total of 134 CsbHLH proteins were identified in the Camellia sinensis var. sinensis (CSS) genome. They were divided into 17 subgroups according to the Arabidopsis thaliana classification. Phylogenetic tree analysis indicated that members of subgroups IIIc-I and IIIc-II might be associated with trichome formation. The expression patterns of CsbHLH116, CsbHLH133, CsbHLH060, CsbHLH028, CsbHLH024, CsbHLH112 and CsbHLH053 from clusters 1, 3 and 5 were similar to the trichome distribution in tea plants. CsbHLH024 and CsbHLH133 were located in the cell nucleus and possessed transcriptional activation ability. They could interact with CsTTG1, which is a regulator of tea trichome formation. This study provides useful information for further research on the function of CsbHLHs in trichome formation.

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.

MYB transcription factors drive evolutionary innovations in Arabidopsis fruit trichome patterning

Both inter- and intra-specific diversity has been described for trichome patterning in fruits, which is presumably involved in plant adaptation. However, the mechanisms underlying this developmental trait have been hardly addressed. Here we examined natural populations of Arabidopsis (Arabidopsis thaliana) that develop trichomes in fruits and pedicels, phenotypes previously not reported in the Arabidopsis genus. Genetic analyses identified five loci, MALAMBRUNO 1-5 (MAU1-5), with MAU2, MAU3, and MAU5 showing strong epistatic interactions that are necessary and sufficient to display these traits. Functional characterization of these three loci revealed cis-regulatory mutations in TRICHOMELESS1 and TRIPTYCHON, as well as a structural mutation in GLABRA1. Therefore, the multiple mechanisms controlled by three MYB transcription factors of the core regulatory network for trichome patterning have jointly been modulated to trigger trichome development in fruits. Furthermore, analyses of worldwide accessions showed that these traits and mutations only occur in a highly differentiated relict lineage from the Iberian Peninsula. In addition, these traits and alleles were associated with low spring precipitation, which suggests that trichome development in fruits and pedicels might be involved in climatic adaptation. Thus, we show that the combination of synergistic mutations in a gene regulatory circuit has driven evolutionary innovations in fruit trichome patterning in Arabidopsis.

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.

Mapping of Genetic Locus for Leaf Trichome Formation in Chinese Cabbage Based on Bulked Segregant Analysis

Chinese cabbage is a leafy vegetable, and its leaves are the main edible organs. The formation of trichomes on the leaves can significantly affect its taste, so studying this phenomenon is of great significance for improving the quality of Chinese cabbage. In this study, two varieties of Chinese cabbage, W30 with trichome leaves and 082 with glabrous leaves, were crossed to generate F₁ and F₁ plants, which were self-fertilized to develop segregating populations with trichome or glabrous morphotypes. The two bulks of the different segregating populations were used to conduct bulked segregant analysis (BSA). A total of 293.4 M clean reads were generated from the samples, and plants from the trichome leaves (AL) bulk and glabrous leaves (GL) bulk were identified. Between the two DNA pools generated from the trichome and glabrous plants, 55,048 SNPs and 272 indels were generated. In this study, three regions (on chromosomes 6, 10 and scaffold000100) were identified, and the annotation revealed three candidate genes that may participate in the formation of leaf trichomes. These findings suggest that the three genes-Bra025087 encoding a cyclin family protein, Bra035000 encoding an ATP-binding protein/kinase/protein kinase/protein serine/threonine kinase and Bra033370 encoding a WD-40 repeat family protein-influence the formation of trichomes by participating in trichome morphogenesis (GO: 0010090). These results demonstrate that BSA can be used to map genes associated with traits and provide new insights into the molecular mechanism of leafy trichome formation in Chinese cabbage.

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.

Looking into 'hair tonics' for cotton fiber initiation

Cotton fiber is the most important source of cellulose for the global textile industry. These hair-like single-celled trichomes develop from ovule epidermis. They are classified into long spinnable lint and short fuzz. A key objective in the cotton industry is to breed elite cultivars with fuzzless seeds carrying high lint yield. Molecular basis underlying lint and fuzz initiation remains obscure. Recent studies indicate fiber initiation is under the control of MYB-bHLH-WDR (MBW) transcription factor complex. Based on molecular genetic studies and gene expression patterns linking fiber phenotypes, we propose that specific but different sets of MBW genes are required to precisely regulate the initiation of the lint and fuzz fibers. Emerging evidence further points to sugar signaling as a 'hair-tonic' to boost fiber initiation through interaction with MBW complex and auxin signaling. An integrative model is provided as a conceptual framework for future studies to dissect the molecular network responsible for cotton fiber initiation.

The Formation of Fruit Quality in Cucumis sativus L

Cucumber is one of the most widely grown vegetables in China and an indispensable fresh fruit in the diet. With the development of society, the demand of people for cucumber quality is higher and higher. Therefore, cultivating high-quality cucumber varieties is one of the main goals of cucumber breeding. With the rapid development of biotechnology such as molecular marker, cucumber quality control network is becoming clear. In this review, we describe the formation mechanism of cucumber fruit quality from three aspects: (1) the commercial quality of cucumber fruit, (2) nutritional quality formation, and (3) flavor quality of cucumber fruit. In addition, the determinants of cucumber fruit quality were summarized from two aspects of genetic regulation and cultivation methods in order to provide ideas for cucumber researchers and cultivators to improve fruit quality.

RNA-Seq Analysis of Prickled and Prickle-Free Epidermis Provides Insight into the Genetics of Prickle Development in Red Raspberry (Rubus ideaus L.)

Red raspberry (Rubus idaeus L.) is a globally commercialized specialty crop with growing demand worldwide. The presence of prickles on the stems, petioles and undersides of the leaves complicates both the field management and harvesting of raspberries. An RNA sequencing analysis was used to identify differentially expressed genes in the epidermal tissue of prickled “Caroline” and prickle-free “Joan J.” and their segregating progeny. Expression patterns of differentially expressed genes (DEGs) in prickle-free plants revealed the downregulation of some vital development-related transcription factors (TFs), including a MIXTA-like R2R3-MYB family member; MADS-box; APETALA2/ETHYLENE RESPONSIVE FACTOR (AP2/ERF) and NAM, ATAF1/2 and CUC2 (NAC) in prickle-free epidermis tissue. The downregulation of these TFs was confirmed by qRT-PCR analysis, indicating a key regulatory role in prickle development. This study adds to the understanding of prickle development mechanisms in red raspberries needed for utilizing genetic engineering strategies for developing prickle-free raspberry cultivars and, possibly, other Rubus species, such as blackberry (Rubus sp.) and black raspberry (R. occidentalis L.).

Melon short internode (CmSi) encodes an ERECTA-like receptor kinase regulating stem elongation through auxin signaling

Plant height is one of the most important agronomic traits that directly determines plant architecture, and compact or dwarf plants can allow for increased planting density and land utilization as well as increased lodging resistance and economic yield. At least four dwarf/semidwarf genes have been identified in different melon varieties, but none of them have been cloned, and little is known about the molecular mechanisms underlying internode elongation in melon. Here, we report map-based cloning and functional characterization of the first semidwarf gene short internode (Cmsi) in melon, which encodes an ERECTA-like receptor kinase regulating internode elongation. Spatial-temporal expression analyses revealed that CmSI exhibited high expression in the vascular bundle of the main stem during internode elongation. The expression level of CmSI was positively correlated with stem length in the different melon varieties examined. Ectopic expression of CmSI in Arabidopsis and cucumber suggested CmSI as a positive regulator of internode elongation in both species. Phytohormone quantitation and transcriptome analysis showed that the auxin content and the expression levels of a number of genes involved in the auxin signaling pathway were altered in the semidwarf mutant, including several well-known auxin transporters, such as members of the ABCB family and PIN-FORMED genes. A melon polar auxin transport protein CmPIN2 was identified by protein-protein interaction assay as physically interacting with CmSI to modulate auxin signaling. Thus, CmSI functions in an auxin-dependent regulatory pathway to control internode elongation in melon. Our findings revealed that the ERECTA family gene CmSI regulates stem elongation in melon through auxin signaling, which can directly affect polar auxin transport.

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.

The HD-ZIP IV transcription factor GL2-LIKE regulates male flowering time and fertility in cucumber

Cucumber is dioecious by nature, having both male and female flowers, and is a model system for unisexual flower development. Knowledge related to male flowering is limited, but it is reported to be regulated by transcription factors and hormone signals. Here, we report functional characterization of the cucumber (Cucumis sativus) GL2-LIKE gene, which encodes a homeodomain leucine zipper (HD-ZIP) IV transcription factor that plays an important role in regulating male flower development. Spatial-temporal expression analyses revealed high-level expression of CsGL2-LIKE in the male flower buds and anthers. CsGL2-LIKE is closely related to AtGL2, which is known to play a key role in trichome development. However, ectopic expression of CsGL2-LIKE in Arabidopsis gl2-8 mutant was unable to rescue the gl2-8 phenotype. Interestingly, the silencing of CsGL2-LIKE delayed male flowering by inhibiting the expression of the florigen gene FT and reduced pollen vigor and seed viability. Protein-protein interaction assays showed that CsGL2-LIKE interacts with the jasmonate ZIM domain protein CsJAZ1 to form a HD-ZIP IV-CsJAZ1 complex. Collectively, our study indicates that CsGL2-LIKE regulates male flowering in cucumber, and reveals a novel function of a HD-ZIP IV transcription factor in regulating male flower development of cucumber.

Comparative transcriptomics analysis revealing flower trichome development during flower development in two Lonicera japonica Thunb. cultivars using RNA-seq

BACKGROUND

Lonicera japonica Thunb. (L. japonica) has the functions of clearing away heat and detoxifying, broad-spectrum antibacterial and anti-virus, etc. More than 70% of anti-inflammatory and cold Chinese patent medicines contain L. japonica. Trichomes comprise specialized multicellular structures that have the capacity to synthesize and secrete secondary metabolites and protect plants from biotic and abiotic stresses. The extraction of trichome secretions has great commercial value. However, little is known about the trichome formation mechanism in L. japonica. Therefore, the study of trichome development between different varieties provides a basis for selecting suitable planting resources.

RESULTS

Here, we present a genome-wide comparative transcriptome analysis between two L. japonica cultivars, toward the identification of biological processes and functional gene activities that occur during flowering stage trichome development. In this study, the density and average lengths of flower trichomes were at their highest during three-green periods (S2). Using the Illumina RNA-Seq method, we obtained 134,304 unigenes, 33,733 of which were differentially expressed. In an analysis of 40 differentially expressed unigenes (DEGs) involved in trichome development, 29 of these were transcription factors. The DEGs analysis of plant hormone signal transduction indicated that plant growth and development may be independent of gibberellin (GA) and cytokinine (CTK) signaling pathways, and plant stress may be independent of jasmonic acid (JA) and ethylene (ET) signaling pathways. We screened several genes involved in the floral biosynthesis of odors, tastes, colors, and plant hormones, and proposed biosynthetic pathways for sesquiterpenoid, triterpenoid, monoterpenoid, flavonoid, and plant hormones. Furthermore, 82 DEGs were assigned to cell cycles and 2616 were predicted as plant resistance genes (PRGs).

CONCLUSIONS

This study provides a comprehensive characterization of the expression profiles of flower development during the seven developmental stages of L. japonica, thereby offering valuable insights into the molecular networks that underly flower development in L. japonica.

Insights into the Diversification and Evolution of R2R3-MYB Transcription Factors in Plants

As one of the largest families of transcription factors (TFs) in plants, R2R3-MYB proteins play crucial roles in regulating a series of plant-specific biological processes. Although the diversity of plant R2R3-MYB TFs has been studied previously, the processes and mechanisms underlying the expansion of these proteins remain unclear. Here, we performed evolutionary analyses of plant R2R3-MYB TFs with dense coverage of streptophyte algae and embryophytes. Our analyses revealed that ancestral land plants exhibited 10 subfamilies of R2R3-MYB proteins, among which orthologs of seven subfamilies were present in chlorophytes and charophycean algae. We found that asymmetric gene duplication events in different subfamilies account for the expansion of R2R3-MYB proteins in embryophytes. We further discovered that the largest subfamily of R2R3-MYBs in land plants, subfamily VIII, emerged in the common ancestor of Zygnematophyceae and embryophytes. During plant terrestrialization, six duplication events gave rise to seven clades of subfamily VIII. Subsequently, this TF subfamily showed a tendency for expansion in bryophytes, lycophytes, and ferns and extensively diversified in ancestral gymnosperms and angiosperms in clades VIII-A-1, VIII-D, and VIII-E. In contrast to subfamily VIII, other subfamilies of R2R3-MYB TFs have remained less expanded across embryophytes. The findings regarding phylogenetic analyses, auxiliary motifs, and DNA-binding specificities provide insight into the evolutionary history of plant R2R3-MYB TFs and shed light on the mechanisms underlying the extensive expansion and subsequent sub- and neofunctionalization of these proteins.

A Scanning Electron Micrograph-based Resource for Identification of Loci Involved in Epidermal Development in Tomato: Elucidation of a New Function for the Mixta-like Transcription Factor in Leaves

The aerial epidermis of plants plays a major role in environmental interactions, yet the development of the cellular components of the aerial epidermis-trichomes, stomata, and pavement cells-is still not fully understood. We have performed a detailed screen of the leaf epidermis in two generations of the well-established Solanum lycopersicum cv M82 × Solanum pennellii ac. LA716 introgression line (IL) population using a combination of scanning electron microscopy (SEM) techniques. Quantification of trichome and stomatal densities in the ILs revealed four genomic regions with a consistently low trichome density. This study also found ILs with abnormal proportions of different trichome types and aberrant trichome morphologies. This work has led to the identification of new, unexplored genomic regions with roles in trichome formation in tomato. This study investigated one interval in IL2-6 in more detail and identified a new function for the transcription factor SlMixta-like in determining trichome patterning in leaves. This illustrates how these SEM images, publicly available to the research community, provide an important dataset for further studies on epidermal development in tomato and other species of the Solanaceae family.

Genetic Control of Glandular Trichome Development

Plant glandular trichomes are epidermal secretory structures producing various specialized metabolites. These metabolites are involved in plant adaptation to its environment and many of them have remarkable properties exploited by fragrance, flavor, and pharmaceutical industries. The identification of genes controlling glandular trichome development is of high interest to understand how plants produce specialized metabolites. Our knowledge about this developmental process is still limited, but genes controlling glandular trichome initiation and morphogenesis have recently been identified. In particular, R2R3-MYB and HD-ZIP IV transcription factors appear to play essential roles in glandular trichome initiation in Artemisia annua and tomato. In this review, we focus on the results obtained in these two species and we propose genetic regulation models integrating these data.