GLABROUS INFLORESCENCE STEMS (GIS) is required for trichome branching through gibberellic acid signaling in Arabidopsis

Artemisia annua ZFP8L regulates glandular trichome development

Trichomes are known to be important biofactories that contribute to the production of secondary metabolites, such as terpenoids. C2H2-zinc finger proteins (C2H2-ZFPs) are vital transcription factors of plants' trichome development. However, little is known about the function of Artemisia annua C2H2-ZFPs in trichome development. To explore the roles of this gene family in trichome development, two C2H2-ZFP transcription factors, named AaZFP8L and AaGIS3, were identified; both are hormonally regulated in A. annua. Overexpression of AaZFP8L in tobacco led to a significant increase in the density and length of glandular trichomes, and improved terpenoid content. In contrast, AaGIS3 was found to positively regulate non-glandular trichome initiation and elongation, which reduces terpenoid accumulation. In addition, ABA contents significantly increased in AaZFP8L-overexpressing tobacco lines and AaZFP8L also can directly bind the promoter of the ABA biosynthesis genes. This study lays the foundation for further investigating A. annua C2H2-ZFPs in trichome development and terpenoid accumulation.

Identification of bZIP Transcription Factors That Regulate the Development of Leaf Epidermal Cells in Arabidopsis thaliana by Single-Cell RNA Sequencing

Epidermal cells are the main avenue for signal and material exchange between plants and the environment. Leaf epidermal cells primarily include pavement cells, guard cells, and trichome cells. The development and distribution of different epidermal cells are tightly regulated by a complex transcriptional regulatory network mediated by phytohormones, including jasmonic acid, and transcription factors. How the fate of leaf epidermal cells is determined, however, is still largely unknown due to the diversity of cell types and the complexity of their regulation. Here, we characterized the transcriptional profiles of epidermal cells in 3-day-old true leaves of Arabidopsis thaliana using single-cell RNA sequencing. We identified two genes encoding BASIC LEUCINE-ZIPPER (bZIP) transcription factors, namely bZIP25 and bZIP53, which are highly expressed in pavement cells and early-stage meristemoid cells. Densities of pavement cells and trichome cells were found to increase and decrease, respectively, in bzip25 and bzip53 mutants, compared with wild-type plants. This trend was more pronounced in the presence of jasmonic acid, suggesting that these transcription factors regulate the development of trichome cells and pavement cells in response to jasmonic acid.

Dual-function C2H2-type zinc-finger transcription factor GmZFP7 contributes to isoflavone accumulation in soybean

Isoflavones are a class of secondary metabolites produced by legumes and play important roles in human health and plant stress tolerance. The C2H2 zinc-finger transcription factor (TF) functions in plant stress tolerance, but little is known about its function in isoflavone regulation in soybean (Glycine max). Here, we report a C2H2 zinc-finger TF gene, GmZFP7, which regulates isoflavone accumulation in soybean. Overexpressing GmZFP7 increased the isoflavone concentration in both transgenic hairy roots and plants. By contrast, silencing GmZFP7 expression significantly reduced isoflavone levels. Metabolomic and qRT-PCR analysis revealed that GmZFP7 can increase the flux of the phenylpropanoid pathway. Furthermore, dual-luciferase and electrophoretic mobility shift assays showed that GmZFP7 regulates isoflavone accumulation by influencing the expression of Isoflavone synthase 2 (GmIFS2) and Flavanone 3 β-hydroxylase 1 (GmF3H1). In this study, we demonstrate that GmZFP7 contributes to isoflavone accumulation by regulating the expression of the gateway enzymes (GmIFS2 and GmF3H1) of competing phenylpropanoid pathway branches to direct the metabolic flux into isoflavone. A haplotype analysis indicated that important natural variations were present in GmZFP7 promoters, with P-Hap1 and P-Hap3 being the elite haplotypes. Our findings provide insight into how GmZFP7 regulates the phenylpropanoid pathway and enhances soybean isoflavone content.

Genetic and Transcriptome Analysis of Leaf Trichome Development in Chinese Cabbage (Brassica rapa L. subsp. pekinensis) and Molecular Marker Development

Chinese cabbage (Brassica rapa L. subsp. pekinensis) is one of the vegetables with the largest cultivated area in China and has been a great addition to the daily diet of Chinese people. A genetic map has been constructed in our previous study using the F₂ population of two inbred lines of Chinese cabbage, namely "G291" (a hairy line) and "ZHB" (a hairless line), based on which a candidate gene related to trichome traits was identified on chromosome A06 with a phenotypic variance of 47%. A molecular marker was found to co-segregate with the trichome traits of the F₂ population, which is in the 5'-flanking region of BrGL1, and a corresponding patent has been granted (NO. CN 108545775 B). Transcriptome analysis was carried out on the cotyledon, the first true leaf and the leaf closest to each inflorescence of F₂ individuals of "G291 × ZHB" with or without trichomes, respectively. Ten pathways, including 189 DEGs, were identified to be involved in the development of trichomes in Chinese cabbage, which may be specifically related to the development of leaf trichomes. Most of the pathways were related to the biosynthesis of the secondary metabolites, which may help plants to adapt to the ever-changing external environment. DEGs also enriched the "plant-pathogen interaction" pathway, which is consistent with the conclusion that trichomes are related to the disease resistance of plants. Our study provides a basis for future research on the occurrence and development of trichomes in Chinese cabbage.

Plant Development and Crop Yield: The Role of Gibberellins

Gibberellins have been classically related to a few key developmental processes, thus being essential for the accurate unfolding of plant genetic programs. After more than a century of research, over one hundred different gibberellins have been described. There is a continuously increasing interest in gibberellins research because of their relevant role in the so-called "Green Revolution", as well as their current and possible applications in crop improvement. The functions attributed to gibberellins have been traditionally restricted to the regulation of plant stature, seed germination, and flowering. Nonetheless, research in the last years has shown that these functions extend to many other relevant processes. In this review, the current knowledge on gibberellins homeostasis and mode of action is briefly outlined, while specific attention is focused on the many different responses in which gibberellins take part. Thus, those genes and proteins identified as being involved in the regulation of gibberellin responses in model and non-model species are highlighted. The present review aims to provide a comprehensive picture of the state-of-the-art perception of gibberellins molecular biology and its effects on plant development. This picture might be helpful to enhance our current understanding of gibberellins biology and provide the know-how for the development of more accurate research and breeding programs.

TRY intron2 determined its expression in inflorescence activated by SPL9 and MADS-box genes in Arabidopsis

Plant trichomes are specialized epidermal cells that protect plants from insects and pathogens. In Arabidopsis, epidermal hairs decrease as internodes increase in height, with only few epidermal hairs produced on the sepals abaxial surface of the early flowers. TRIPTYCHON (TRY) is known to be a negative regulator of epidermal hair development in Arabidopsis, suppressing the formation of ectopic epidermal hairs in the inflorescence. Here, we reported that the second intron of TRY gene plays a critical role in trichome spatial distribution in Arabidopsis. The expression of TRY rises with the increasing stem nodes and reaches the peak in the inflorescence, while the trichomes distribution decrease. The transgenic plants showed that TRY promoter could only drive the genomic instead of coding sequences combined with GUS reporter gene, which indicates that the regulatory elements of TRY expression in inflorescence could be located in the intron regions. Multiple SPLs and MADS-box binding sites were found in the TRY intron2 sequence. Further genetic and biochemistry assays revealed that the flowering-related genes such as SPL9 could bind to these cis-elements directly, contributing to the TRY spatial expression. Since cotton fiber and Arabidopsis trichomes share similar regulatory mechanism, extended analysis showed that the intron2 of cotton TRY genes also contain the cis-elements. Thus, the introns harboring the transcription element may be the general way to regulate the gene expression in different plants and provides molecular clues for the related crops' traits design.

Tomato ARPC1 regulates trichome morphology and density and terpene biosynthesis

Based on transcriptomic analysis of wild-type and mutant tomato plants, ARPC1 was found to be important for trichome formation and development and it plays a key role in terpene synthesis. Trichomes are protruding epidermal cells in plant species. They function as the first defense layer against biotic and abiotic stresses. Despite the essential role of tomato trichomes in defense against herbivores, the understanding of their development is still incomplete. Therefore, the aim of this study was to identify genes involved in trichome formation and morphology and terpene synthesis, using transcriptomic techniques. To achieve this, we examined leaf morphology and compared the expression levels of some putative genes involved in trichome formation between wild-type (WT) and hairless-3 (hl-3) tomato mutant. The hl-3 plants displayed swollen and distorted trichomes and reduced trichome density (type I and IV) and terpene synthesis compared with that of the WT plants. Gene expression analysis showed that Actin-Related Protein Component1 (ARPC1) was expressed more highly in the WT than in the hl-3 mutant, indicating its critical role in trichome morphology and density. Additionally, the expression of MYC1 and several terpene synthase genes (TPS9, 12, 20), which are involved in type VI trichome initiation and terpene synthesis, was lower in the hl-3 mutant than in the WT plants. Moreover, transformation of the hl-3 mutant with WT ARPC1 restored normal trichome structure and density, and terpene synthesis. Structural and amino acid sequence analysis showed that there was a missplicing mutation in the hl-3 mutant, which was responsible for the abnormal trichome structure and density, and impaired terpene synthesis. Overall, the findings of this study demonstrated that ARPC1 is involved in regulating trichome structure and terpene synthesis in tomato.

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.

An advanced method for the release, enrichment and purification of high-quality Arabidopsis thaliana rosette leaf trichomes enables profound insights into the trichome proteome

BACKGROUND

Rosette leaf trichomes of Arabidopsis thaliana have been broadly used to study cell development, cell differentiation and, more recently, cell wall biogenesis. However, trichome-specific biochemical or -omics analyses require a proper separation of trichomes from residual plant tissue. Thus, different strategies were proposed in the past for trichome isolation, which mostly rely on harsh conditions and suffer from low yield, thereby limiting the spectrum of downstream analyses.

RESULTS

To take trichome-leaf separation to the next level, we revised a previously proposed method for isolating A. thaliana trichomes by optimizing the mechanical and biochemical specifications for trichome release. We additionally introduced a density gradient centrifugation step to remove residual plant debris. We found that prolonged, yet mild seedling agitation increases the overall trichome yield by more than 60% compared to the original protocol. We noticed that subsequent density gradient centrifugation further visually enhances trichome purity, which may be advantageous for downstream analyses. Gene expression analysis by quantitative reverse transcriptase-polymerase chain reaction validated a substantial enrichment upon purification of trichomes by density gradient centrifugation. Histochemical and biochemical investigation of trichome cell wall composition indicated that unlike the original protocol gentle agitation during trichome release largely preserves trichome integrity. We used enriched and density gradient-purified trichomes for proteomic analysis in comparison to trichome-depleted leaf samples and present a comprehensive reference data set of trichome-resident and -enriched proteins. Collectively we identified 223 proteins that are highly enriched in trichomes as compared to trichome-depleted leaves. We further demonstrate that the procedure can be applied to retrieve diverse glandular and non-glandular trichome types from other plant species.

CONCLUSIONS

We provide an advanced method for the isolation of A. thaliana leaf trichomes that outcompetes previous procedures regarding yield and purity. Due to the large amount of high-quality trichomes our method enabled profound insights into the so far largely unexplored A. thaliana trichome proteome. We anticipate that our protocol will be of use for a variety of downstream analyses, which are expected to shed further light on the biology of leaf trichomes in A. thaliana and possibly other plant species.

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.

A HAT1-DELLA signaling module regulates trichome initiation and leaf growth by achieving gibberellin homeostasis

Trichome initiation and leaf growth are two critical developmental processes in the plant life cycle, which need to be optimized in accordance with developmental stage and immediate surroundings. To a large extent, this optimization is achieved by fine-tuning of hormonal pathways, including the gibberellin (GA) pathway. However, the mechanism by which plants control GA homeostasis to optimize these two developmental processes is unknown. Here, we report that HAT1, a HD-ZIP II transcription factor, negatively regulates GA-mediated trichome initiation and cotyledon expansion. Both protein and transcript levels indicated that HAT1 was induced by GA, while an increased abundance of HAT1, in turn, was found to suppress GA biosynthesis and signaling, thus forming a regulatory negative feedback loop that controls GA homeostasis to fine-tune trichome development and cotyledon expansion. We also found that HAT1 interacts with DELLAs, including GAI and RGA. GAI inhibits both protein stability and the binding activity of HAT1 to its target genes. Overexpression of HAT1 in della5 can completely suppress the enhanced trichome initiation and enlarged cotyledon of della5. Our findings demonstrate that HAT1 functions as a critical repressor to regulate GA-mediated trichome initiation and cotyledon growth; in addition, we describe a novel mechanism by which the plant regulates trichome initiation and cotyledon expansion through a HAT1-DELLA regulatory module under various GA concentrations.

Large-scale whole-genome resequencing unravels the domestication history of Cannabis sativa

Cannabis sativa has long been an important source of fiber extracted from hemp and both medicinal and recreational drugs based on cannabinoid compounds. Here, we investigated its poorly known domestication history using whole-genome resequencing of 110 accessions from worldwide origins. We show that C. sativa was first domesticated in early Neolithic times in East Asia and that all current hemp and drug cultivars diverged from an ancestral gene pool currently represented by feral plants and landraces in China. We identified candidate genes associated with traits differentiating hemp and drug cultivars, including branching pattern and cellulose/lignin biosynthesis. We also found evidence for loss of function of genes involved in the synthesis of the two major biochemically competing cannabinoids during selection for increased fiber production or psychoactive properties. Our results provide a unique global view of the domestication of C. sativa and offer valuable genomic resources for ongoing functional and molecular breeding research.

Genome-Wide Identification and Expression Patterns of the C2H2-Zinc Finger Gene Family Related to Stress Responses and Catechins Accumulation in Camellia sinensis [L.] O. Kuntze

The C2H2-zinc finger protein (C2H2-ZFP) is essential for the regulation of plant development and widely responsive to diverse stresses including drought, cold and salt stress, further affecting the late flavonoid accumulation in higher plants. Tea is known as a popular beverage worldwide and its quality is greatly dependent on the physiological status and growing environment of the tea plant. To date, the understanding of C2H2-ZFP gene family in Camellia sinensis [L.] O. Kuntze is not yet available. In the present study, 134 CsC2H2-ZFP genes were identified and randomly distributed on 15 chromosomes. The CsC2H2-ZFP gene family was classified into four clades and gene structures and motif compositions of CsC2H2-ZFPs were similar within the same clade. Segmental duplication and negative selection were the main forces driving the expansion of the CsC2H2-ZFP gene family. Expression patterns suggested that CsC2H2-ZFPs were responsive to different stresses including drought, salt, cold and methyl jasmonate (MeJA) treatment. Specially, several C2H2-ZFPs showed a significant correlation with the catechins content and responded to the MeJA treatment, which might contribute to the tea quality and specialized astringent taste. This study will lay the foundations for further research of C2H2-type zinc finger proteins on the stress responses and quality-related metabolites accumulation in C. sinensis.

Construction of a Full-Length cDNA Over-Expressing Library to Identify Valuable Genes from Populus tomentosa

Poplar wood is the main source of renewable biomass energy worldwide, and is also considered to be a model system for studying woody plants. The Full-length cDNA Over-eXpressing (FOX) gene hunting system is an effective method for generating gain-of-function mutants. Large numbers of novel genes have successfully been identified from many herbaceous plants according to the phenotype of gain-of-function mutants under normal or abiotic stress conditions using this system. However, the system has not been used for functional gene identification with high-throughput mutant screening in woody plants. In this study, we constructed a FOX library from the Chinese white poplar, Populus tomentosa. The poplar cDNA library was constructed into the plant expression vector pEarleyGate101 and further transformed into Arabidopsis thaliana (thale cress). We collected 1749 T1 transgenic plants identified by PCR. Of these, 593 single PCR bands from different transgenic lines were randomly selected for sequencing, and 402 diverse sequences of poplar genes were isolated. Most of these genes were involved in photosynthesis, environmental adaptation, and ribosome biogenesis based on Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway annotation. We characterized in detail two mutant lines carrying PtoCPCa or PtoWRKY13 cDNA insertions. Phenotypic characterization showed that overexpression of these genes in A. thaliana affected trichome development or secondary cell wall (SCW) deposition, respectively. Together, the Populus-FOX-Arabidopsis library generated in our experiments will be helpful for efficient discovery of novel genes in poplar.

Analysis and review of trichomes in plants

BACKGROUND

Trichomes play a key role in the development of plants and exist in a wide variety of species.

RESULTS

In this paper, it was reviewed that the structure and morphology characteristics of trichomes, alongside the biological functions and classical regulatory mechanisms of trichome development in plants. The environment factors, hormones, transcription factor, non-coding RNA, etc., play important roles in regulating the initialization, branching, growth, and development of trichomes. In addition, it was further investigated the atypical regulation mechanism in a non-model plant, found that regulating the growth and development of tea (Camellia sinensis) trichome is mainly affected by hormones and the novel regulation factors.

CONCLUSIONS

This review further displayed the complex and differential regulatory networks in trichome initiation and development, provided a reference for basic and applied research on trichomes in plants.

Phytohormone-Based Regulation of Trichome Development

Phytohormones affect plant growth and development. Many phytohormones are involved in the initiation of trichome development, which can help prevent damage from UV radiation and insect bites and produce fragrance, flavors, and compounds used as pharmaceuticals. Phytohormones promote the participation of transcription factors in the initiation of trichome development; for example, the transcription factors HDZIP, bHLH and MYB interact and form transcriptional complexes to regulate trichome development. Jasmonic acid (JA) mediates the progression of the endoreduplication cycle to increase the number of multicellular trichomes or trichome size. Moreover, there is crosstalk between phytohormones, and some phytohormones interact with each other to affect trichome development. Several new techniques, such as the CRISPR-Cas9 system and single-cell transcriptomics, are available for investigating gene function, determining the trajectory of individual trichome cells and elucidating the regulatory network underlying trichome cell lineages. This review discusses recent advances in the modulation of trichome development by phytohormones, emphasizes the differences and similarities between phytohormones initially present in trichomes and provides suggestions for future research.

Transcriptomic Analysis and Specific Expression of Transcription Factor Genes in the Root and Sporophyll of Dryopteris fragrans (L.) Schott

Background: Dryopteris fragrans, which is densely covered with glandular trichomes, is considered to be one of the ferns with the most medicinal potential. The transcriptomes from selected tissues of D. fragrans were collected and analyzed for functional and comparative genomic studies. The aim of this study was to determine the transcriptomic characteristics of wild D. fragrans sporangium in tissues from the SR (root), SL (sporophyll), and TRL (sporophyll with glandular trichomes removed). Results: Cluster analysis identified genes that were highly expressed in an organ-specific manner according to read mapping, feature counting, and normalization. The functional map identified gene clusters that can uniquely describe the function of each tissue. We identified a group of three tissue-specific transcription factors targeting the SL, SR, and TRL. In addition, highly expressed transcription factors (TFs) were found in each tissue-specific gene cluster, where ERF and bHLH transcription factors were the two types showing the most distinct expression patterns between the three different tissues. The specific expression of transcription factor genes varied between the different types of tissues. The numbers of transcription factors specifically expressed in the roots and sporophylls were 60 and 30, respectively, while only seven were found for the sporophylls with glandular trichomes removed. The expression of genes known to be associated with the development of glandular trichomes in flowering plants, including MIXTA, ATML1, and MYB106, were also validated and are discussed. In particular, a unigene encoding MIXTA was identified and exhibited the highest expression level in SL in D. fragrans. Conclusions: This study is the first report of global transcriptomic analysis in different tissues of D. fragrans, and the first to discuss these findings in the context of the development of homologous glandular trichomes. These results set the stage for further research on the development, stress resistance, and secondary metabolism of D. fragrans glandular trichomes.

Class I TCP transcription factors regulate trichome branching and cuticle development in Arabidopsis

Trichomes and the cuticle are two specialized structures of the aerial epidermis that are important for plant organ development and interaction with the environment. In this study, we report that Arabidopsis thaliana plants affected in the function of the class I TEOSINTE BRANCHED 1, CYCLOIDEA, PCF (TCP) transcription factors TCP14 and TCP15 show overbranched trichomes in leaves and stems and increased cuticle permeability. We found that TCP15 regulates the expression of MYB106, a MIXTA-like transcription factor involved in epidermal cell and cuticle development, and overexpression of MYB106 in a tcp14 tcp15 mutant reduces trichome branch number. TCP14 and TCP15 are also required for the expression of the cuticle biosynthesis genes CYP86A4, GPAT6, and CUS2, and of SHN1 and SHN2, two AP2/EREBP transcription factors required for cutin and wax biosynthesis. SHN1 and CUS2 are also targets of TCP15, indicating that class I TCPs influence cuticle formation acting at different levels, through the regulation of MIXTA-like and SHN transcription factors and of cuticle biosynthesis genes. Our study indicates that class I TCPs are coordinators of the regulatory network involved in trichome and cuticle development.

miR319a/TCP module and DELLA protein regulate trichome initiation synergistically and improve insect defenses in Populus tomentosa

Trichomes are specialized epidermal cells that contribute to plant resistance against herbivores. Their formation is controlled precisely by multiple genetic and environmental signals. Previous studies have shown that microRNA319 (miR319) and gibberellin (GA) signaling are involved in trichome development in Arabidopsis, but little is known about their interaction between these factors. Here we reported that the miR319a/TEOSINTE BRANCHED/CYCLOIDEA/PCF (TCP) module participates in trichome initiation synergistically with GA signaling in Populus tomentosa. We demonstrated that overexpression of miR319a decreased transcription levels of its targeted TCPs and significantly elevated leaf trichome density in transgenic poplar, resulting in decreasing insect herbivory. Conversely, repressing miR319a by short tandem target mimics (STTM) elevated TCP expression levels and decreased trichome density in transgenic plants. The trichome phenotype of 35S:miR319a plants could be abolished by introducing a miR319a-resistant form of TCP19. Furthermore, the miR319a-targeted TCP19 interacted directly with REPRESSOR OF ga1-3 (RGA), a downstream repressor of GA signaling. TCP19 and RGA synergistically inhibited the GLABROUS1 (GL1)-induced expression of trichome marker gene GLABRA2 (GL2), thereby repressing leaf trichome initiation. Our results provide an insight into the molecular mechanism by which miR319/TCP19 module and GA signaling coordinated regulating trichome initiation in P. tomentosa.

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.

The Dynamic Genetic-Hormonal Regulatory Network Controlling the Trichome Development in Leaves

Plant trichomes are outgrowths developed from an epidermal pavement cells of leaves and other organs. Trichomes (also called 'hairs') play well-recognized roles in defense against insect herbivores, forming a physical barrier that obstructs insect movement and mediating chemical defenses. In addition, trichomes can act as a mechanosensory switch, transducing mechanical stimuli (e.g., insect movement) into physiological signals, helping the plant to respond to insect attacks. Hairs can also modulate plant responses to abiotic stresses, such as water loss, an excess of light and temperature, and reflect light to protect plants against UV radiation. The structure of trichomes is species-specific and this trait is generally related to their function. These outgrowths are easily analyzed and their origin represents an outstanding subject to study epidermal cell fate and patterning in plant organs. In leaves, the developmental control of the trichomatous complement has highlighted a regulatory network based on four fundamental elements: (i) genes that activate and/or modify the normal cell cycle of epidermal pavement cells (i.e., endoreduplication cycles); (ii) transcription factors that create an activator/repressor complex with a central role in determining cell fate, initiation, and differentiation of an epidermal cell in trichomes; (iii) evidence that underlines the interplay of the aforesaid complex with different classes of phytohormones; (iv) epigenetic mechanisms involved in trichome development. Here, we reviewed the role of genes in the development of trichomes, as well as the interaction between genes and hormones. Furthermore, we reported basic studies about the regulation of the cell cycle and the complexity of trichomes. Finally, this review focused on the epigenetic factors involved in the initiation and development of hairs, mainly on leaves.

The evolution of gene regulatory networks controlling Arabidopsis thaliana L. trichome development

BACKGROUND

The variation in structure and function of gene regulatory networks (GRNs) participating in organisms development is a key for understanding species-specific evolutionary strategies. Even the tiniest modification of developmental GRN might result in a substantial change of a complex morphogenetic pattern. Great variety of trichomes and their accessibility makes them a useful model for studying the molecular processes of cell fate determination, cell cycle control and cellular morphogenesis. Nowadays, a large number of genes regulating the morphogenesis of A. thaliana trichomes are described. Here we aimed at a study the evolution of the GRN defining the trichome formation, and evaluation its importance in other developmental processes.

RESULTS

In study of the evolution of trichomes formation GRN we combined classical phylogenetic analysis with information on the GRN topology and composition in major plants taxa. This approach allowed us to estimate both times of evolutionary emergence of the GRN components which are mainly proteins, and the relative rate of their molecular evolution. Various simplifications of protein structure (based on the position of amino acid residues in protein globula, secondary structure type, and structural disorder) allowed us to demonstrate the evolutionary associations between changes in protein globules and speciations/duplications events. We discussed their potential involvement in protein-protein interactions and GRN function.

CONCLUSIONS

We hypothesize that the divergence and/or the specialization of the trichome-forming GRN is linked to the emergence of plant taxa. Information about the structural targets of the protein evolution in the GRN may predict switching points in gene networks functioning in course of evolution. We also propose a list of candidate genes responsible for the development of trichomes in a wide range of plant species.

A New Allele of the SPIKE1 Locus Reveals Distinct Regulation of Trichome and Pavement Cell Development and Plant Growth

The single-celled trichomes of Arabidopsis thaliana have long served as an elegant model for elucidating the mechanisms of cell differentiation and morphogenesis due to their unique growth patterns. To identify new components in the genetic network that governs trichome development, we carried out exhaustive screens for additional Arabidopsis mutants with altered trichome morphology. Here, we report one mutant, aberrantly branched trichome1-1 (abt1-1), with a reduced trichome branching phenotype. After positional cloning, a point mutation in the SPIKE1 (SPK1) gene was identified in abt1-1. Further genetic complementation experiments confirmed that abt1-1 is a new allele of SPK1, so abt1-1 was renamed as spk1-7 according to the literatures. spk1-7 and two other spk1 mutant alleles, covering a spectrum of phenotypic severity, highlighted the distinct responses of developmental programs to different SPK1 mutations. Although null spk1 mutants are lethal and show defects in plant stature, trichome and epidermal pavement cell development, only trichome branching is affected in spk1-7. Surprisingly, we found that SPK1 is involved in the positioning of nuclei in the trichome cells. Lastly, through double mutant analysis, we found the coordinated regulation of trichome branching between SPK1 and two other trichome branching regulators, ANGUSTIFOLIA (AN) and ZWICHEL (ZWI). SPK1 might serve for the precise positioning of trichome nuclei, while AN and ZWI contribute to the formation of branch points through governing the cMTs dynamics. In summary, this study presented a fully viable new mutant allele of SPK1 and shed new light on the regulation of trichome branching and other developmental processes by SPK1.

Comparative transcriptome study of hairy and hairless tea plant (Camellia sinensis) shoots

Trichome (also referred to as 'háo' in tea) is a key feature in both tea products and tea plant (Camellia sinensis) selection breeding. Although trichomes are used as a model for studying cell differentiation and have been well studied in many plant species, the regulation of trichome formation at the molecular level is poorly understood in tea plants. In the present study, the hairy and hairless tea plant cultivars Fudingdabaicha (FDDB) and Rongchunzao (RCZ), respectively, were used to study this mechanism. We characterised tea plant trichomes as unicellular and unbranched structures. High-throughput Illumina sequencing yielded approximately 277.0 million high-quality clean reads from the FDDB and RCZ cultivars. After de novo assembly, 161,444 unigenes were generated, with an average length of 937 bp. Among these unigenes, 81,425 were annotated using public databases, and 55,201 coding sequences and 4004 transcription factors (TFs) were identified. In total, 21,599 differentially expressed genes (DEGs) were identified between RCZ and FDDB, of which 10,785 DEGs were up-regulated and 10,814 DEGs were down-regulated. Genes involved in the DNA replication pathway were significantly enriched. Furthermore, between FDDB and RCZ, DEGs related to TFs, phytohormone signals, and cellulose synthesis were identified, suggesting that certain genes involved in these pathways are crucial for trichome initiation in tea plants. Together, the results of this study provide novel data to improve our understanding of the potential molecular mechanisms of trichome formation and lay a foundation for additional trichome studies in tea plants.

SPATULA regulates floral transition and photomorphogenesis in a PHYTOCHROME B-dependent manner in Arabidopsis

Light is the most important exogenous stimulus regulating plant growth and various developmental processes. Phytochromes, especially PHYTOCHROME B (PHYB) mediates the various light-mediated processes in Arabidopsis. SPATULA (SPT) is an important transcription factor, which has been reported previously to participate in temperature-mediated transition from seed dormancy to germination. Here we investigate the function of SPT in the floral transition under long day conditions and photomorphogenesis in Arabidopsis. In this study, spt-2 shows significantly delayed flowering time. But mutation of SPT in the background of phyb-1 rescues the phenotype of spt-2. The flowering time of double mutant of spt-2/phyb-1 is similar with the wild type. These results indicate that SPT promotes the transition from vegetative stage to floral stage and it regulates this transition in a PHYB-dependent manner. With qRT-PCR analysis, it is found that SPT regulates flowering time via FLC, SVP, FT and SOC1. Furthermore, SPT also controls photomorphogenesis. spt-2 displays shortened hypocotyls and increased chlorophylls contents compared with the wild type. These phenotypes are also rescued in the double mutant of spt-2/phyb-1. These results indicate that SPT is also involved in photomorphogenic development in Arabidopsis and SPT regulates photomorphogenesis in a PHYB-dependent manner. Collectively, SPT is not only a temperature responder but it is also an important light regulator during plant growth and development.

NbGIS regulates glandular trichome initiation through GA signaling in tobacco

KEY MESSAGE

A novel gene NbGIS positively regulates glandular trichome initiation through GA Signaling in tobacco. NbMYB123-like regulates glandular trichome initiation by acting downstream of NbGIS in tobacco. Glandular trichome is a specialized multicellular structure which has capability to synthesize and secrete secondary metabolites and protects plants from biotic and abiotic stresses. Our previous results revealed that a C2H2 zinc-finger transcription factor GIS and its sub-family genes act upstream of GL3/EGL3-GL1-TTG1 transcriptional activator complex to regulate trichome initiation in Arabidopsis. In this present study, we found that NbGIS could positively regulate glandular trichome development in Nicotiana benthamiana (tobacco). Our result demonstrated that 35S:NbGIS lines exhibited much higher densities of trichome on leaves, main stems, lateral branches and sepals than WT plants, while NbGIS:RNAi lines had the opposite phenotypes. Furthermore, our results also showed that NbGIS was required in response to GA signal to control glandular trichome initiation in Nicotiana benthamiana. In addition, our results also showed that NbGIS significantly influenced GA accumulation and expressions of marker genes of the GA biosynthesis, might result in the changes of growth and maturation in tobacco. Lastly, our results also showed that NbMYB123-like regulated glandular trichome initiation in tobacco by acting downstream of NbGIS. These findings provide new insights to discover the molecular mechanism by which C2H2 transcriptional factors regulates glandular trichome initiation through GA signaling pathway in tobacco.

CIN-TCP transcription factors: Transiting cell proliferation in plants

Leaves are the most conspicuous planar organs in plants, designed for efficient capture of sunlight and its conversion to energy that is channeled into sustaining the entire biosphere. How a few founder cells derived from the shoot apical meristem give rise to diverse leaf forms has interested naturalists and developmental biologists alike. At the heart of leaf morphogenesis lie two simple cellular processes, division and expansion, that are spatially and temporally segregated in a developing leaf. In leaves of dicot model species, cell division occurs predominantly at the base, concomitant with the expansion and differentiation of cells at the tip of the lamina that drives increase in leaf surface area. The timing of the transition from one cell fate (division) to the other (expansion) within a growing leaf lamina is a critical determinant of final leaf shape, size, complexity and flatness. The TCP proteins, unique to plant kingdom, are sequence-specific DNA-binding transcription factors that control several developmental and physiological traits. A sub-group of class II TCPs, called CINCINNATA-like TCPs (CIN-TCPs henceforth), are key regulators of the timing of the transition from division to expansion in dicot leaves. The current review highlights recent advances in our understanding of how the pattern of CIN-TCP activity is translated to the dynamic spatio-temporal control of cell-fate transition through the transactivation of cell-cycle regulators, growth-repressing microRNAs, and interactions with the chromatin remodeling machinery to modulate phytohormone responses. Unravelling how environmental inputs influence CIN-TCP-mediated growth control is a challenge for future studies. © 2018 IUBMB Life, 70(8):718-731, 2018.

Regulation of trichome development in tobacco by JcZFP8, a C2H2 zinc finger protein gene from Jatropha curcas L

Trichomes are epidermal outgrowths of plant tissues that can secrete or store large quantities of secondary metabolites, which contribute to plant defense responses against stress. The use of bioengineering methods for regulating the development of trichomes and metabolism is a widely researched topic. In the present study, we demonstrate that JcZFP8, a C2H2 zinc finger protein gene from Jatropha curcas L., can regulate trichome development in transgenic tobacco. To understand the underlying mechanisms, we performed transcriptome profiling of overexpression JcZFP8 transgenic plants and wild-type tobacco. Based on the analysis of differentially expressed genes, we determined that genes of the plant hormone signal transduction pathway was significantly enriched, suggesting that these pathways were modulated in the transgenic plants. In addition, the transcript levels of the known trichome-related genes in Arabidopsis were not significantly changed, whereas CycB2 and MYB genes were differentially expressed in the transgenic plants. Despite tobacco and Arabidopsis have different types of trichomes, all the pathways were associated with C2H2 zinc finger protein genes. Our findings help us to understand the regulation of multicellular trichome formation and suggest a new metabolic engineering method for the improvement of plants.

Quantitative Trait Loci Controlling Fruit Size and Other Horticultural Traits in Bell Pepper (Capsicum annuum)

Bell pepper ( L.) is a group of fruit vegetables that has large variation in fruit shape, fruit size, and horticultural traits. Using unadapted sources of germplasm to bring in novel alleles while maintaining favorable quality and horticultural traits is challenging for breeding in pepper. A genetic map with 318 loci from genotype-by-sequencing (GBS) and single nucleotide polymorphism assays was generated from a recombinant inbred line population derived from a cultivated bell-type 'Maor' and a landrace highly resistant to , 'Criollo de Morelos-334'. Forty-nine quantitative trait loci (QTLs) were detected for fruit, leaf, and horticultural traits with the scantwo permutation and stepwiseqtl methods from R/qtl. With the availability of a pepper reference genome and GBS data, candidate genes for pepper organ size and other horticultural traits were predicted. , , and genes were candidate genes for controlling organ sizes on chromosome 1, 2, and 3, respectively. Two candidate genes controlling trichome formation in pepper are located at chromosome 10: and . The locus on chromosome 10, which encodes a member of the R2R3 MYB-domain family of proteins, has a function in anthocyanin accumulation. These QTL results and the candidate genes for each trait emphasize the genetic basis of the important traits for breeding with unadapted parents in bell pepper.

The TCP4 transcription factor regulates trichome cell differentiation by directly activating GLABROUS INFLORESCENCE STEMS in Arabidopsis thaliana

Trichomes are the first cell type to be differentiated during the morphogenesis of leaf epidermis and serve as an ideal model to study cellular differentiation. Many genes involved in the patterning and differentiation of trichome cells have been studied over the past decades, and the majority of these genes encode transcription factors that specifically regulate epidermal cell development. However, the upstream regulators of these genes that link early leaf morphogenesis with cell type differentiation are less studied. The TCP proteins are the plant-specific transcription factors involved in regulating diverse aspects of plant development including lateral organ morphogenesis by modulating cell proliferation and differentiation. Here, we show that the miR319-regulated class II TCP proteins, notably TCP4, suppress trichome branching in Arabidopsis leaves and inflorescence stem by direct transcriptional activation of GLABROUS INFLORESCENCE STEMS (GIS), a known negative regulator of trichome branching. The trichome branch number is increased in plants with reduced TCP activity and decreased in the gain-of-function lines of TCP4. Biochemical analyses show that TCP4 binds to the upstream regulatory region of GIS and activates its expression. Detailed genetic analyses show that GIS and TCP4 work in same pathway and GIS function is required for TCP4-mediated regulation of trichome differentiation. Taken together, these results identify a role for the class II TCP genes in trichome differentiation, thus providing a connection between organ morphogenesis and cellular differentiation.

AtGIS, a C2H2 zinc-finger transcription factor from Arabidopsis regulates glandular trichome development through GA signaling in tobacco

Glandular trichome is specialized multicellular structures that have capability to synthesize and secrete secondary metabolites and protect plants from biotic and abiotic stresses. Our previous results revealed that the C2H2 zinc-finger transcription factors (GIS) acts upstream of GL3/EGL3-GL1-TTG1transcriptional activator complex to regulate trichome initiation through GA signal in Arabidopsis. In the present study, we are reporting that ectopic expression of AtGIS could regulate glandular trichome development through GA signaling in tobacco. X-gluc staining of various organs from transgenic plants showed that AtGIS expressed mainly in the glandular trichomes. Statistical analysis demonstrated that over expression of GIS increased significantly glandular trichome production on the leaf, stem, branch, and sepal in tobacco. After PAC treatment, reduction of glandular trichome production in transgenic plants was more severe with compared to wild type plants. Furthermore, GA treatment could induce expression of AtGIS. More importantly, our results also demonstrated that overexpressed AtGIS significantly affect the main components of trichome exudates, such as significantly increase the content of nicotine, Cembratriene-4, 6-diol. Taken together, these results suggest that ectopic expression of AtGIS regulates glandular trichome development and may play a key role in compounds secretion in tobacco.

Involvement of C2H2 zinc finger proteins in the regulation of epidermal cell fate determination in Arabidopsis

Cell fate determination is a basic developmental process during the growth of multicellular organisms. Trichomes and root hairs of Arabidopsis are both readily accessible structures originating from the epidermal cells of the aerial tissues and roots respectively, and they serve as excellent models for understanding the molecular mechanisms controlling cell fate determination and cell morphogenesis. The regulation of trichome and root hair formation is a complex program that consists of the integration of hormonal signals with a large number of transcriptional factors, including MYB and bHLH transcriptional factors. Studies during recent years have uncovered an important role of C2H2 type zinc finger proteins in the regulation of epidermal cell fate determination. Here in this minireview we briefly summarize the involvement of C2H2 zinc finger proteins in the control of trichome and root hair formation in Arabidopsis.

GLABROUS INFLORESCENCE STEMS regulates trichome branching by genetically interacting with SIM in Arabidopsis

Arabidopsis trichomes are large branched single cells that protrude from the epidermis. The first morphological indication of trichome development is an increase in nuclear content resulting from an initial cycle of endoreduplication. Our previous study has shown that the C2H2 zinc finger protein GLABROUS INFLORESCENCE STEMS (GIS) is required for trichome initiation in the inflorescence organ and for trichome branching in response to gibberellic acid signaling, although GIS gene does not play a direct role in regulating trichome cell division. Here, we describe a novel role of GIS, controlling trichome cell division indirectly by interacting genetically with a key endoreduplication regulator SIAMESE (SIM). Our molecular and genetic studies have shown that GIS might indireclty control cell division and trichome branching by acting downstream of SIM. A loss of function mutation of SIM signficantly reduced the expression of GIS. Futhermore, the overexpression of GIS rescued the trichome cluster cell phenotypes of sim mutant. The gain or loss of function of GIS had no significant effect on the expression of SIM. These results suggest that GIS may play an indirect role in regulating trichome cell division by genetically interacting with SIM.

Zinc Finger Protein 6 (ZFP6) regulates trichome initiation by integrating gibberellin and cytokinin signaling in Arabidopsis thaliana

The Arabidopsis trichome is a model system for studying cell development, cell differentiation and the cell cycle in plants. Our previous studies have shown that the ZINC FINGER PROTEIN5 (ZFP5) controls shoot maturation and epidermal cell fate through GA signaling in Arabidopsis. We have identified a novel C2H2 zinc finger protein ZINC FINGER PROTEIN 6 (ZFP6) which plays a key role in regulating trichome development in Arabidopsis. Overexpression of ZFP6 results in ectopic trichomes on carpels and other inflorescence organs. Gain- and loss-of-function analyses have shown that the zfp6 mutant exhibits a reduced number of trichomes in sepals of flowers, cauline leaves, lateral branch and main inflorescence stems in comparison to wild-type plants. Molecular and genetic analyses suggest that ZFP6 functions upstream of GIS, GIS2, ZFP8, ZFP5 and key trichome initiation regulators GL1 and GL3.We reveal that ZFP6 and ZFP5 mediate the regulation of trichome initiation by integrating GA and cytokinin signaling in Arabidopsis. These findings provide new insights into the molecular mechanism of plant hormone control of epidermal trichome patterning through C2H2 transcriptional factors.