Hormone-mediated promotion of trichome initiation in plants is conserved but utilizes species- and trichome-specific regulatory mechanisms

Transcriptome Analysis Reveals Key Genes Involved in Trichome Formation in Pepper (Capsicum annuum L.)

Trichomes are specialized organs located in the plant epidermis that play important defense roles against biotic and abiotic stresses. However, the mechanisms regulating the development of pepper epidermal trichomes and the related regulatory genes at the molecular level are not clear. Therefore, we performed transcriptome analyses of A114 (less trichome) and A115 (more trichome) to dig deeper into the genes involved in the regulatory mechanisms of epidermal trichome development in peppers. In this study, the epidermal trichome density of A115 was found to be higher by phenotypic observation and was highest in the leaves at the flowering stage. A total of 39,261 genes were quantified by RNA-Seq, including 11,939 genes not annotated in the previous genome analysis and 18,833 differentially expressed genes. Based on KEGG functional enrichment, it was found that DEGs were mainly concentrated in three pathways: plant-pathogen interaction, MAPK signaling pathway-plant, and plant hormone signal transduction. We further screened the DEGs associated with the development of epidermal trichomes in peppers, and the expression of the plant signaling genes GID1B-like (Capana03g003488) and PR-6 (Capana09g001847), the transcription factors MYB108 (Capana05g002225) and ABR1-like (Capana04g001261), and the plant resistance genes PGIP-like (Capana09g002077) and At5g49770 (Capana08g001721) in the DEGs were higher at A115 compared to A114, and were highly expressed in leaves at the flowering stage. In addition, based on the WGCNA results and the establishment of co-expression networks showed that the above genes were highly positively correlated with each other. The transcriptomic data and analysis of this study provide a basis for the study of the regulatory mechanisms of pepper epidermal trichomes.

A large presence/absence variation in the promotor of the ClLOG gene determines trichome elongation in watermelon

KEY MESSAGE

The ClLOG gene encoding a cytokinin riboside 5'-monophosphate phosphoribohydrolase determines trichome length in watermelon, which is associated with its promoter variations. Trichomes, which are differentiated from epidermal cells, are special accessory structures that cover the above-ground organs of plants and possibly contribute to biotic and abiotic stress resistance. Here, a bulked segregant analysis (BSA) of an F2 population with significant variations in trichome length was undertaken. A 1.84-Mb candidate region on chromosome 10 was associated with trichome length. Resequencing and fine-mapping analyses indicated that a 12-kb structural variation in the promoter of Cla97C10G203450 (ClLOG) led to a significant expression difference in this gene in watermelon lines with different trichome lengths. In addition, a virus-induced gene silencing analysis confirmed that ClLOG positively regulated trichome elongation. These findings provide new information and identify a potential target gene for controlling multicellular trichome elongation in watermelon.

A review of the types, functions and regulatory mechanisms of plant spines

Over a long period of evolution, plants have developed self-protection mechanisms, such as leaving seeds, dropping leaves, growing thorns, producing specific substances or emitting special odors to repel insects. Although studies on the taxonomic characteristics, functions and application of spines in spiny plants have been reported in China and abroad, a systematic overview of plant spines is currently lacking. This study therefore identifies the characteristics and types of plant spines based on domestic and international research on plant spines to provide clear criteria or bases for determining the types of plant spines. In addition, the functions, regulatory mechanisms, and factors influencing the formation of spines and the prospects for their development and application are described and summarized. This study will help to improve the understanding of the types, functions and regulatory mechanisms of plant spines and provide new ideas for the genetic improvement of plants from spiny to nonspiny varieties.

Transcription factors NtHD9 and NtHD12 control long glandular trichome formation via jasmonate signaling

Glandular trichomes are universal epidermal structures that produce abundant specialized metabolites. However, knowledge of the initiation of glandular heads in glandular trichomes is limited. Herein, we found an intrinsic link of morphogenesis between glandular trichomes and non-glandular trichomes. Two novel homeodomain leucine zipper II members in tobacco (Nicotiana tabacum), NtHD9 and NtHD12, played important roles in long glandular trichome formation: NtHD9 was responsible for glandular head formation, while NtHD12 simultaneously controlled the formation of stalks and glandular heads. DAP-seq analysis suggested that NtHD9 can bind to the KKGCATTWAWTR motif of the cytochromes P450 94C1 (NtCYP94C1) promoter, which is involved in jasmonoyl-isoleucine oxidation. RNA-seq analysis of non-transformed tobacco and nthd9 plants revealed that NtHD9 modulates the expression of jasmonate (JA) signaling- and six trichome development-related genes. Notably, MeJA treatment restored the morphogenesis of long glandular trichomes in nthd9 and nthd12 plants, and the size of glandular heads increased with increasing MeJA concentration. However, the phenotype of long glandular trichome absence in double mutants of NtHD9 and NtHD12 could not be restored by MeJA. Our data demonstrate that NtHD9 and NtHD12 have distinct major functions yet overlapping roles in long glandular trichome formation via JA signaling.

Exogenous application of stress-related signaling molecules affect growth and cannabinoid accumulation in medical cannabis (Cannabis sativa L.)

Medical cannabis (Cannabis sativa L.) is a source of bioactive phytochemicals with promising pharmacological and therapeutic applications. Enhancing the accumulation of valuable bioactive compounds is potentially a way of increasing the economic importance of this crop. Signaling molecules like salicylic acid (SA), jasmonic acid (JA), and γ-aminobutyric acid (GABA) are involved in the regulation of plant development and responses to biotic and abiotic stresses. Moreover, several phytohormones regulate plant trichome formation and elicit the synthesis of secondary metabolites in many plant species in both in vitro and in vivo systems. Therefore, exogenously delivered plant signaling molecules have the potential to modify the chemical profiles of medical cannabis. In this study, we found that the foliar application of SA, methyl jasmonate (MeJA), and GABA produces changes in the accumulation of the two major cannabinoids, cannabidiolic acid (CBDA) and Δ⁹- tetrahydrocannabinolic acid (THCA), in leaves and inflorescences of a medical cannabis variety. MeJA at 0.1 mM increased the CBDA content in inflorescences by 15.6%, while SA and MeJA at 0.1 mM increased CBDA and THCA accumulation in leaves by up to 57.3%. Treatments did not change the expression of genes participating in the final steps of the biosynthetic pathway of cannabinoids: olivetolic acid cyclase (CsOAC-1 and CsOAC-2), 2-acylphloroglucinol 4-prenyltransferase (CsPT4), cannabidiolic acid synthase (CsCBDAS), and tetrahydrocannabinolic acid synthase (CsTHCAS). Trichome density was not significantly different from the control plants in any treatment. Besides, we found strong correlations between several plant growth parameters and cannabinoid yields, showing a direct link between plant fitness and the production of cannabinoids.

Chromosome-level assembly and analysis of the Thymus genome provide insights into glandular secretory trichome formation and monoterpenoid biosynthesis in thyme

Thyme has medicinal and aromatic value because of its potent antimicrobial and antioxidant properties. However, the absence of a fully sequenced thyme genome limits functional genomic studies of Chinese native thymes. Thymus quinquecostatus Čelak., which contains large amounts of bioactive monoterpenes such as thymol and carvacrol, is an important wild medicinal and aromatic plant in China. Monoterpenoids are abundant in glandular secretory trichomes. Here, high-fidelity and chromatin conformation capture technologies were used to assemble and annotate the T. quinquecostatus genome at the chromosome level. The 13 chromosomes of T. quinquecostatus had a total length of 528.66 Mb, a contig N50 of 8.06 Mb, and a BUSCO score of 97.34%. We found that T. quinquecostatus had experienced two whole-genome duplications, with the most recent event occurring ∼4.34 million years ago. Deep analyses of the genome, in conjunction with comparative genomic, phylogenetic, transcriptomic, and metabonomic studies, uncovered many regulatory factors and genes related to monoterpenoids and glandular secretory trichome development. Genes encoding terpene synthase (TPS), cytochrome P450 monooxygenases (CYPs), short-chain dehydrogenase/reductase (SDR), R2R3-MYB, and homeodomain-leucine zipper (HD-ZIP) IV were among those present in the T. quinquecostatus genome. Notably, Tq02G002290.1 (TqTPS1) was shown to encode the terpene synthase responsible for catalyzing production of the main monoterpene product γ-terpinene from geranyl diphosphate (GPP). Our study provides significant insight into the mechanisms of glandular secretory trichome formation and monoterpenoid biosynthesis in thyme. This work will facilitate the development of molecular breeding tools to enhance the production of bioactive secondary metabolites in Lamiaceae.

Arbuscular mycorrhizal fungi and production of secondary metabolites in medicinal plants

Medicinal plants are an important source of therapeutic compounds used in the treatment of many diseases since ancient times. Interestingly, they form associations with numerous microorganisms developing as endophytes or symbionts in different parts of the plants. Within the soil, arbuscular mycorrhizal fungi (AMF) are the most prevalent symbiotic microorganisms forming associations with more than 70% of vascular plants. In the last decade, a number of studies have reported the positive effects of AMF on improving the production and accumulation of important active compounds in medicinal plants.In this work, we reviewed the literature on the effects of AMF on the production of secondary metabolites in medicinal plants. The major findings are as follows: AMF impact the production of secondary metabolites either directly by increasing plant biomass or indirectly by stimulating secondary metabolite biosynthetic pathways. The magnitude of the impact differs depending on the plant genotype, the AMF strain, and the environmental context (e.g., light, time of harvesting). Different methods of cultivation are used for the production of secondary metabolites by medicinal plants (e.g., greenhouse, aeroponics, hydroponics, in vitro and hairy root cultures) which also are compatible with AMF. In conclusion, the inoculation of medicinal plants with AMF is a real avenue for increasing the quantity and quality of secondary metabolites of pharmacological, medical, and cosmetic interest.

NbJAZ3 is required for jasmonate-meditated glandular trichome development in Nicotiana benthamiana

Exogenous methyl jasmonate (MeJA) treatment induces glandular trichome development in Nicotiana benthamiana, but the function of JAZ proteins, acting as core repressors, and their downstream genes have not been clearly shown in plants. Here, a bioinformatics analysis of 71 JAZ genes from tobacco, Arabidopsis thaliana, and tomato was carried out and shown to share highly conserved domains. Then, the expression profile of 17 NbJAZs in different tissues was analyzed, and NbJAZ3 was highly expressed in trichome. Through transgenic technology, we demonstrated that the glandular trichome density of NbJAZ3-overexpression lines significantly decreased with lower expression levels of NbWo, NbCycB2, and NbMIXTA. In contrast, the trichome density of NbJAZ3 RNAi lines slightly increased with higher expression level of NbWo. Given the negative protein feedback regulation relationship between NbCycB2 and NbWo, we verified that MeJA induced NbWo expression. NbWo was a direct target gene of NbJAZ3 and further demonstrated that NbJAZ3 inhibited the transcriptional activation of NbCycB2 by NbWo. Together, our findings outline a novel JA-meditated glandular trichome development model consisting of the NbJAZ3-NbWo-NbCycB2 axis.

H and HL synergistically regulate jasmonate-triggered trichome formation in tomato

The development of trichomes, which protect plants against herbivores, is affected by various stresses. In tomato, previous studies showed that stress triggered JA signaling influences trichome formation, but the underlying mechanism is not fully resolved. Here, we found two C2H2 zinc finger proteins synergistically regulate JA-induced trichome formation in tomato. The naturally occurring mutations in H and its close homolog H-like gene in a spontaneous mutant, LA3172 cause severely affected trcihome development. Compared with respective single mutant, h/hl double mutant displayed more severe trichome defects in all tissues. Despite the partially redundant function, H and HL genes regulate the trichome formation in the spatially distinct manner, with HL more involved in hypocotyls and leaves, while H more involved in stems and sepals. Furthermore，the activity of H/HL is essential for JA-triggered trichome formation. JA signaling inhibitor SlJAZ2 represses the activity of H and HL via physical interaction, resulting in the activation of THM1, a negative regulator of trichome formation. Our results provide novel insight into the mechanism of the trichome formation in response to stress induced JA signaling in tomato.

Rewilding staple crops for the lost halophytism: Toward sustainability and profitability of agricultural production systems

Abiotic stress tolerance has been weakened during the domestication of all major staple crops. Soil salinity is a major environmental constraint that impacts over half of the world population; however, given the increasing reliance on irrigation and the lack of available freshwater, agriculture in the 21st century will increasingly become saline. Therefore, global food security is critically dependent on the ability of plant breeders to create high-yielding staple crop varieties that will incorporate salinity tolerance traits and account for future climate scenarios. Previously, we have argued that the current agricultural practices and reliance on crops that exclude salt from uptake is counterproductive and environmentally unsustainable, and thus called for a need for a major shift in a breeding paradigm to incorporate some halophytic traits that were present in wild relatives but were lost in modern crops during domestication. In this review, we provide a comprehensive physiological and molecular analysis of the key traits conferring crop halophytism, such as vacuolar Na⁺ sequestration, ROS desensitization, succulence, metabolic photosynthetic switch, and salt deposition in trichomes, and discuss the strategies for incorporating them into elite germplasm, to address a pressing issue of boosting plant salinity tolerance.

Methyl jasmonate treatment, aphid resistance assay, and transcriptomic analysis revealed different herbivore defensive roles between tobacco glandular and non-glandular trichomes

KEY MESSAGE

Methyl jasmonate treatment and aphid resistance assays reveal different roles in herbivore defensive responses between tobacco glandular and non-glandular trichomes. These roles correlate with trichome gene expression patterns. In plants, trichomes greatly contribute to biotic stress resistance. To better understand the different defensive functions between glandular and non-glandular trichomes, we used Nicotiana tabacum as a model. This species bears three types of trichomes: long and short stalk glandular trichomes (LGT and SGT, respectively), and non-glandular trichomes (NGT). Tobacco accession T.I.1068 (lacking NGT) and T.I.1112 (lacking LGT) were used for the experiment. After methyl jasmonate (MeJA) treatment, LGT formation was promoted not only in T.I.1068, but also in T.I.1112, whereas NGT remained absent in T.I.1068, and was slightly reduced in T.I.1112. Diterpenoids, which play important roles in herbivore resistance, accumulated abundantly in T.I.1068 and were elevated by MeJA; however, they were not found in T.I.1112 but became detectable after MeJA treatment. The aphid resistance of T.I.1068 was higher than that of T.I.1112, and both were enhanced by MeJA, which was closely correlated with LGT density. Trichomes detached from T.I.1068 and T.I.1112 were used for RNA-Seq analysis, the results showed that pentose phosphate, photosynthesis, and diterpenoid biosynthesis genes were much more expressed in T.I.1068 than in T.I.1112, which was consistent with the vigorous diterpenoid biosynthesis in T.I.1068. In T.I.1112, citrate cycle, propanoate, and glyoxylate metabolism processes were enriched, and some defensive protein genes were expressed at higher levels than those in T.I.1068.These results suggested that LGT plays a predominant role in aphid resistance, whereas NGT could strengthen herbivore resistance by accumulating defensive proteins, and the roles of LGT and NGT are associated with their gene expression patterns.

A SlMYB75-centred transcriptional cascade regulates trichome formation and sesquiterpene accumulation in tomato

Tomato trichomes act as a mechanical and chemical barrier against pests. An R2R3 MYB transcription factor gene, SlMYB75, is highly expressed in type II, V, and VI trichomes. SlMYB75 protein is located in the nucleus and possesses transcriptional activation activity. Down-regulation of SlMYB75 increased the formation of type II, V, and VI trichomes, accumulation of δ-elemene, β-caryophyllene, and α-humulene in glandular trichomes, and tolerance to spider mites in tomato. In contrast, overexpression of SlMYB75 inhibited trichome formation and sesquiterpene accumulation, and increased plant sensitivity to spider mites. RNA-Seq analyses of the SlMYB75 RNAi line indicated massive perturbation of the transcriptome, with a significant impact on several classes of transcription factors. Expression of the MYB genes SlMYB52 and SlTHM1 was strongly reduced in the RNAi line and increased in the SlMYB75-overexpressing line. SlMYB75 protein interacted with SlMYB52 and SlTHM1 and activated their expression. SlMYB75 directly targeted the promoter of the cyclin gene SlCycB2, increasing its activity. The auxin response factor SlARF4 directly targeted the promoter of SlMYB75 and inhibited its expression. SlMYB75 also bound to the promoters of the terpene synthase genes SlTPS12, SlTPS31, and SlTPS35, inhibiting their transcription. Our findings indicate that SlMYB75 perturbation affects several transcriptional circuits, resulting in altered trichome density and metabolic content.

R2R3 MYB-dependent auxin signalling regulates trichome formation, and increased trichome density confers spider mite tolerance on tomato

Unicellular and multicellular tomato trichomes function as mechanical and chemical barriers against herbivores. Auxin treatment increased the formation of II, V and VI type trichomes in tomato leaves. The auxin response factor gene SlARF4, which was highly expressed in II, V and VI type trichomes, positively regulated the auxin-induced formation of II, V and VI type trichomes in the tomato leaves. SlARF4 overexpression plants with high densities of these trichomes exhibited tolerance to spider mites. Two R2R3 MYB genes, SlTHM1 and SlMYB52, were directly targeted and inhibited by SlARF4. SlTHM1 was specifically expressed in II and VI type trichomes and negatively regulated the auxin-induced formation of II and VI type trichomes in the tomato leaves. SlTHM1 down-regulation plants with high densities of II and VI type trichomes also showed tolerance to spider mites. SlMYB52 was specifically expressed in V type trichomes and negatively regulated the auxin-induced formation of V type trichome in the tomato leaves. The regulation of SlARF4 on the formation of II, V and VI type trichomes depended on SlTHM1 and SlMYB52, which directly targeted cyclin gene SlCycB2 and increased its expression. In conclusion, our data indicates that the R2R3 MYB-dependent auxin signalling pathway regulates the formation of II, V and VI type trichomes in tomato leaves. Our study provides an effective method for improving the tolerance of tomato to spider mites.

Cultivar Variation in Tomato Seed Coat Permeability Is an Important Determinant of Jasmonic Acid Elicited Defenses Against Western Flower Thrips

Induction of defenses is one of the most widely accepted eco-friendly approaches for management of pests and diseases. Seeds are receptive to resistance-inducing chemicals and could offer broad-spectrum protection at the early stages of development. However, seed treatment with elicitors has previously been shown to differentially influence induced defense responses among cultivars and thus, could hamper commercial exploitation. In this context, the objective of the present study was to evaluate the genotype-dependent ability of jasmonic acid (JA) to induce resistance against western flower thrips (WFT) at the seed stage. We examined the variation in inducibility of resistance in eight commercial tomato cultivars. Causal factors accounting for discrepancies in JA-induced responses at the seed stage were phenotypically and biochemically evaluated. Seed receptivity to exogenous JA appeared to be cultivar dependent. Thrips associated silver damage was only reduced in JA seed-treated plants of cultivar Carousel. Enhancement of resistance, was not associated with activation of defense-related traits such as polyphenol oxidase activity (PPO), trichomes or volatiles. Sulfuric acid scarification, prior to JA seed incubation, significantly augmented the embryonic responsiveness to JA in cv. Moneymaker without an adverse effect on growth. Hence, these results support the hypothesis that seed coat permeability is a key factor for successfully inducing JA mediated thrips defenses. The outcome of our study is of translational value as it creates opportunities for the seed industry to perform pre-treatments on non-responsive cultivars as well as for tomato breeding programs to select for genetic traits that affect seed permeability.

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.

Glandular trichomes: micro-organs with model status?

Glandular trichomes are epidermal outgrowths that are the site of biosynthesis and storage of large quantities of specialized metabolites. Besides their role in the protection of plants against biotic and abiotic stresses, they have attracted interest owing to the importance of the compounds they produce for human use; for example, as pharmaceuticals, flavor and fragrance ingredients, or pesticides. Here, we review what novel concepts investigations on glandular trichomes have brought to the field of specialized metabolism, particularly with respect to chemical and enzymatic diversity. Furthermore, the next challenges in the field are understanding the metabolic network underlying the high productivity of glandular trichomes and the transport and storage of metabolites. Another emerging area is the development of glandular trichomes. Studies in some model species, essentially tomato, tobacco, and Artemisia, are now providing the first molecular clues, but many open questions remain: How is the distribution and density of different trichome types on the leaf surface controlled? When is the decision for an epidermal cell to differentiate into one type of trichome or another taken? Recent advances in gene editing make it now possible to address these questions and promise exciting discoveries in the near future.

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

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

Functional Analysis of the Rice Type-B Response Regulator RR22

The phytohormone cytokinin plays a critical role in regulating growth and development throughout the life cycle of the plant. The primary transcriptional response to cytokinin is mediated by the action of the type-B response regulators (RRs), with much of our understanding for their functional roles being derived from studies in the dicot Arabidopsis. To examine the roles played by type-B RRs in a monocot, we employed gain-of-function and loss-of-function mutations to characterize RR22 function in rice. Ectopic overexpression of RR22 in rice results in an enhanced cytokinin response based on molecular and physiological assays. Phenotypes associated with enhanced activity of RR22 include effects on leaf and root growth, inflorescence architecture, and trichome formation. Analysis of four Tos17 insertion alleles of RR22 revealed effects on inflorescence architecture, trichomes, and development of the stigma brush involved in pollen capture. Both loss- and gain-of-function RR22 alleles affected the number of leaf silica-cell files, which provide mechanical stability and improve resistance to pathogens. Taken together, these results indicate that a delicate balance of cytokinin transcriptional activity is necessary for optimal growth and development in rice.

Co-suppression of NbClpC1 and NbClpC2 alters plant morphology with changed hormone levels in Nicotiana benthamiana

KEY MESSAGE

Co-suppression of chaperonic ClpC1 and ClpC2 in Nicotiana benthamiana significantly affect the development and exogenous application of gibberellin partially rescue the developmental defects. Over the past decade, the Clp protease complex has been identified as being implicated in plastid protein quality control in plant cells. CLPC1 and CLPC2 proteins form the chaperone subunits of the Clp protease complex and unfold protein substrates to thread them into the ClpP complex. Here, using the technique of virus-induced gene silencing (VIGS), we suppressed both Nicotiana benthamiana ClpC1 and ClpC2 (NbClpC1/C2) functioning as chaperone subunits in the protease complex. Co-suppression of NbClpC1/C2 caused chlorosis and retarded-growth phenotype with no seed formation and significantly reduced root length. We found that co-suppression of NbClpC1/C2 also affected stomata and trichome formation and vascular bundle differentiation and patterning. Analysis of phytohormones revealed significant alteration and imbalance of major hormones in the leaves of NbClpC1/C2 co-suppressed plant. We also found that application of gibberellin (GA3) partially rescued the developmental defects. Co-suppression of NbClpC1/C2 significantly affected the development of N. benthamiana and exogenous application of GA3 partially rescued the developmental defects. Overall, our findings demonstrate that CLPC1 and CLPC2 proteins have a pivotal role in plant growth and development.

Induction of essential oil production in Mentha x piperita by plant growth promoting bacteria was correlated with an increase in jasmonate and salicylate levels and a higher density of glandular trichomes

Plant growth promoting bacteria (PGPB) are agriculturally important soil bacteria that increase plant growth. We subjected peppermint to inoculation with three species of PGPB. After inoculation, the plants were sprayed with methyl jasmonate solution (MeJA) or SA (salicylic acid). Then, the plants were harvested and the plant growth parameters, trichome density, EO content and endogenous phytohormones were measured. Shoot fresh weight was reduced in plants inoculated and treated with MeJA whereas EO content varied depending on the MeJA concentration applied. Plants inoculated and treated with MeJA 2 mM showed the maximum increase in EO production, revealing a synergism between PGPB and MeJA. SA treatments also enhanced EO yield. The increased growth and EO production observed upon PGPB application were at least partly due to an increase in the JA and SA concentrations in the plant, as well as to an associated rise in the glandular trichome density.

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.

Type-B response regulators of rice play key roles in growth, development and cytokinin signaling

Cytokinins are plant hormones with crucial roles in growth and development. Although cytokinin signaling is well characterized in the model dicot Arabidopsis, we are only beginning to understand its role in monocots, such as rice (Oryza sativa) and other cereals of agronomic importance. Here, we used primarily a CRISPR/Cas9 gene-editing approach to characterize the roles of a key family of transcription factors, the type-B response regulators (RRs), in cytokinin signaling in rice. Results from the analysis of single rr mutants as well as higher-order rr21/22/23 mutant lines revealed functional overlap as well as subfunctionalization within members of the gene family. Mutant phenotypes associated with decreased activity of rice type-B RRs included effects on leaf and root growth, inflorescence architecture, flower development and fertilization, trichome formation and cytokinin sensitivity. Development of the stigma brush involved in pollen capture was compromised in the rr21/22/23 mutant, whereas anther development was compromised in the rr24 mutant. Novel as well as conserved roles for type-B RRs in the growth and development of a monocot compared with dicots were identified.

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

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

Type VI glandular trichome density and their derived volatiles are differently induced by jasmonic acid in developing and fully developed tomato leaves: Implications for thrips resistance

Variation in the induction of plant defenses along the plant canopy can determine distribution and colonization of arthropod herbivores within the plant. In tomato, type VI glandular trichomes, which are epidermal defensive structures, and their derived volatiles are induced by the phytohormone jasmonic acid (JA). How JA-mediated induction of these trichome-associated chemical defenses depends on the leaf developmental stage and correlates with resistance against herbivory is unknown. We showed that application of JA reduced thrips-associated damage, however the amplitude of this response was reduced in the fully developed leaves compared to those still developing. Although JA increased type-VI trichome densities in all leaf developmental stages, as well as JA-inducible defensive proteins, these increases were stronger in developing leaves. Remarkably, the concentration of trichome-derived volatiles was induced by JA to a larger degree in developing leaves than in fully developed leaves. In fully developed leaves, the increase in trichome-derived volatiles was explained by an enhanced production per trichome, while in developing leaves this was mainly caused by increases in type-VI trichome densities. Together, we showed that JA-mediated induction of trichome density and chemistry depends on leaf development stage, and it might explain the degree of thrips-associated leaf damage in tomato.

Formation of α- and β-Cembratriene-Diols in Tobacco (Nicotiana tabacum L.) Is Regulated by Jasmonate-Signaling Components via Manipulating Multiple Cembranoid Synthetic Genes

Cembranoids are a group of natural diterpenoid compounds with pharmaceutical potentials, and the cembratriene-diols produced by Nicotiana (tobacco) species display activities in anti-nicotine addiction and neuron protection. Although the enzymes catalyzing cembratriene-diols' formation in tobacco have been investigated, the regulatory mechanism underlying this physiological process remains unknown. This study has investigated the roles of phytohormone jasmonic acid (JA) in regulating cembratriene-diol formation in N. tabacum cv. TN90 and found that JA and COI1, the receptor protein of the bioactive derivative of JA (i.e., JA-Ile), display critical roles in regulating cembratriene-diols' formation and the expression of cembranoid synthetic genes CBTS, P450 and NtLTP1. Further studies showed that over-expressing either the gene encoding bHLH transcription factor MYC2a or that encoding MYB transcription factor MYB305 could upregulate the cembranoid synthetic genes and enhance the cembranoid production in plants with dysfunction of COI1. Further studies suggest that COI1 and its downstream regulators MYC2a and MYB305 also modulate the trichome secretion, which is correlated with cembranoid formation. Taken together, this study has demonstrated a critical role of JA-signaling components in governing the cembratriene-diol formation and the transcription of cembratriene-diol synthetic genes in tobacco. Findings in this study are of great importance to reveal the molecular regulatory mechanism underlying cembranoid synthesis.

Floral homeotic proteins modulate the genetic program for leaf development to suppress trichome formation in flowers

As originally proposed by Goethe in 1790, floral organs are derived from leaf-like structures. The conversion of leaves into different types of floral organ is mediated by floral homeotic proteins, which, as described by the ABCE model of flower development, act in a combinatorial manner. However, how these transcription factors bring about this transformation process is not well understood. We have previously shown that floral homeotic proteins are involved in suppressing the formation of branched trichomes, a hallmark of leaf development, on reproductive floral organs of Arabidopsis Here, we present evidence that the activities of the C function gene AGAMOUS (AG) and the related SHATTERPROOF1/2 genes are superimposed onto the regulatory network that controls the distribution of trichome formation in an age-dependent manner. We show that AG regulates cytokinin responses and genetically interacts with the organ polarity gene KANADI1 to suppress trichome initiation on gynoecia. Thus, our results show that parts of the genetic program for leaf development remain active during flower formation but have been partially rewired through the activities of the floral homeotic proteins.

Induced Resistance Against Western Flower Thrips by the Pseudomonas syringae-Derived Defense Elicitors in Tomato

Western flower thrips (WFT) Frankliniella occidentalis (Pergande) is a key agricultural pest of cultivated tomatoes. Induced host plant resistance by activating jasmonic acid (JA) signaling pathway constitutes a promising method for WFT control. The phytotoxin coronatine (COR), produced by Pseudomonas syringae pv. tomato DC3000 (Pst), mimics the plant hormone JA-Isoleucine and can promote resistance against herbivorous arthropods. Here we determined the effect of Pst and COR on tomato resistance against WFT, induction of JA and salicylic acid (SA) associated defenses, and plant chemistry. Additionally, we investigated the presence of other components in Pst-derived and filtered culture medium, and their interactive effect with COR on tomato resistance to WFT. Our results showed that infiltration of COR or Pst reduced WFT feeding damage in tomato plants. COR and Pst induced the expression of JA-associated gene and protein marker. COR also induced expression of a SA-related responsive gene, although at much less magnitude. Activation of JA defenses in COR and Pst infiltrated plants did not affect density of type VI leaf trichomes, which are defenses reported to be induced by JA. An untargeted metabolomic analysis showed that both treatments induced strong changes in infiltrated leaves, but leaf responses to COR or Pst slightly differed. Application of the Pst-derived and filtered culture medium, containing COR but not viable Pst, also increased tomato resistance against WFT confirming that the induction of tomato defenses does not require a living Pst population to be present in the plant. Infiltration of tomato plants with low concentrations of COR in diluted Pst-derived and filtered culture medium reduced WFT feeding damage in a greater magnitude than infiltration with an equivalent amount of pure COR indicating that other elicitors are present in the medium. This was confirmed by the fact that the medium from a COR-mutant of Pst also strongly reduced silver damage. In conclusion, our results indicate that induction of JA defenses by COR, Pst infection, the medium of Pst and the medium of a Pst COR^- mutant increased resistance against WFT. This was not mediated by the reinforcement of leaf trichome densities, but rather the induction of chemical defenses.

Induction of Jasmonic Acid-Associated Defenses by Thrips Alters Host Suitability for Conspecifics and Correlates with Increased Trichome Densities in Tomato

Plant defenses inducible by herbivorous arthropods can determine performance of subsequent feeding herbivores. We investigated how infestation of tomato (Solanum lycopersicum) plants with the Western flower thrips (Frankliniella occidentalis) alters host plant suitability and foraging decisions of their conspecifics. We explored the role of delayed-induced jasmonic acid (JA)-mediated plant defense responses in thrips preference by using the tomato mutant def-1, impaired in JA biosynthesis. In particular, we investigated the effect of thrips infestation on trichome-associated tomato defenses. The results showed that when offered a choice, thrips preferred non-infested plants over infested wild-type plants, while no differences were observed in def-1. Exogenous application of methyl jasmonate restored the repellency effect in def-1. Gene expression analysis showed induction of the JA defense signaling pathway in wild-type plants, while activating the ethylene signaling pathway in both genotypes. Activation of JA defenses led to increases in type-VI leaf glandular trichome densities in the wild type, augmenting the production of trichome-associated volatiles, i.e. terpenes. Our study revealed that plant-mediated intraspecific interactions between thrips are determined by JA-mediated defenses in tomato. We report that insects can alter not only trichome densities but also the allelochemicals produced therein, and that this response might depend on the magnitude and/or type of the induction.

Synthesis and Functions of Jasmonates in Maize

Of the over 600 oxylipins present in all plants, the phytohormone jasmonic acid (JA) remains the best understood in terms of its biosynthesis, function and signaling. Much like their eicosanoid analogues in mammalian system, evidence is growing for the role of the other oxylipins in diverse physiological processes. JA serves as the model plant oxylipin species and regulates defense and development. For several decades, the biology of JA has been characterized in a few dicot species, yet the function of JA in monocots has only recently begun to be elucidated. In this work, the synthesis and function of JA in maize is presented from the perspective of oxylipin biology. The maize genes responsible for catalyzing the reactions in the JA biosynthesis are clarified and described. Recent studies into the function of JA in maize defense against insect herbivory, pathogens and its role in growth and development are highlighted. Additionally, a list of JA-responsive genes is presented for use as biological markers for improving future investigations into JA signaling in maize.

A Jasmonate-Inducible Defense Trait Transferred from Wild into Cultivated Tomato Establishes Increased Whitefly Resistance and Reduced Viral Disease Incidence

Whiteflies damage tomatoes mostly via the viruses they transmit. Cultivated tomatoes lack many of the resistances of their wild relatives. In order to increase protection to its major pest, the whitefly Bemisia tabaci and its transmitted Tomato Yellow Leaf Curl Virus (TYLCV), we introgressed a trichome-based resistance trait from the wild tomato Solanum pimpinellifolium into cultivated tomato, Solanum lycopersicum. The tomato backcross line BC₅S₂ contains acylsucrose-producing type-IV trichomes, unlike cultivated tomatoes, and exhibits increased, yet limited protection to whiteflies at early development stages. Treatment of young plants with methyl jasmonate (MeJA) resulted in a 60% increase in type-IV trichome density, acylsucrose production, and enhanced resistance to whiteflies, leading to 50% decrease in the virus disease incidence compared to cultivated tomato. Using transcriptomics, metabolite analysis, and insect bioassays we established the basis of this inducible resistance. We found that MeJA activated the expression of the genes involved in the biosynthesis of the defensive acylsugars in young BC₅S₂ plants leading to enhanced chemical defenses in their acquired type-IV trichomes. Our results show that not only constitutive but also these inducible defenses can be transferred from wild into cultivated crops to aid sustainable protection, suggesting that conventional breeding strategies provide a feasible alternative to increase pest resistance in tomato.

Auxin Response Gene SlARF3 Plays Multiple Roles in Tomato Development and is Involved in the Formation of Epidermal Cells and Trichomes

The auxin response factor (ARF) genes encode a large family of proteins involved in auxin signaling transduction. SlARF3, a member of the ARF gene family, encodes a protein containing two conserved domains, B3 and ARF, and lacking an Aux/IAA domain. Expression analysis showed that SlARF3 has a particularly high expression level in trichomes. In situ hybridization also detected the SlARF3 transcripts in epidermal pavement cells of leaves. The physiological function of SlARF3 was studied by using the RNA interference (RNAi) strategy. SlARF3-down-regulated plants exhibited decreased density of epidermal pavement cells and obviously reduced density of type I, V and VI trichomes of leaves, which indicates the important role of SlARF3 in the formation of trichomes and epidermal cells in tomato. The number of shoot xylem cells was also decreased in SlARF3-down-regulated lines. Furthermore, RNA-sequencing (RNA-Seq) analysis identified 51 differentially expressed genes (DEGs) belonging to 14 transcription factor (TF) families, such as MYB, bHLH, WD40 and C2H2 zinc finger. Twenty-seven DEGs were involved in the metabolism and signaling transduction of phytohormones, such as auxin, ethylene and gibberellin. These results indicated the important roles of the TFs and hormones in auxin-dependent transcriptional regulation of trichome formation in tomato. Taken together, our results demonstrate that SlARF3 plays an important role in the formation of epidermal cells and trichomes and reveal novel and specific functions for ARFs in tomato developmental processes.

Subcellular compartmentalization in protoplasts from Artemisia annua cell cultures: engineering attempts using a modified SNARE protein

Plants are ideal bioreactors for the production of macromolecules but transport mechanisms are not fully understood and cannot be easily manipulated. Several attempts to overproduce recombinant proteins or secondary metabolites failed. Because of an independent regulation of the storage compartment, the product may be rapidly degraded or cause self-intoxication. The case of the anti-malarial compound artemisinin produced by Artemisia annua plants is emblematic. The accumulation of artemisinin naturally occurs in the apoplast of glandular trichomes probably involving autophagy and unconventional secretion thus its production by undifferentiated tissues such as cell suspension cultures can be challenging. Here we characterize the subcellular compartmentalization of several known fluorescent markers in protoplasts derived from Artemisia suspension cultures and explore the possibility to modify compartmentalization using a modified SNARE protein as molecular tool to be used in future biotechnological applications. We focused on the observation of the vacuolar organization in vivo and the truncated form of AtSYP51, 51H3, was used to induce a compartment generated by the contribution of membrane from endocytosis and from endoplasmic reticulum to vacuole trafficking. The artificial compartment crossing exocytosis and endocytosis may trap artemisinin stabilizing it until extraction; indeed, it is able to increase total enzymatic activity of a vacuolar marker (RGUSChi), probably increasing its stability. Exploring the 51H3-induced compartment we gained new insights on the function of the SNARE SYP51, recently shown to be an interfering-SNARE, and new hints to engineer eukaryote endomembranes for future biotechnological applications.

Transgenic approach to increase artemisinin content in Artemisia annua L

Artemisinin, the endoperoxide sesquiterpene lactone, is an effective antimalarial drug isolated from the Chinese medicinal plant Artemisia annua L. Due to its effectiveness against multi-drug-resistant cerebral malaria, it becomes the essential components of the artemisinin-based combination therapies which are recommended by the World Health Organization as the preferred choice for malaria tropica treatments. To date, plant A. annua is still the main commercial source of artemisinin. Although semi-synthesis of artemisinin via artemisinic acid in yeast is feasible at present, another promising approach to reduce the price of artemisinin is using plant metabolic engineering to obtain a higher content of artemisinin in transgenic plants. In the past years, an Agrobacterium-mediated transformation system of A. annua has been established by which a number of genes related to artemisinin biosynthesis have been successfully transferred into A. annua plants. In this review, the progress on increasing artemisinin content in A. annua by transgenic approach and its future prospect are summarized and discussed.

Usual and unusual development of the dicot leaf: involvement of transcription factors and hormones

Morphological diversity exhibited by higher plants is essentially related to the tremendous variation of leaf shape. With few exceptions, leaf primordia are initiated postembryonically at the flanks of a group of undifferentiated and proliferative cells within the shoot apical meristem (SAM) in characteristic position for the species and in a regular phyllotactic sequence. Auxin is critical for this process, because genes involved in auxin biosynthesis, transport, and signaling are required for leaf initiation. Down-regulation of transcription factors (TFs) and cytokinins are also involved in the light-dependent leaf initiation pathway. Furthermore, mechanical stresses in SAM determine the direction of cell division and profoundly influence leaf initiation suggesting a link between physical forces, gene regulatory networks and biochemical gradients. After the leaf is initiated, its further growth depends on cell division and cell expansion. Temporal and spatial regulation of these processes determines the size and the shape of the leaf, as well as the internal structure. A complex array of intrinsic signals, including phytohormones and TFs control the appropriate cell proliferation and differentiation to elaborate the final shape and complexity of the leaf. Here, we highlight the main determinants involved in leaf initiation, epidermal patterning, and elaboration of lamina shape to generate small marginal serrations, more deep lobes or a dissected compound leaf. We also outline recent advances in our knowledge of regulatory networks involved with the unusual pattern of leaf development in epiphyllous plants as well as leaf morphology aberrations, such as galls after pathogenic attacks of pests.

Trichomes as models for studying plant cell differentiation

Trichomes, originating from epidermal cells, are present on nearly all terrestrial plants. They exist in diverse forms, are readily accessible, and serve as an excellent model system for analyzing the molecular mechanisms in plant cell differentiation, including cell fate choices, cell cycle control, and cell morphogenesis. In Arabidopsis, two regulatory models have been identified that function in parallel in trichome formation; the activator-inhibitor model and the activator-depletion model. Cotton fiber, a similar unicellular structure, is controlled by some functional homologues of Arabidopsis trichome-patterning genes. Multicellular trichomes, as in tobacco and tomato, may form through a distinct pathway from unicellular trichomes. Recent research has shown that cell cycle control participates in trichome formation. In this review, we summarize the molecular mechanisms involved in the formation of unicellular and multicellular trichomes, and discuss the integration of the cell cycle in its initiation and morphogenesis.

Effect of gibberellic acid and calliterpenone on plant growth attributes, trichomes, essential oil biosynthesis and pathway gene expression in differential manner in Mentha arvensis L

Extensive research is going on throughout the world to find out new molecules from natural sources to be used as plant growth promoter. Mentha arvensis L. is the main source of menthol rich essential oil used commercially in various food, pharmaceutical and other preparations. Experiments were conducted on field grown plants for understanding the effect of calliterpenone (CA), a stereo-isomer of abbeokutone, in comparison to gibberellic acid (GA3) on growth attributes, trichomes, essential oil biosynthesis and expression of some oil biosynthetic pathway genes. The exogenous application of CA (1 μM, 10 μM and 100 μM) was found to be better in improving plant biomass and stolon yield, leaf area, branching and leaf stem ratio than with counterpart GA3 at the same concentrations. CA treated plants showed higher glandular trichome number, density and diameter and also correlated with enhanced oil biogenetic capacity as revealed by feeding labeled (14)C-sucrose for 72 h to excised shoots. Semi-quantitative PCR analysis of key pathway genes revealed differential up regulation under CA treatments. Transcript level of menthol dehydrogenase/menthone reductase was found highly up regulated in CA treated plants with increased content of menthone and menthol in oil. These findings demonstrate that CA positively regulated the yields by enhanced branching and higher density of trichomes resulting into higher accumulation of essential oil. The results suggest CA as a novel plant derived diterpenoid with growth promoting action and opens up new possibilities for improving the crop yields and essential oil biosynthesis in qualitative and quantitative manner.

Pathways for epidermal cell differentiation via the homeobox gene GLABRA2: update on the roles of the classic regulator

Recent plant development studies have identified regulatory pathways for epidermal cell differentiation in Arabidopsis thaliana. Interestingly, some of such pathways contain transcriptional networks with a common structure in which the homeobox gene GLABLA2 (GL2) is downstream of the transactivation complex consisting of MYB, bHLH, and WD40 proteins. Here, we review the role of GL2 as an output device of the conserved network, and update the knowledge of epidermal cell differentiation pathways downstream of GL2. Despite the consistent position of GL2 within the network, its role in epidermal tissues varies; in the root epidermis, GL2 promotes non-hair cell differentiation after cell pattern formation, whereas in the leaf epidermis, it is likely to be involved in both pattern formation and differentiation of trichomes. GL2 expression levels act as quantitative factors for initiation of cell differentiation in the root and leaf epidermis; the quantity of hairless cells in non-root hair cell files is reduced by gl2 mutations in a semi-dominant manner, and entopically additive expression of GL2 and a heterozygous gl2 mutation increase and decrease the number of trichomes, respectively. Although few direct target genes have been identified, evidence from genetic and expression analyses suggests that GL2 directly regulates genes with various hierarchies in epidermal cell differentiation pathways.

Response of sunflower (Helianthus annuus L.) leaf surface defenses to exogenous methyl jasmonate

Helianthus annuus, the common sunflower, produces a complex array of secondary compounds that are secreted into glandular trichomes, specialized structures found on leaf surfaces and anther appendages of flowers. The primary components of these trichome secretions are sesquiterpene lactones (STL), a diverse class of compounds produced abundantly by the plant family Compositae and believed to contribute to plant defense against herbivory. We treated wild and cultivated H. annuus accessions with exogenous methyl jasmonate, a plant hormone that mediates plant defense against insect herbivores and certain classes of fungal pathogens. The wild sunflower produced a higher density of glandular trichomes on its leaves than the cultivar. Comparison of the profiles of glandular trichome extracts obtained by liquid chromatography–mass spectroscopy (LC-MS) showed that wild and cultivated H. annuus were qualitatively similar in surface chemistry, although differing in the relative size and proportion of various compounds detected. Despite observing consistent transcriptional responses to methyl jasmonate treatment, we detected no significant effect on glandular trichome density or LC-MS profile in cultivated or wild sunflower, with wild sunflower exhibiting a declining trend in overall STL production and foliar glandular trichome density of jasmonate-treated plants. These results suggest that glandular trichomes and associated compounds may act as constitutive defenses or require greater levels of stimulus for induction than the observed transcriptional responses to exogenous jasmonate. Reduced defense investment in domesticated lines is consistent with predicted tradeoffs caused by selection for increased yield; future research will focus on the development of genetic resources to explicitly test the ecological roles of glandular trichomes and associated effects on plant growth and fitness.

Functional characterization of TRICHOMELESS2, a new single-repeat R3 MYB transcription factor in the regulation of trichome patterning in Arabidopsis

BACKGROUND

Single-repeat R3 MYB transcription factors (single-repeat MYBs) play important roles in controlling trichome patterning in Arabidopsis. It was proposed that single-repeat MYBs negatively regulate trichome formation by competing with GLABRA1 (GL1) for binding GLABRA3/ENHANCER OF GLABRA3 (GL3/EGL3), thus inhibiting the formation of activator complex TTG1(TRANSPARENT TESTA GLABRA1)-GL3/EGL3-GL1 that is required for the activation of GLABRA2 (GL2), whose product is a positive regulator of trichome formation. Previously we identified a novel single-repeat MYB transcription factor, TRICHOMELESS1 (TCL1), which negatively regulates trichome formation on the inflorescence stems and pedicels by directly suppressing the expression of GL1.

RESULTS

We analyzed here the role of TRICHOMELESS2 (TCL2), a previously-uncharacterized single-repeat MYB transcription factor in trichome patterning in Arabidopsis. We showed that TCL2 is closely related to TCL1, and like TCL1 and other single-repeat MYBs, TCL2 interacts with GL3. Overexpression of TCL2 conferred glabrous phenotype while knockdown of TCL2 via RNAi induced ectopic trichome formation on the inflorescence stems and pedicels, a phenotype that was previously observed in tcl1 mutants. These results suggested that TCL2 may have overlapping function with TCL1 in controlling trichome formation on inflorescences. On the other hand, although the transcription of TCL2, like TCL1, is not controlled by the activator complex formed by GL1 and GL3, and TCL2 and TCL1 proteins are more than 80% identical at the amino acid level, the expression of TCL2 under the control of TCL1 promoter only partially recovered the mutant phenotype of tcl1, implying that TCL2 and TCL1 are not fully functional equivalent.

CONCLUSIONS

TCL2 function redundantly with TCL1 in controlling trichome formation on inflorescences, but they are not fully functional equivalent. Transcription of TCL2 is not controlled by activator complex formed by GL1 and GL3, but MIR156 controlled SQUAMOSA PROMOTER BINDING PROTEIN LIKE (SPL) transcription factors. However, SPLs might require co-activators to regulate the expression of their target genes, including TCL1, TRY and possibly, TCL2.

Dissection of the phytohormonal regulation of trichome formation and biosynthesis of the antimalarial compound artemisinin in Artemisia annua plants

• Biosynthesis of the sesquiterpene lactone and potent antimalarial drug artemisinin occurs in glandular trichomes of Artemisia annua plants and is subjected to a strict network of developmental and other regulatory cues. • The effects of three hormones, jasmonate, gibberellin and cytokinin, were studied at the structural and molecular levels in two different A. annua chemotypes by microscopic analysis of gland development, and by targeted metabolite and transcript profiling. Furthermore, a genome-wide cDNA-amplified fragment length polymorphism (AFLP)-based transcriptome profiling was carried out of jasmonate-elicited leaves at different developmental stages. • Although cytokinin and gibberellin positively affected at least one aspect of gland formation, these two hormones did not stimulate artemisinin biosynthesis. Only jasmonate simultaneously promoted gland formation and coordinated transcriptional activation of biosynthetic gene expression, which ultimately led to increased sesquiterpenoid accumulation with chemotype-dependent effects on the distinct pathway branches. Transcriptome profiling revealed a trichome-specific fatty acyl- coenzyme A reductase, trichome-specific fatty acyl-CoA reductase 1 (TFAR1), the expression of which correlates with trichome development and sesquiterpenoid biosynthesis. • TFAR1 is potentially involved in cuticular wax formation during glandular trichome expansion in leaves and flowers of A. annua plants. Analysis of phytohormone-modulated transcriptional regulons provides clues to dissect the concerted regulation of metabolism and development of plant trichomes.

New insights into the mechanism of development of Arabidopsis root hairs and trichomes

Epidermis cell differentiation in Arabidopsis thaliana is a model system for understanding the mechanisms leading to the developmental end state of plant cells. Both root hairs and trichomes differentiate from epidermal cells and molecular genetic analyses using Arabidopsis mutants have demonstrated that the differentiation of root hairs and trichomes is regulated by similar molecular mechanisms. Molecular-genetic approaches have led to the identification of many genes that are involved in epidermal cell differentiation, most of which encode transcription factors that induce the expression of genes active in both root hair and trichome development. Control of cell growth after fate determination has also been studied using Arabidopsis mutants.