AaGL3-like is jasmonate-induced bHLH transcription factor that positively regulates trichome density in Artemisia annua

The leaves of Artemisia annua contain GSTs (Glandular secretory trichomes) that can secrete and store artemisinin, the drug most effective for treating uncomplicated malaria. Therefore, increasing the density of GSTs in A. annua is an efficient way to enhance artemisinin content. However, our understanding of how GSTs develop still needs to be improved. Here, we isolated an A. annua homolog of AtGL3 (GLABRA3), known as AaGL3-like, that positively regulates trichome density in A. annua. AaGL3-like is nuclear-localized and transcriptionally active. It is least expressed in roots and most prominently in aerial components like leaves, stems, and inflorescence. Under JA and GA hormonal treatments, AaGL3-like expression is significantly increased. In transgenic over-expression AaGL3-like lines, trichome developmental genes such as AaHD1 and AaGSW2 showed much increased expression. The AaGL3RNAi line exhibited considerably lower levels of AaHD1 and AaGSW2 transcripts. As a result, the AaGL3-RNAi lines showed reduced levels of artemisinin content and trichome density compared to wild-type and overexpression lines. Additionally, we have found that when co-expressed with AaJAZ8, the induction of trichome developmental genes was reduced as compared to individual OEAaGL3-like lines. Further, AaJAZ8 directly binds to AaGL3-like in the Y2H assay. These findings suggest that AaGL3-like is a jasmonate-induced bHLH transcription factor that drastically increases the final accumulation of artemisinin content by regulating trichome density in A. annua.

Tomato root specialized metabolites evolved through gene duplication and regulatory divergence within a biosynthetic gene cluster

Tremendous plant metabolic diversity arises from phylogenetically restricted specialized metabolic pathways. Specialized metabolites are synthesized in dedicated cells or tissues, with pathway genes sometimes colocalizing in biosynthetic gene clusters (BGCs). However, the mechanisms by which spatial expression patterns arise and the role of BGCs in pathway evolution remain underappreciated. In this study, we investigated the mechanisms driving acylsugar evolution in the Solanaceae. Previously thought to be restricted to glandular trichomes, acylsugars were recently found in cultivated tomato roots. We demonstrated that acylsugars in cultivated tomato roots and trichomes have different sugar cores, identified root-enriched paralogs of trichome acylsugar pathway genes, and characterized a key paralog required for root acylsugar biosynthesis, SlASAT1-LIKE (SlASAT1-L), which is nested within a previously reported trichome acylsugar BGC. Last, we provided evidence that ASAT1-L arose through duplication of its paralog, ASAT1, and was trichome-expressed before acquiring root-specific expression in the Solanum genus. Our results illuminate the genomic context and molecular mechanisms underpinning metabolic diversity in plants.

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.

Characterization and expression analysis of GLABRA3 (GL3) genes in cotton: insights into trichome development and hormonal regulation

BACKGROUND

GLABRA3 (GL3) and ENHANCER OF GLABRA3 (EGL3) genes encode a typical helix-loop-helix (bHLH) transcription factors that primarily regulate trichome branching and root hair development, DNA endoreduplication, trichoblast size, and stomatal formation. The functions of GL3 genes in cotton crop have been poorly characterized. In this study, we performed comprehensive genome-wide scans for GL3 and EGL3 homologs to enhance our comprehension of their potential roles in trichome and fiber development in cotton crop.

METHODS AND RESULTS

Our findings paraded that Gossypium hirsutum and G. barbadense have 6 GL3s each, unevenly distributed on 4 chromosomes whereas, G. arboreum, and G. raimondii have 3 GL3s each, unevenly distributed on 2 chromosomes. Gh_A08G2088 and Gb_A09G2187, despite having the same bHLH domain as the other GL3 genes, were excluded due to remarkable short sequences and limited number of motifs, indicating a lack of potential functional activity. The phylogenetic analysis categorized remaining 16 GL3s into three subfamilies (Group I-III) closely related to A. thaliana. The 16 GL3s have complete bHLH domain, encompassing 590-631 amino acids, with molecular weights (MWs) ranging from 65.92 to 71.36 kDa. Within each subfamily GL3s depicted shared similar gene structures and motifs, indicating conserved characteristics within respective groups. Promoter region analysis revealed 27 cis-acting elements, these elements were responsive to salicylic acid, abscisic acid (ABA), methyl jasmonate (MeJA), and gibberellin. The expression of GL3 genes was analyzed across 12 tissues in both G. barbadense and G. hirsutum using the publicly available RNA-seq data. Among GL3s, Gb_D11G0219, Gb_D11G0214, and Gb_D08G2182, were identified as relatively highly expressed across different tissues, consequently selected for hormone treatment and expression validation in G. barbadense. RT-qPCR results demonstrated significant alterations in the expression levels of Gb_D11G0219 and Gb_D11G0214 following MeJA, GA, and ABA treatment. Subcellular localization prediction revealed that most GL3 proteins were predominantly expressed in the nucleus, while a few were localized in the cytoplasm and chloroplasts.

CONCLUSIONS

In summary, this study lays the foundation for subsequent functional validation of GL3 genes by identifying hormonal regulation patterns and probable sites of action in cotton trichome formation and fiber development. The results stipulate a rationale to elucidate the roles and regulatory mechanisms of GL3 genes in the intricate process of cotton fibre and trichome development.

Identification of a cis-element for long glandular trichome-specific gene expression, which is targeted by a HD-ZIP IV protein

Glandular trichomes are epidermal outgrowths that secret a variety of secondary metabolites, which not only help plants adapt to environmental stresses but also have important commercial value in fragrances, pharmaceuticals, and pesticides. In Nicotiana tabacum, it has been confirmed that a B-type cyclin, CycB2, negatively regulates the formation of long glandular trichomes (LGTs). This study aimed to identify the upstream regulatory gene involved in LGT formation by screening LGT-specific cis-elements within the NtCycB2 promoter. Using GUS as a reporter gene, the tissue-driven ability of NtCycB2 promoter showed that NtCycB2 promoter could drive GUS expression specifically in LGTs. Function analysis of a series of successive 5' truncations and synthetic segments of the NtCycB2 promoter indicated that the 87-bp region from -1221 to -1134 of the NtCycB2 promoter was required for gene expression in LGTs, and the L1-element (5'-AAAATTAATAAGAG-3') located in the 87-bp region contributed to the gene expression in the stalk of LGTs. Further Y1H and LUC assays confirmed that this L1-element exclusively binds to a HD-Zip IV protein, NtHD13. Gene function analysis revealed that NtHD13 positively controlled LGT formation, as overexpression of NtHD13 resulted in a high number of LGTs, whereas knockout of NtHD13 led to a decrease in LGTs. These findings demonstrate that NtHD13 can bind to an L1-element within the NtCycB2 promoter to regulate LGT formation.

Interplay of EXO70 and MLO proteins modulates trichome cell wall composition and susceptibility to powdery mildew

Exocyst component of 70-kDa (EXO70) proteins are constituents of the exocyst complex implicated in vesicle tethering during exocytosis. MILDEW RESISTANCE LOCUS O (MLO) proteins are plant-specific calcium channels and some MLO isoforms enable fungal powdery mildew pathogenesis. We here detected an unexpected phenotypic overlap of Arabidopsis thaliana exo70H4 and mlo2 mlo6 mlo12 triple mutant plants regarding the biogenesis of leaf trichome secondary cell walls. Biochemical and Fourier transform infrared spectroscopic analyses corroborated deficiencies in the composition of trichome cell walls in these mutants. Transgenic lines expressing fluorophore-tagged EXO70H4 and MLO exhibited extensive colocalization of these proteins. Furthermore, mCherry-EXO70H4 mislocalized in trichomes of the mlo triple mutant and, vice versa, MLO6-GFP mislocalized in trichomes of the exo70H4 mutant. Expression of GFP-marked PMR4 callose synthase, a known cargo of EXO70H4-dependent exocytosis, revealed reduced cell wall delivery of GFP-PMR4 in trichomes of mlo triple mutant plants. In vivo protein-protein interaction assays in plant and yeast cells uncovered isoform-preferential interactions between EXO70.2 subfamily members and MLO proteins. Finally, exo70H4 and mlo6 mutants, when combined, showed synergistically enhanced resistance to powdery mildew attack. Taken together, our data point to an isoform-specific interplay of EXO70 and MLO proteins in the modulation of trichome cell wall biogenesis and powdery mildew susceptibility.

The GaKAN2, a KANADI transcription factor, modulates stem trichomes in Gossypium arboreum

KEY MESSAGE

GaKAN2, a member of the KANADI family, was found to be widely expressed in the cotton tissues and regulates trichome development through complex pathways. Cotton trichomes are believed to be the defense barrier against insect pests. Cotton fiber and trichomes are single-cell epidermal extensions with shared regulatory mechanisms. Despite several studies underlying mechanism of trichome development remains elusive. The KANADI is one of the key transcription factors (TFs) family, regulating Arabidopsis trichomes growth. However, the function of KANADI genes in cotton remains unknown. In the current study genome-wide scanning, transcriptomic analysis, gene silencing, subcellular localization, and yeast two-hybrid techniques were employed to decipher the function of KANADI TFs family genes in cotton crop. A total of 7 GaKAN genes were found in the Gossypium arboreum. Transcriptomic data revealed that these genes were significantly expressed in stem and root. Moreover, GaKAN2 was widely expressed in other tissues also. Subsequently, we selected GaKAN2 to validate the function of KANADI genes. Silencing of GaKAN2 resulted in a 24.99% decrease in single-cell trichomes and an 11.33% reduction in internodal distance, indicating its potential role in regulating trichomes and plant growth. RNA-Seq analysis elucidated that GaSuS and GaERS were the downstream genes of GaKAN2. The transcriptional activation and similarity in silencing phenotype between GaKAN2 and GaERS suggested that GaKAN2 regulates trichomes development through GaERS. Moreover, KEGG analysis revealed that a significant number of genes were enriched in the biosynthesis of secondary metabolites and plant hormone signal transduction pathways, thereby suggesting that GaKAN2 regulates the stem trichomes and plant growth. The GFP subcellular localization and yeast transcriptional activation analysis elucidated that GaKAN2 was located in the nucleus and capable of regulating the transcription of downstream genes. This study elucidated the function and characteristics of the KANADI gene family in cotton, providing a fundamental basis for further research on GaKAN2 gene in cotton plant trichomes and plant developmental processes.

Identification of quantitative trait loci (QTLs) regulating leaf SPAD value and trichome density in mungbean (Vigna radiata L.) using genotyping-by-sequencing (GBS) approach

Quantitative trait loci (QTL) mapping is used for the precise localization of genomic regions regulating various traits in plants. Two major QTLs regulating Soil Plant Analysis Development (SPAD) value (qSPAD-7-1) and trichome density (qTric-7-2) in mungbean were identified using recombinant inbred line (RIL) populations (PMR-1×Pusa Baisakhi) on chromosome 7. Functional analysis of QTL region identified 35 candidate genes for SPAD value (16 No) and trichome (19 No) traits. The candidate genes regulating trichome density on the dorsal leaf surface of the mungbean include VRADI07G24840, VRADI07G17780, and VRADI07G15650, which encodes for ZFP6, TFs bHLH DNA-binding superfamily protein, and MYB102, respectively. Also, candidate genes having vital roles in chlorophyll biosynthesis are VRADIO7G29860, VRADIO7G29450, and VRADIO7G28520, which encodes for s-adenosyl-L-methionine, FTSHI1 protein, and CRS2-associated factor, respectively. The findings unfolded the opportunity for the development of customized genotypes having high SPAD value and high trichome density having a possible role in yield and mungbean yellow vein mosaic India virus (MYMIV) resistance in mungbean.

QTL Mapping of Trichome Traits and Analysis of Candidate Genes in Leaves of Wheat (Triticum aestivum L.)

Trichome plays an important role in heat dissipation, cold resistance, water absorption, protection of leaves from mechanical damage, and direct exposure to ultraviolet rays. It also plays an important role in the photosynthesis, transpiration, and respiration of plants. However, the genetic basis of trichome traits is not fully understood in wheat. In this study, wheat DH population (Hanxuan 10 × Lumai 14) was used to map quantitative trait loci (QTL) for trichome traits in different parts of flag leaf at 10 days after anther with growing in Zhao County, Hebei Province, and Taigu County, Shanxi Province, respectively. The results showed that trichome density (TD) was leaf center > leaf tip > leaf base and near vein > middle > edge, respectively, in both environments. The trichome length (TL) was leaf tip > leaf center > leaf base and edge > middle > near vein. Significant phenotypic positive correlations were observed between the trichome-related traits of different parts. A total of 83 QTLs for trichome-related traits were mapped onto 18 chromosomes, and each one accounted for 2.41 to 27.99% of the phenotypic variations. Two QTL hotspots were detected in two marker intervals: AX-95232910~AX-95658735 on 3A and AX-94850949~AX-109507404 on 7D. Six possible candidate genes (TraesCS3A02G406000, TraesCS3A02G414900, TraesCS3A02G440900, TraesCS7D02G145200, TraesCS7D02G149200, and TraesCS7D02G152400) for trichome-related traits of wheat leaves were screened out according to their predicted expression levels in wheat leaves. The expression of these genes may be induced by a variety of abiotic stresses. The results provide the basis for further validation and functional characterization of the candidate genes.

Effects of exogenous indole-3-acetic acid on the density of trichomes, expression of artemisinin biosynthetic genes, and artemisinin biosynthesis in Artemisia annua

Artemisinin is the most practical medication for the treatment of malaria, but is only very minimally synthesized in Artemisia annua, significantly less than the market needs. In this study, indole-3-acetic acid (IAA) was used to investigate its effects on trichomes, artemisinin accumulation, and biosynthetic gene expression in A. anuua. The results showed that exogenous IAA could contribute to the growth and development of A. annua and increase the density of trichomes. Analysis using liquid chromatography-tandem mass spectrometry (LC-MS/MS) indicated that artemisinin and dihydroartemisinic acid (DHAA) contents were increased by 1.9-fold (1.1 mg/g) and 2.1-fold (0.51 mg/g) after IAA treatment in comparison with control lines (CK), respectively. Furthermore, quantitative real-time PCR results showed that AaADS, AaCYP71AV1, AaALDH1, and AaDBR2, four critical enzyme genes for the biosynthesis of artemisinin, had relatively high transcription levels in leaves of A. annua treated with IAA. In summary, this study indicated that exogenous IAA treatment was a feasible strategy to enhance artemisinin production, which paves the way for further metabolic engineering of artemisinin biosynthesis.

AXR1 modulates trichome morphogenesis through mediating ROP2 stability in Arabidopsis

Cell differentiation and morphogenesis are crucial for the establishment of diverse cell types and organs in multicellular organisms. Trichome cells offer an excellent paradigm for dissecting the regulatory mechanisms of plant cell differentiation and morphogenesis due to their unique growth characteristics. Here, we report the isolation of an Arabidopsis mutant, aberrantly branched trichome 3-1 (abt3-1), with a reduced trichome branching phenotype. Positional cloning and molecular complementation experiments confirmed that abt3-1 is a new mutant allele of Auxin resistant 1 (AXR1), which encodes the N-terminal half of ubiquitin-activating enzyme E1 and functions in auxin signaling pathway. Meanwhile, we found that transgenic plants expressing constitutively active version of ROP2 (CA-ROP2) caused a reduction of trichome branches, resembling that of abt3-1. ROP2 is a member of Rho GTPase of plants (ROP) family, serving as versatile signaling switches involved in a range of cellular and developmental processes. Our genetic and biochemical analyses showed AXR1 genetically interacted with ROP2 and mediated ROP2 protein stability. The loss of AXR1 aggravated the trichome defects of CA-ROP2 and induced the accumulation of steady-state ROP2. Consistently, elevated AXR1 expression levels suppressed ROP2 expression and partially rescued trichome branching defects in CA-ROP2 plants. Together, our results presented a new mutant allele of AXR1, uncovered the effects of AXR1 and ROP2 during trichome development, and revealed a pathway of ROP2-mediated regulation of plant cell morphogenesis in Arabidopsis.

SMALLER TRICHOMES WITH VARIABLE BRANCHES (SVB) and its homolog SVBL act downstream of transcription factor NAC089 and function redundantly in Arabidopsis unfolded protein response

The unfolded protein response (UPR) is a cellular mechanism that alleviates endoplasmic reticulum stress to maintain protein homeostasis. Although SMALLER TRICHOMES WITH VARIABLE BRANCHES (SVB) is characterized as an emerging UPR factor downstream of the IRE-bZIP60 pathway, whether its homologs participate in the plant UPR remains unknown. Here, we showed that an SVB homolog, SVB-like (SVBL), functions redundantly with SVB in endoplasmic reticulum stress tolerance. The svb-1 svbl-1 double mutant showed a hypersensitivity phenotype and had higher UPR gene expression under endoplasmic reticulum stress than single mutants and the wild type. SVB responded to endoplasmic reticulum stress by accumulating in the root epidermis and phloem cells, but SVBL did not. Ectopic expression of the UPR factor NAC089 up-regulated both SVB and SVBL genes, suggesting that SVB and SVBL work downstream of NAC089. Thus, SVB and SVBL play distinct roles that are modulated by the common upstream regulator NAC089 to cope with endoplasmic reticulum stress in Arabidopsis.

Metabolic engineering of a 1,8-cineole synthase enhances aphid repellence and increases trichome density in transgenic tobacco (Nicotiana tabacum L.)

BACKGROUND

The green peach aphid (Myzus persicae Sulzer) is a harmful agricultural pest that causes severe crop damage by directly feeding or indirectly vectoring viruses. 1,8-cineole synthase (CINS) is a multiproduct enzyme that synthesizes monoterpenes, with 1,8-cineole dominating the volatile organic compound profile. However, the relationship between aphid preference and CINS remains elusive.

RESULTS

Here, we present evidence that SoCINS, a protein from garden sage (Salvia officinalis), enhanced aphid repellence and increased trichome density in transgenic tobacco. Our results demonstrated that overexpression of SoCINS (SoCINS-OE) led to the emission of 1,8-cineole at a level of up to 181.5 ng per g fresh leaf. Subcellular localization assay showed that SoCINS localized to chloroplasts. A Y-tube olfactometer assay and free-choice assays revealed that SoCINS-OE plants had a repellent effect on aphids, without incurring developmental or fecundity-related penalties. Intriguingly, the SoCINS-OE plants displayed an altered trichome morphology, showing increases in trichome density and in the relative proportion of glandular trichomes, as well as enlarged glandular cells. We also found that SoCINS-OE plants had significantly higher jasmonic acid (JA) levels than wild-type plants. Furthermore, application of 1,8-cineole elicited increased JA content and trichome density.

CONCLUSION

Our results demonstrate that SoCINS-OE plants have a repellent effect on aphids, and suggest an apparent link between 1,8-cineole, JA and trichome density. This study presents a viable and sustainable approach for aphid management by engineering the expression of 1,8-cineole synthase gene in plants, and underscores the potential usefulness of monoterpene synthase for pest control. © 2023 Society of Chemical Industry.

The Light- and Jasmonic Acid-Induced AaMYB108-like Positive Regulates the Initiation of Glandular Secretory Trichome in Artemisia annua L

The plant Artemisia annua L. is famous for producing "artemisinin", which is an essential component in the treatment of malaria. The glandular secretory trichomes (GSTs) on the leaves of A. annua secrete and store artemisinin. Previous research has demonstrated that raising GST density can effectively raise artemisinin content. However, the molecular mechanism of GST initiation is not fully understood yet. In this study, we identified an MYB transcription factor, the AaMYB108-like, which is co-induced by light and jasmonic acid, and positively regulates glandular secretory trichome initiation in A. annua. Overexpression of the AaMYB108-like gene in A. annua increased GST density and enhanced the artemisinin content, whereas anti-sense of the AaMYB108-like gene resulted in the reduction in GST density and artemisinin content. Further experiments demonstrated that the AaMYB108-like gene could form a complex with AaHD8 to promote the expression of downstream AaHD1, resulting in the initiation of GST. Taken together, the AaMYB108-like gene is a positive regulator induced by light and jasmonic acid for GST initiation in A. annua.

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

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

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.

Structural variation of GL1 gene determines the trichome formation in Brassica juncea

A 448 kb region on chromosome B02 was delimited to be associated with trichome trait in Brassica juncea, in which the BjuVB02G54610 gene with a structural variation of 3 kb structure variation (SV) encoding a MYB transcription factor was predicted as the possible candidate gene. Mustards (Brassica juncea) are allopolyploid crops in the worldwide, and trichomes are essential quality attributes that significantly influence its taste and palpability in vegetable-use cultivars. As important accessory tissues from specialized epidermal cells, trichomes also play an important role in mitigating biotic and abiotic stresses. In this study, we constructed a F₂ segregating population using YJ27 with intensive trichome leaves and 03B0307 with glabrous leaves as parents. By bulked segregant analysis (BSA-seq), we obtained a 2.1 Mb candidate region on B02 chromosome associated with the trichome or glabrous trait formation. Then, we used 13 Kompetitive Allele Specific PCR (KASP) markers for fine mapping and finally narrowed down the candidate region to about 448 kb in length. Interestingly, among the region, there was a 3 kb sequence deletion that located on the BjuVB02G54610 gene in the F₂ individuals with trichome leaves. Genotyping results of F₂ populations confirmed this deletion (R² = 81.44%) as a major QTL. Natural population re-sequencing analysis and genotyping results further validated the key role of the 3 kb structure variation (SV) of insertion/deletion type in trichome development in B. juncea. Our findings provide important information on the formation of trichomes and potential target gene for breeding vegetable mustards.

Engineering Nicotiana tabacum trichomes for triterpenic acid production

In this work, we aimed at implementing the biosynthesis of triterpenic acids in Nicotiana tabacum glandular trichomes. Although endogenous genes coding for enzymes involved in such biosynthetic pathway are found in the Nicotiana tabacum genome, implementing such pathway specifically in glandular trichomes required to boost endogenous enzymatic activities. Five transgenes coding for a farnesyl-diphosphate synthase, a squalene synthase, a squalene epoxidase, a beta-amyrin synthase and a beta-amyrin 28-monooxygenase were introduced in N.tabacum, their expression being driven by pMALD1, a trichome-specific transcriptional promoter. This study aimed at testing whether sinking isoprenoid precursors localized in plastids, by exploiting potential cross-talks allowing the exchange of terpenoid pools from the chloroplast to the cytosol, could be a way to improve overall yield. By analyzing metabolites extracted from entire leaves, a low amount of ursolic acid was detected in plants expressing the five transgenes. Our study shows that the terpene biosynthetic pathway could be, in part, redirected in N.tabacum glandular trichomes with no deleterious phenotype at the whole plant level (chlorosis, dwarfism,…). In light of our results, possible ways to improve the final yield are discussed.

A HD-ZIP transcription factor specifies fates of multicellular trichomes via dosage-dependent mechanisms in tomato

Hair-like structures are shared by most living organisms. The hairs on plant surfaces, commonly referred to as trichomes, form diverse types to sense and protect against various stresses. However, it is unclear how trichomes differentiate into highly variable forms. Here, we show that a homeodomain leucine zipper (HD-ZIP) transcription factor named Woolly controls the fates of distinct trichomes in tomato via a dosage-dependent mechanism. The autocatalytic reinforcement of Woolly is counteracted by an autoregulatory negative feedback loop, creating a circuit with a high or low Woolly level. This biases the transcriptional activation of separate antagonistic cascades that lead to different trichome types. Our results identify the developmental switch of trichome formation and provide mechanistic insights into the progressive fate specification in plants, as well as a path to enhancing plant stress resistance and the production of beneficial chemicals.

Specialized metabolism by trichome-enriched Rubisco and fatty acid synthase components

Acylsugars, specialized metabolites with defense activities, are secreted by trichomes of many solanaceous plants. Several acylsugar metabolic genes (AMGs) remain unknown. We previously reported multiple candidate AMGs. Here, using multiple approaches, we characterized additional AMGs. First, we identified differentially expressed genes between high- and low-acylsugar-producing F2 plants derived from a cross between cultivated tomato (Solanum lycopersicum) and a wild relative (Solanum pennellii), which produce acylsugars that are ∼1% and ∼20% of leaf dry weight, respectively. Expression levels of many known and candidate AMGs positively correlated with acylsugar amounts in F2 individuals. Next, we identified lycopersicum-pennellii putative orthologs with higher nonsynonymous to synonymous substitutions. These analyses identified four candidate genes, three of which showed enriched expression in stem trichomes compared to underlying tissues (shaved stems). Virus-induced gene silencing confirmed two candidates, Sopen05g009610 [beta-ketoacyl-(acyl-carrier-protein) reductase; fatty acid synthase component] and Sopen07g006810 (Rubisco small subunit), as AMGs. Phylogenetic analysis indicated that Sopen05g009610 is distinct from specialized metabolic cytosolic reductases but closely related to two capsaicinoid biosynthetic reductases, suggesting evolutionary relationship between acylsugar and capsaicinoid biosynthesis. Analysis of publicly available datasets revealed enriched expression of Sopen05g009610 orthologs in trichomes of several acylsugar-producing species. Similarly, orthologs of Sopen07g006810 were identified as solanaceous trichome-enriched members, which form a phylogenetic clade distinct from those of mesophyll-expressed "regular" Rubisco small subunits. Furthermore, δ13C analyses indicated recycling of metabolic CO2 into acylsugars by Sopen07g006810 and showed how trichomes support high levels of specialized metabolite production. These findings have implications for genetic manipulation of trichome-specialized metabolism in solanaceous crops.

Single-cell RNA-seq reveals fate determination control of an individual fibre cell initiation in cotton (Gossypium hirsutum)

Cotton fibre is a unicellular seed trichome, and lint fibre initials per seed as a factor determines fibre yield. However, the mechanisms controlling fibre initiation from ovule epidermis are not understood well enough. Here, with single-cell RNA sequencing (scRNA-seq), a total of 14 535 cells were identified from cotton ovule outer integument of Xu142_LF line at four developmental stages (1.5, 1, 0.5 days before anthesis and the day of anthesis). Three major cell types, fibre, non-fibre epidermis and outer pigment layer were identified and then verified by RNA in situ hybridization. A comparative analysis on scRNA-seq data between Xu142 and its fibreless mutant Xu142 fl further confirmed fibre cluster definition. The developmental trajectory of fibre cell was reconstructed, and fibre cell was identified differentiated at 1 day before anthesis. Gene regulatory networks at four stages revealed the spatiotemporal pattern of core transcription factors, and MYB25-like and HOX3 were demonstrated played key roles as commanders in fibre differentiation and tip-biased diffuse growth respectively. A model for early development of a single fibre cell was proposed here, which sheds light on further deciphering mechanism of plant trichome and the improvement of cotton fibre yield.

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.

Glandular trichome specificity of menthol biosynthesis pathway gene promoters from Mentha × piperita

Several cis-elements including Myb-binding motifs together confer glandular trichome specificity as revealed from heterologous expression and analysis of menthol biosynthesis pathway gene promoters. Glandular Trichomes (GTs) are result of division of epidermal cells that produce diverse metabolites. Species of mint family are important for their essential oil containing many high-value terpenoids, biosynthesized and stored in these GTs. Hence, GTs constitute attractive targets for metabolic engineering and GT-specific promoters are important. In this investigation, the upstream regions of the Mentha × piperita menthol biosynthetic pathway genes (-)-limonene synthase, (-)-P450 limonene-3- hydroxylase, (-)-trans-isopiperitenol dehydrogenase, (-)-Isopiperitenone reductase, ( +)-Pulegone reductase, (-)-Menthone reductase/ (-)-Menthol dehydrogenase and a branched pathway gene ( +)-menthofuran synthase were isolated and characterized. These fragments, fused to β-glucuronidase (GUS) reporter gene of pBI101 binary vector, are able to drive high level gene expression in transgenic tobacco trichomes with strong signals in GTs, except for (-)-Isopiperitenone reductase. The GT-enriched tissue from transformed plants were analysed for GUS enzyme activity and RNA expression which correlates the GUS staining. To characterize the cis-elements responsible for GT-specific expression, a series of 5' deletion constructs for MpPLS and MpPMFS were cloned and analysed in stable transgenic tobacco lines. The specificity of trichome expression was located to -  797 to-  598 bp sequence for (-)-limonene synthase and-  629 to -   530 bp for ( +)-menthofuran synthase promoters containing specific Myb-binding motifs in addition to other unique motifs described for developmental regulation without any defined pattern. All other pathway promoters also recruits specific but different Myb factors as indicated by this analysis.

Genetic and spatial variation in vegetative and floral traits across a hybrid zone

PREMISE

Genetic variation influences the potential for evolution to rescue populations from impacts of environmental change. Most studies of genetic variation in fitness-related traits focus on either vegetative or floral traits, with few on floral scent. How vegetative and floral traits compare in potential for adaptive evolution is poorly understood.

METHODS

We measured variation across source populations, planting sites, and genetic families for vegetative and floral traits in a hybrid zone. Seeds from families of Ipomopsis aggregata, I. tenuituba, and F₁ and F₂ hybrids of the two species were planted into three common gardens. Measured traits included specific leaf area (SLA), trichomes, water-use efficiency (WUE), floral morphology, petal color, nectar, and floral volatiles.

RESULTS

Vegetative traits SLA and WUE varied greatly among planting sites, while showing weak or no genetic variation among source populations. Specific leaf area and trichomes responded plastically to snowmelt date, and SLA exhibited within-population genetic variation. All aspects of floral morphology varied genetically among source populations, and corolla length, corolla width, and sepal width varied genetically within populations. Heritability was not detected for volatiles due to high environmental variation, although one terpene had high evolvability, and high emission of two terpenes, a class of compounds emitted more strongly from the calyx than the corolla, correlated genetically with sepal width. Environmental variation across sites was weak for floral morphology and stronger for volatiles and vegetative traits. The inheritance of three of four volatiles departed from additive.

CONCLUSIONS

Results indicate stronger genetic potential for evolutionary responses to selection in floral morphology compared with scent and vegetative traits and suggest potentially adaptive plasticity in some vegetative traits.

[Cloning of StHD1 and StHD8 from Schizonepeta tenuifolia and function of regulating glandular trichome development]

Hd-Zip, a unique transcription factor in plant kingdom, influences the growth, development, and secondary metabolism of plants. Hd-zip Ⅳ is thought to play an important role in trichome development of Schizonepeta tenuifolia. This study aims to explore the functions of StHD1 and StHD8 in Hd-zip Ⅳ subfamily in peltate glandular trichome development. To be specific, the expression patterns of the two genes and interaction between the proteins encoded by them were analyzed based on transcriptome sequencing and two-hybrid screening. The subcellular localization was performed and functions of the genes were verified in tobacco and S. tenuifolia. The results showed that StHD1 and StHD8 had high similarity to HD-Zip Ⅳ proteins of other plants and they all had the characteristic conserved domains of HD-Zip Ⅳ subfamily. They were located in the nucleus. The two genes mainly expressed in young tissues and spikes, and StHD1 and StHD8 proteins interacted with each other. The density and length of glandular trichomes increased significantly in tobacco plants with the overexpression of StHD1 and StHD8. Inhibiting the expression of StHD1 and StHD8 by VIGS(virus-induced gene silencing) in S. tenuifolia resulted in the reduction in the density of peltate glandular trichomes, the expression of key genes related to mono-terpene synthesis, and the relative content of limonene and pulegone, the main components of monoterpene. These results suggested that StHD1 and StHD8 of S. tenuifolia formed a complex to regulate glandular trichomes and affect the biosynthesis of monoterpenes.

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.

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

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

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.

The MYB transcription factor MYB103 acts upstream of TRICHOME BIREFRINGENCE-LIKE27 in regulating aluminum sensitivity by modulating the O-acetylation level of cell wall xyloglucan in Arabidopsis thaliana

Modification of the O-acetylation level of xyloglucan (XyG) appears to affect aluminum (Al) sensitivity in Arabidopsis by modulating its binding capacity to Al. However, the transcriptional regulation of this process remains largely unknown. In our previous studies, we found that the expression of TRICHOME BIREFRINGENCE-LIKE27 (TBL27), which is responsible for the O-acetylation of XyG, was downregulated under Al stress. In the present study, we showed that the expression of an R2R3-type transcription factor-encoding gene, MYB103, was also inhibited by Al exposure and exhibited a co-expression pattern with TBL27 in roots and siliques, suggesting a potential link between MYB103 and TBL27. The loss of function of MYB103 resulted in increased Al sensitivity, as indicated by more inhibited root growth and elevated root Al content compared with the wild type. Moreover, we also detected increased Al accumulation in the root cell wall and the hemicellulose fraction, which was attributed to the changes in the O-acetylation level of XyG rather than the XyG content itself. In addition, further analysis revealed that MYB103 positively activated TBL27 expression by directly binding to the TBL27 promoter region, and TBL27 overexpression in the myb103 mutant rescued the Al-sensitive phenotype of the mutant to the wild-type level. Taken together, we conclude that MYB103 acts upstream of TBL27 to positively regulate Al resistance by modulating the O-acetylation of the cell wall XyG.

Characterization of trichome-specific BAHD acyltransferases involved in acylsugar biosynthesis in Nicotiana tabacum

Glandular trichomes of tobacco (Nicotiana tabacum) produce blends of acylsucroses that contribute to defence against pathogens and herbivorous insects, but the mechanism of assembly of these acylsugars has not yet been determined. In this study, we isolated and characterized two trichome-specific acylsugar acyltransferases that are localized in the endoplasmic reticulum, NtASAT1 and NtASAT2. They sequentially catalyse two additive steps of acyl donors to sucrose to produce di-acylsucrose. Knocking out of NtASAT1 or NtASAT2 resulted in deficiency of acylsucrose; however, there was no effect on acylsugar accumulation in plants overexpressing NtASAT1 or NtASAT2. Genomic analysis and profiling revealed that NtASATs originated from the T subgenome, which is derived from the acylsugar-producing diploid ancestor N. tomentosiformis. Our identification of NtASAT1 and NtASAT2 as enzymes involved in acylsugar assembly in tobacco potentially provides a new approach and target genes for improving crop resistance against pathogens and insects.

The transcription factors TLR1 and TLR2 negatively regulate trichome density and artemisinin levels in Artemisia annua

The important antimalarial drug artemisinin is biosynthesized and stored in Artemisia annua glandular trichomes and the artemisinin content correlates with trichome density; however, the factors affecting trichome development are largely unknown. Here, we demonstrate that the A. annua R2R3 MYB transcription factor TrichomeLess Regulator 1 (TLR1) negatively regulates trichome development. In A. annua, TLR1 overexpression lines had 44.7%-64.0% lower trichome density and 11.5%-49.4% lower artemisinin contents and TLR1-RNAi lines had 33%-93.3% higher trichome density and 32.2%-84.0% higher artemisinin contents compared with non-transgenic controls. TLR1 also negatively regulates the expression of anthocyanin biosynthetic pathway genes in A. annua. When heterologously expressed in Arabidopsis thaliana, TLR1 interacts with GLABROUS3a, positive regulator of trichome development, and represses trichome development. Yeast two-hybrid and pull-down assays indicated that TLR1 interacts with the WUSCHEL homeobox (WOX) protein AaWOX1, which interacts with the LEAFY-like transcription factor TLR2. TLR2 overexpression in Arabidopsis and A. annua showed that TLR2 reduces trichome development by reducing gibberellin levels. Furthermore, artemisinin contents were 19%-43% lower in TLR2-overexpressing A. annua plants compared to controls. These data indicate that TLR1 and TLR2 negatively regulate trichome density by lowering gibberellin levels and may enable approaches to enhance artemisinin yields.

Transcriptomic and functional analysis provides molecular insights into multicellular trichome development

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

Cotton genes GhMML1 and GhMML2 control trichome branching when ectopically expressed in tobacco

Trichomes exhibit extraordinary diversity in shape, ultrastructure, distribution, secretion capability, biological functions, and morphological differences, which are strongly associated with their multifunction. Previous researches showed MIXTA-like transcription factors involved in regulating trichome initiation and patterning via forming MYB-bHLH-WD40 transcriptional activator complex to induce the expression of downstream genes. Here, we report the characteristics and role of GhMML1 and GhMML2, members of subgroup 9 of the R2R3-type MYB TFs. GhMML1 and GhMML2 were preferentially targeted to the nucleus and prominently expressed in the early stage during fiber development. Ectopic expression of GhMML1 and GhMML2 respectively in the transgenic tobacco plants changed the morphological characteristics of leaf trichomes; that is, the unbranched trichomes turned into multiple branched, and in the meantime, the density of trichomes was reduced on the surface of the leaf. Y2H and LCI assay revealed that both GhMML1 and GhMML2 could physically interact with a bZIP transcription factor family protein (GhbZIP) in vivo and in vitro. It has been reported that GhbZIP's homolog TAG3 in Arabidopsis is involved in the asymmetric growth of leaves and flowers via direct interaction with BOP1. Taken together, our results demonstrated that two MYB MIXTA-like proteins, GhMML1 and GhMML2, together with GhbZIP might form a multimeric complex to involve in trichome development. This study highlights the importance of MIXTA-like genes from TF subgroup 9 and will help to uncover the molecular mechanism underlying differential trichomes and their development.

Control of resource allocation between primary and specialized metabolism in glandular trichomes

Plant specialized metabolites are often synthesized and stored in dedicated morphological structures such as glandular trichomes, resin ducts, or laticifers where they accumulate in large concentrations. How this high productivity is achieved is still elusive, in particular, with respect to the interface between primary and specialized metabolism. Here, we focus on glandular trichomes to survey recent progress in understanding how plant metabolic cell factories manage to balance homeostasis of essential central metabolites while producing large quantities of compounds that constitute a metabolic sink. In particular, we review the role of gene duplications, transcription factors and photosynthesis.

Two R2R3-MYB genes cooperatively control trichome development and cuticular wax biosynthesis in Prunus persica

The fruit surface has an enormous impact on the external appearance and postharvest shelf-life of fruit. Here, we report two functionally redundant genes, PpMYB25 and PpMYB26, involved in regulation of fruit skin texture in peach. PpMYB25 can activate transcription of PpMYB26 and they both induce trichome development and cuticular wax accumulation, resulting in peach fruit with a fuzzy and dull appearance. By contrast, nonfunctional mutation of PpMYB25 caused by an insertional retrotransposon in the last exon in nectarine fails to activate transcription of PpMYB26, resulting in nectarine fruit with a smooth and shiny appearance due to loss of trichome initiation and decreased cuticular wax accumulation. Secondary cell wall biosynthesis in peach fruit pubescence is controlled by a transcriptional regulatory network, including the master regulator PpNAC43 and its downstream MYB transcription factors such as PpMYB42, PpMYB46 and PpMYB83. Our results show that PpMYB25 and PpMYB26 coordinately regulate fruit pubescence and cuticular wax accumulation and their simultaneous perturbation results in the origin of nectarine, which is botanically classified as a subspecies of peach.

ROOT HAIR DEFECTIVE 2 vesicular delivery to the apical plasma membrane domain during Arabidopsis root hair development

Arabidopsis (Arabidopsis thaliana) root hairs develop as long tubular extensions from the rootward pole of trichoblasts and exert polarized tip growth. The establishment and maintenance of root hair polarity is a complex process involving the local apical production of reactive oxygen species generated by A. thaliana nicotinamide adenine dinucleotide phosphate (NADPH) oxidase respiratory burst oxidase homolog protein C/ROOT HAIR-DEFECTIVE 2 (AtRBOHC/RHD2). Loss-of-function root hair defective 2 (rhd2) mutants have short root hairs that are unable to elongate by tip growth, and this phenotype is fully complemented by GREEN FLUORESCENT PROTEIN (GFP)-RHD2 expressed under the RHD2 promoter. However, the spatiotemporal mechanism of AtRBOHC/RHD2 subcellular redistribution and delivery to the plasma membrane (PM) during root hair initiation and tip growth are still unclear. Here, we used advanced microscopy for detailed qualitative and quantitative analysis of vesicular compartments containing GFP-RHD2 and characterization of their movements in developing bulges and growing root hairs. These compartments, identified by an independent molecular marker mCherry-VTI12 as the trans-Golgi network (TGN), deliver GFP-RHD2 to the apical PM domain, the extent of which corresponds with the stage of root hair formation. Movements of TGN/early endosomes, but not late endosomes, were affected in the bulging domains of the rhd2-1 mutant. Finally, we revealed that structural sterols might be involved in the accumulation, docking, and incorporation of TGN compartments containing GFP-RHD2 to the apical PM of root hairs. These results help in clarifying the mechanism of polarized AtRBOHC/RHD2 targeting, maintenance, and recycling at the apical PM domain, coordinated with different developmental stages of root hair initiation and growth.

Flavonoid deficiency disrupts redox homeostasis and terpenoid biosynthesis in glandular trichomes of tomato

Glandular trichomes (GTs) are epidermal structures that provide the first line of chemical defense against arthropod herbivores and other biotic threats. The most conspicuous structure on leaves of cultivated tomato (Solanum lycopersicum) is the type-VI GT (tVI-GT), which accumulates both flavonoids and volatile terpenoids. Although these classes of specialized metabolites are derived from distinct metabolic pathways, previous studies with a chalcone isomerase 1 (CHI1)-deficient mutant called anthocyanin free (af) showed that flavonoids are required for terpenoid accumulation in tVI-GTs. Here, we combined global transcriptomic and proteomic analyses of isolated trichomes as a starting point to show that the lack of CHI1 is associated with reduced levels of terpenoid biosynthetic transcripts and enzymes. The flavonoid deficiency in af trichomes also resulted in the upregulation of abiotic stress-responsive genes associated with DNA damage and repair. Several lines of biochemical and genetic evidence indicate that the terpenoid defect in af mutants is specific for the tVI-GT and is associated with the absence of bulk flavonoids rather than loss of CHI1 per se. A newly developed genome-scale model of metabolism in tomato tVI-GTs helped identify metabolic imbalances caused by the loss of flavonoid production. We provide evidence that flavonoid deficiency in this cell type leads to increased production of reactive oxygen species (ROS), which may impair terpenoid biosynthesis. Collectively, our findings support a role for flavonoids as ROS-scavenging antioxidants in GTs.

PaNAC089 is a membrane-tethered transcription factor (MTTF) that modulates flowering, chlorophyll breakdown and trichome initiation

Flowering and senescence are essential developmental stages of green plants, which are governed by complex molecular regulatory networks. However, the connection between flowering regulation and senescence regulation in London plane tree (Platanus acerifolia ) remains unknown. In this study, we identified a gene PaNAC089 from London plane tree, which encodes a membrane-tethered transcription factor (MTTF) belonging to the NAC (NAM, ATAF1/2, CUC2) transcription factor family. We investigated the functions of PaNAC089 in the regulation of flowering and senescence through the analysis of expression profiles and transgenic phenotypes. Heterologous overexpression of ΔPaNAC089 delayed flowering and inhibited chlorophyll breakdown to produce dark green rosette leaves in Arabidopsis . In addition, the trichome density of rosette leaves was decreased in transgenic lines. In ΔPaNAC089 overexpression plants, a series of functional genes with inhibited expression were identified by quantitative real-time polymerase chain reaction (qRT-PCR), including genes that regulate flowering, chlorophyll decomposition, and trichome initiation. Furthermore, Δ PaNAC089 directly binds to the promoter of CONSTANS (CO ) and NON-YELLOWING2 (NYE2 ) in the yeast one-hybrid assay. Consistent with this, luciferase (LUC) transient expression assays also showed that Δ PaNAC089 could inhibit the activity of NYE2 . To summarise, our data suggests that PaNAC089 is an MTTF that modulates flowering, chlorophyll breakdown and trichome initiation.

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.

Ectopic Expression of JcCPL1, 2, and 4 Affects Epidermal Cell Differentiation, Anthocyanin Biosynthesis and Leaf Senescence in Arabidopsis thaliana

The CAPRICE (CPC)-like (CPL) genes belong to a single-repeat R3 MYB family, whose roles in physic nut (Jatropha curcas L.), an important energy plant, remain unclear. In this study, we identified a total of six CPL genes (JcCPL1–6) in physic nut. The JcCPL3, 4, and 6 proteins were localized mainly in the nucleus, while proteins JcCPL1, 2, and 5 were localized in both the nucleus and the cytoplasm. Ectopic overexpression of JcCPL1, 2, and 4 in Arabidopsis thaliana resulted in an increase in root hair number and decrease in trichome number. Consistent with the phenotype of reduced anthocyanin in shoots, the expression levels of anthocyanin biosynthesis genes were down-regulated in the shoots of these three transgenic A. thaliana lines. Moreover, we observed that OeJcCPL1, 2, 4 plants attained earlier leaf senescence, especially at the late developmental stage. Consistent with this, the expression levels of several senescence-associated and photosynthesis-related genes were, respectively, up-regulated and down-regulated in leaves. Taken together, our results indicate functional divergence of the six CPL proteins in physic nut. These findings also provide insight into the underlying roles of CPL transcription factors in leaf senescence.

Trichome Transcripts as Efficiency Control for Synthetic Biology and Molecular Farming

A variety of methods for studying glandular leaf hairs (trichomes) as multicellular micro-organs are well established for synthetic biology platforms like tobacco or tomato but rather rare for nonglandular and usually single-celled trichomes of the model plant Arabidopsis thaliana. A thorough isolation of-ideally intact-trichomes is decisive for further biochemical and genomic analyses of primary and secondary metabolic compounds, enzymes, and especially transcripts to monitor initial success of an engineering approach. While isolation of tomato or tobacco trichomes is rather easy, by simply freezing whole plants in liquid nitrogen and brushing off trichomes, this approach does not work for Arabidopsis. This is mainly due to damage of trichome cells during the collection procedure and very low yield. Here, we provide a robust method for a virtually epithelial cell-free isolation of Arabidopsis trichomes. This method is then joined with an RNA isolation protocol to perform mRNA analysis on extracts of the isolated trichomes using a semi-quantitative RT-PCR setup.

Root hairs vs. trichomes: Not everyone is straight!

Trichomes show 47 morphological phenotypes, while literature reports only two root hair phenotypes in all plants. However, could hair-like structures exist below-ground in a similar wide range of morphologies like trichomes? Genetic mutants and root hair stress phenotypes point to the possibility of uncharacterized morphological variation existing belowground. For example, such root hairs in Arabidopsis (Arabidopsis thaliana) can be wavy, curled, or branched. We found hints in the literature about hair-like structures that emerge before root hairs belowground. As such, these early emerging hair structures can be potential exceptions to the contrasting morphological variation between trichomes and root hairs. Here, we show a previously unreported 'hooked' hair structure growing below-ground in common bean. The unique 'hooking' shape distinguishes the 'hooked hair' morphologically from root hairs. Currently, we cannot fully characterize the phenotype of our observation due to the lack of automated methods for phenotyping root hairs. This phenotyping bottleneck also handicaps the discovery of more morphology types that might exist below-ground as manual screening across species is slower than computer-assisted high-throughput screening.

SlHair2 Regulates the Initiation and Elongation of Type I Trichomes on Tomato Leaves and Stems

Trichomes are hair-like structures that are essential for abiotic and biotic stress responses. Tomato Hair (H), encoding a C2H2 zinc finger protein, was found to regulate the multicellular trichomes on stems. Here, we characterized Solyc10g078990 (hereafter Hair2, H2), its closest homolog, to examine whether it was involved in trichome development. The H2 gene was highly expressed in the leaves, and its protein contained a single C2H2 domain and was localized to the nucleus. The number and length of type I trichomes on the leaves and stems of knock-out h2 plants were reduced when compared to the wild-type, while overexpression increased their number and length. An auto-activation test with various truncated forms of H2 using yeast two-hybrid (Y2H) suggested that H2 acts as a transcriptional regulator or co-activator and that its N-terminal region is important for auto-activation. Y2H and pull-down analyses showed that H2 interacts with Woolly (Wo), which regulates the development of type I trichomes in tomato. Luciferase complementation imaging assays confirmed that they had direct interactions, implying that H2 and Wo function together to regulate the development of trichomes. These results suggest that H2 has a role in the initiation and elongation of type I trichomes in tomato.

A pair of DUF538 domain-containing proteins modulates plant growth and trichome development through the transcriptional regulation of GLABRA1 in Arabidopsis thaliana

SMALLER TRICHOMES WITH VARIABLE BRANCHES (SVB) is an emerging plant growth regulator in trichome development, endoplasmic reticulum stress response, and phosphoinositide signaling, and belongs to the land plant-specific DUF538 domain-containing protein family. Despite its multifaceted roles, the functions of this protein family are poorly understood in plant growth and development. Here, we report that SVB-like (SVBL), the closest homolog of SVB, modulates plant growth and trichome development with SVB in Arabidopsis thaliana. Although none of the single mutants showed an obvious growth defect, the double mutants of svb svbl exhibited dwarfed plant growth. In trichome development, the defects in svb mutant were greatly enhanced by the additional mutation in SVBL, despite the single knockout of SVBL showing the mild defects. The double mutation reduced the transcript level of one of the central hub genes for trichome development, GLABRA1 (GL1), which in turn affects the other downstream genes, GLABRA2 (GL2), TRANSPARENT TESTA GLABRA2 (TTG2), TRIPTYCHON (TRY), CAPRICE (CPC), and ENHANCER OF TRY AND CPC1 (ETC1). In situ translational reporter assays showed that SVB and SVBL share highly similar localization patterns both at tissue and subcellular levels. The present study suggests that SVB and SVBL play a pivotal role in plant growth and trichome development by affecting a specific subset of known trichome developmental regulators, highlighting the importance of the DUF538 protein family in higher plants.

Arabidopsis thaliana TCP15 interacts with the MIXTA-like transcription factor MYB106/NOECK

MYB106 and MYB16 are MIXTA-like transcription factors that control trichome maturation and cuticle formation in Arabidopsis. In a recent study, we found that the TEOSINTE BRANCHED 1, CYCLOIDEA and PROLIFERATING CELL FACTORS (TCP) transcription factor TCP15 also acts as an important regulator of aerial epidermis specialization in Arabidopsis through the control of trichome development and cuticle formation. TCP15 and MYB106 regulate the expression of common groups of genes, including genes coding for transcription factors and enzymes of the cuticle biosynthesis pathway. In this study, we report that TCP15 physically interacts with MYB106 when both proteins are expressed in yeast cells or Nicotiana bentamiana leaves. Furthermore, we also observed interaction in leaves of Arabidopsis thaliana. Altogether, our findings raise the possibility that TCP15 and MYB106 bind together to the promoters of target genes to exert their action. Our data provide a base to investigate the role of TCP-MIXTA complexes in the context of cuticle development in Arabidopsis thaliana.

Integrating RNA-seq with functional expression to analyze the regulation and characterization of genes involved in monoterpenoid biosynthesis in Nepeta tenuifolia Briq

Nepeta tenuifolia Briq. (Lamiaceae) is a medicinal plant historically used in the East Asia region to treat cold and fever, and it is currently used as a clinically effective treatment for respiratory diseases. We previously found that monoterpenoids are the dominant volatile secondary metabolites in N. tenuifolia and their biosynthesis occurs in peltate glandular trichomes. To gain an insight into the molecular mechanisms underlying monoterpenoid biosynthesis in N. tenuifolia, we conducted transcriptome sequencing and examined the expression differences in monoterpene molecular pathway-related genes in different tissues and growth stages by qRT-RCR. In total, six p-menthane monoterpene biosynthetic genes in the (+)-menthone pathway were identified and cloned successfully based on transcriptome data. Moreover, the major constituents, including (+)-limonene, (-)-pulegone and (+)-menthone showed greater accumulation in the spikes than in other organs, such as the expression levels of related key enzyme genes. Additionally, the relative expression of pulegone reductase was the highest at 84 days, showing an inverse trend from (-)-pulegone relative content and leading to (+)-menthone accumulation in peltate glandular trichomes. Finished cloning of the gene for limonene 3-hydroxylase in N. tenuifolia (NtL3OH), heterologous expression in yeast, and in vitro assays were performed for functional characterization. Our study provides an important resource for further research of secondary metabolism of monoterpenes in peltate glandular trichomes of N. tenuifolia and other homologous species.

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

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

The Carboxyl-Terminus of TRANSPARENT TESTA GLABRA1 Is Critical for Its Functions in Arabidopsis

The Arabidopsis WD40 repeat protein TRANSPARENT TESTA GLABRA1 (TTG1) regulates cell fate determination, including trichome initiation and root hair formation, as well as secondary metabolism such as flavonoid biosynthesis and seed coat mucilage production. TTG1 regulates different processes via regulating the expression of its downstream target genes by forming MYB-bHLH-WD40 (MBW) activator complexes with different R2R3 MYB and bHLH transcription factors. Here, we report the identification of the carboxyl (C)-terminus as a critical domain for TTG1′s functions in Arabidopsis. We found that the ttg1Δ15aa mutant shows pleiotropic phenotypes identical to a TTG1 loss-of-function mutant. Gene sequencing indicates that a single nucleotide substitution in TTG1 led to a premature stop at the W327 residue, leading to the production of a truncated TTG1 protein with a deletion of the last 15 C-terminal amino acids. The expression of TTG1 under the control of its native promoter fully restored the ttg1Δ15aa mutant phenotypes. Consistent with these observations, the expression levels of TTG1 downstream genes such as GLABRA2 (GL2) and CAPRICE (CPC) were reduced in the ttg1Δ15aa mutant. Assays in Arabidopsis protoplast show that TTG1Δ15aa failed to interact with the bHLH transcription factor GL3, and the deletion of the last 3 C-terminal amino acids or the 339L amino acid alone fully abolished the interaction of TTG1 with GL3. Furthermore, the expression of TTG1Δ3aa under the control of TTG1 native promoter failed to restore the ttg1Δ15aa mutant phenotypes. Taken together, our results suggest that the C-terminal domain of TTG1 is required for its proper function in Arabidopsis.

Tomato transcriptomic response to Tuta absoluta infestation

BACKGROUND

The South America pinworm, Tuta absoluta, is a destructive pest of tomato that causes important losses worldwide. Breeding of resistant/tolerant tomato cultivars could be an effective strategy for T. absoluta management but, despite the economic importance of tomato, very limited information is available about its response to this treat. To elucidate the defense mechanisms to herbivore feeding a comparative analysis was performed between a tolerant and susceptible cultivated tomato at both morphological and transcriptome level to highlight constitutive leaf barriers, molecular and biochemical mechanisms to counter the effect of T. absoluta attack.

RESULTS

The tolerant genotype showed an enhanced constitutive barrier possibly as result of the higher density of trichomes and increased inducible reactions upon mild infestation thanks to the activation/repression of key transcription factors regulating genes involved in cuticle formation and cell wall strength as well as of antinutritive enzymes, and genes involved in the production of chemical toxins and bioactive secondary metabolites.

CONCLUSIONS

Overall, our findings suggest that tomato resilience to the South America pinworm is achieved by a combined strategy between constitutive and induced defense system. A well-orchestrated modulation of plant transcription regulation could ensure a trade-off between defense needs and fitness costs. Our finding can be further exploited for developing T. absoluta tolerant cultivars, acting as important component of integrated pest management strategy for more sustainable production.

VvMYB114 mediated by miR828 negatively regulates trichome development of Arabidopsis

Trichome is a specialized structure differentiated during the morphogenesis of plant leaf epidermal cells. In recent years, with the continuous researches on trichome development of Arabidopsis and other plants, more and more genes related to trichome morphogenesis have been discovered, including R2R3-type MYB genes. In this study, we cloned a R2R3-type MYB family gene from grape, VvMYB114, a target gene of vvi-miR828. qRT-PCR showed that VvMYB114 mRNA accumulated during grape fruit ripening, and VvMYB114 protein had transcriptional activation activity. Heterologous overexpression of VvMYB114 in Arabidopsis reduced the number of trichome on leaves and stems. Mutating the miR828-binding site in VvMYB114 without altering amino-acid sequence had no effect on trichome development in Arabidopsis. The results showed a different role of the regulation of miR828 to VvMYB114 in Arabidopsis from in grape, which indicated the functional divergence of miRNA targeting homoeologous genes in different species played an important roles in evolution and useful trait selection.