Characterisation of the Mentha canadensis R2R3-MYB transcription factor gene McMIXTA and its involvement in peltate glandular trichome development

A gradient of the HD-Zip regulator Woolly regulates multicellular trichome morphogenesis in tomato

Homeodomain (HD) proteins regulate embryogenesis in animals such as the fruit fly (Drosophila melanogaster), often in a concentration-dependent manner. HD-leucine zipper (Zip) IV family genes are unique to plants and often function in the L1 epidermal cell layer. However, our understanding of the roles of HD-Zip IV family genes in plant morphogenesis is limited. In this study, we investigated the morphogenesis of tomato (Solanum lycopersicum) multicellular trichomes, a type of micro-organ in plants. We found that a gradient of the HD-Zip IV regulator Woolly (Wo) coordinates spatially polarized cell division and cell expansion in multicellular trichomes. Moreover, we identified a TEOSINTE BRANCHED1, CYCLOIDEA, and PROLIFERATING CELL NUCLEAR ANTIGEN BINDING FACTOR (TCP) transcription factor-encoding gene, SlBRANCHED2a (SlBRC2a), as a key downstream target of Wo that regulates the transition from cell division to cell expansion. High levels of Wo promote cell division in apical trichome cells, whereas in basal trichome cells, Wo mediates a negative feedback loop with SlBRC2a that forces basal cells to enter endoreduplication. The restricted high and low activities of Wo pattern the morphogenesis of tomato multicellular trichomes. These findings provide insights into the functions of HD-Zip IV genes during plant morphogenesis.

Effects of MhMYB1 and MhMYB2 transcription factors on the monoterpenoid biosynthesis pathway in l-menthol chemotype of Mentha haplocalyx Briq

MAIN CONCLUSION

Transcription factors MhMYB1 and MhMYB2 correlate with monoterpenoid biosynthesis pathway in l-menthol chemotype of Mentha haplocalyx Briq, which could affect the contents of ( -)-menthol and ( -)-menthone. Mentha haplocalyx Briq., a plant with traditional medicinal and edible uses, is renowned for its rich essential oil content. The distinct functional activities and aromatic flavors of mint essential oils arise from various chemotypes. While the biosynthetic pathways of the main monoterpenes in mint are well understood, the regulatory mechanisms governing different chemotypes remain inadequately explored. In this investigation, we identified and cloned two transcription factor genes from the M. haplocalyx MYB family, namely MhMYB1 (PP236792) and MhMYB2 (PP236793), previously identified by our research group. Bioinformatics analysis revealed that MhMYB1 possesses two conserved MYB domains, while MhMYB2 contains a conserved SANT domain. Yeast one-hybrid (Y1H) analysis results demonstrated that both MhMYB1 and MhMYB2 interacted with the promoter regions of MhMD and MhPR, critical enzymes in the monoterpenoid biosynthesis pathway of M. haplocalyx. Subsequent virus-induced gene silencing (VIGS) of MhMYB1 and MhMYB2 led to a significant reduction (P < 0.01) in the relative expression levels of MhMD and MhPR genes in the VIGS groups of M. haplocalyx. In addition, there was a noteworthy decrease (P < 0.05) in the contents of ( -)-menthol and ( -)-menthone in the essential oil of M. haplocalyx. These findings suggest that MhMYB1 and MhMYB2 transcription factors play a positive regulatory role in ( -)-menthol biosynthesis, consequently influencing the essential oil composition in the l-menthol chemotype of M. haplocalyx. This study serves as a pivotal foundation for unraveling the regulatory mechanisms governing monoterpenoid biosynthesis in different chemotypes of M. haplocalyx.

A eudicot MIXTA family ancestor likely functioned in both conical cells and trichomes

The MIXTA family of MYB transcription factors modulate the development of diverse epidermal features in land plants. This study investigates the evolutionary history and function of the MIXTA gene family in the early-diverging eudicot model lineage Thalictrum (Ranunculaceae), with R2R3 SBG9-A MYB transcription factors representative of the pre-core eudicot duplication and thus hereby referred to as "paleoMIXTA" (PMX). Cloning and phylogenetic analysis of Thalictrum paleoMIXTA (ThPMX) orthologs across 23 species reveal a genus-wide duplication coincident with a whole-genome duplication. Expression analysis by qPCR confirmed that the highest expression is found in carpels, while newly revealing high expression in leaves and nuanced differences between paralogs in representative polyploid species. The single-copy ortholog from the diploid species T. thalictroides (TthPMX, previously TtMYBML2), which has petaloid sepals with conical-papillate cells and trichomes on leaves, was functionally characterized by virus-induced gene silencing (VIGS), and its role in leaves was also assessed from heterologous overexpression in tobacco. Another ortholog from a species with conical-papillate cells on stamen filaments, TclPMX, was also targeted for silencing. Overexpression assays in tobacco provide further evidence that the paleoMIXTA lineage has the potential for leaf trichome function in a core eudicot. Transcriptome analysis by RNA-Seq on leaves of VIGS-treated plants suggests that TthPMX modulates leaf trichome development and morphogenesis through microtubule-associated mechanisms and that this may be a conserved pathway for eudicots. These experiments provide evidence for a combined role for paleoMIXTA orthologs in (leaf) trichomes and (floral) conical-papillate cells that, together with data from other systems, makes the functional reconstruction of a eudicot ancestor most likely as also having a combined function.

Morphological, phytochemical, and transcriptome analyses provide insights into the biosynthesis of monoterpenes in Monarda citriodora

MAIN CONCLUSION

Morphological, phytochemical, and transcriptome analyses revealed candidate genes involved in the biosynthesis of volatile monoterpenes and development of glandular trichomes in Monarda citriodora. Monarda citriodora Cerv. ex Lag. is a valuable aromatic plant due to the presence of monoterpenes as major constituents in its essential oil (EO). Thus, it is of sheer importance to gain knowledge about the site of the biosynthesis of these terpenoid compounds in M. citriodora, as well as the genes involved in their biosynthesis. In this study, we studied different types of trichomes and their relative densities in three different developmental stages of leaves, early stage of leaf development (L1), mid-stage of leaf development (L2), and later stage of leaf development (L3) and the histochemistry of trichomes for the presence of lipid and terpenoid compounds. Further, the phytochemical analysis of this plant through GC-MS indicated a higher content of monoterpenes (thymol, thymoquinone, γ-terpinene, p-cymene, and carvacrol) in the L1 stage with a substantial decrease in the L3 stage of leaf development. This considerable decrease in the content of monoterpenes was attributed to the decrease in the trichome density from L1 to L3. Further, we developed a de novo transcriptome assembly by carrying out RNA sequencing of different plant parts of M. citriodora. The transcriptome data revealed several putative unigenes involved in the biosynthesis of specialized terpenoid compounds, as well as regulatory genes involved in glandular trichome development. The data generated in the present study build a strong foundation for further improvement of M. citriodora, in terms of quantity and quality of its essential oil, through genetic engineering.

Formation mechanism of glandular trichomes involved in the synthesis and storage of terpenoids in lavender

BACKGROUND

Lavender (genus Lavandula, family Lamiaceae) is an aromatic plant widely grown as an ornamental plant. The chemical composition of lavender is characterized by monoterpenoids, sesquiterpenoids, and other compounds, which are primarily synthesized and stored in epidermal secretory structures called glandular trichomes (GTs). Volatile organic compounds (VOCs) are responsible for the aroma characteristics of plant oil that drive consumer preference. Aroma is usually regarded as a characteristic trait for the classification of aromatic plants. Interestingly, VOCs are synthesized and stored in GTs. Lamiaceae species such as purple perilla, peppermint, basil, thyme, and oregano usually possess two types of GTs: peltate glandular trichomes (PGTs) and capitate glandular trichomes (CGTs). But the development process of PGTs in lavender has been reported in only a few studies to date.

RESULTS

In this study, we identified and quantified the VOCs in four lavender cultivars by headspace-solid phase micro extraction-gas chromatography mass spectrometry (HS-SPME-GC-MS). A total of 66 VOCs were identified in these four cultivars, the most prominent of which were linalyl acetate and linalool, and flowers were the main site of accumulation of these VOCs. Here, we examined the developmental process of PGTs, including the formation of their base, body, and apex. The apex cells contained secretory cavities, which produced VOCs. Based on the reference genome sequence of the lavender cultivar 'Jingxun 2', several R2R3-MYB subfamily genes related to GT formation were identified. These results will guide the engineering of GTs and molecular breeding of lavender for improving the VOC content.

CONCLUSIONS

In this study, we identified the VOCs in four lavender cultivars. We analyzed the formation of GTs, and compared the number and diameter size of PGTs among four lavender cultivars. Additionally, we identified four candidate genes belonging to the R2R3-MYB family.

The non-specific lipid transfer protein McLTPII.9 of Mentha canadensis is involved in peltate glandular trichome density and volatile compound metabolism

Mentha canadensis L. is an important spice crop and medicinal herb with high economic value. The plant is covered with peltate glandular trichomes, which are responsible for the biosynthesis and secretion of volatile oils. Plant non-specific lipid transfer proteins (nsLTPs) belong to a complex multigenic family involved in various plant physiological processes. Here, we cloned and identified a non-specific lipid transfer protein gene (McLTPII.9) from M. canadensis, which may positively regulate peltate glandular trichome density and monoterpene metabolism. McLTPII.9 was expressed in most M. canadensis tissues. The GUS signal driven by the McLTPII.9 promoter in transgenic Nicotiana tabacum was observed in stems, leaves, and roots; it was also expressed in trichomes. McLTPII.9 was associated with the plasma membrane. Overexpression of McLTPII.9 in peppermint (Mentha piperita. L) significantly increased the peltate glandular trichome density and total volatile compound content compared with wild-type peppermint; it also altered the volatile oil composition. In McLTPII.9-overexpressing (OE) peppermint, the expression levels of several monoterpenoid synthase genes and glandular trichome development-related transcription factors-such as limonene synthase (LS), limonene-3-hydroxylase (L3OH), geranyl diphosphate synthase (GPPS), HD-ZIP3, and MIXTA-exhibited varying degrees of alteration. McLTPII.9 overexpression resulted in both a change in expression of genes for terpenoid biosynthetic pathways which corresponded with an altered terpenoid profile in OE plants. In addition, peltate glandular trichome density was altered in the OE plants as well as the expression of genes for transcription factors that were shown to be involved in trichome development in plants.