The β-1,3-galactosetransferase gene DoGALT2 is essential for stigmatic mucilage production in Dendrobium officinale

Metabolic accumulation and related synthetic genes of O-acetyl groups in mannan polysaccharides of Dendrobium officinale

Mannan polysaccharides (MPs), which contain substituted O-acetyl groups in their backbone, are abundant in the medicinal plant Dendrobium officinale. Acetyl groups can influence the physiological and biochemical properties of polysaccharides, which mainly accumulate in the stems of D. officinale at four developmental stages (S1-S4), showing an increasing trend and a link with water-soluble polysaccharides (WSPs) and mannose. The genes coding for enzymes that catalyze O-acetyl groups to MPs are unknown in D. officinale. The TRICHOME BIREFRINGENCE-LIKE (TBL) gene family contains TBL and DUF231 domains that can transfer O-acetyl groups to various polysaccharides. Based on an established D. officinale genome database, 37 DoTBL genes were identified. Analysis of cis-elements in the promoter region showed that DoTBL genes might respond to different hormones and abiotic stresses. Most of the genes with MeJA-responsive elements were upregulated or downregulated after treatment with MeJA. qRT-PCR results demonstrated that DoTBL genes had significantly higher expression levels in stems and leaves than in roots. Eight DoTBL genes showed relatively higher expression at S2-S4 stages, which showed a link with the content of WSPs and O-acetyl groups. DoTBL35 and its homologous gene DoTBL34 displayed the higher mRNA level in different organs and developmental stages, which might participate in the acetylation of MPs in D. officinale. The subcellular localization of DoTBL34 and DoTBL35 reveals that the endoplasmic reticulum may play an important role in the acetylation of MPs.

Advances in Breeding, Bioprospecting, and In Vitro Culture of Laelia Orchid Species

Orchids (Orchidaceae) are plants that are highly appreciated by their beautiful flowers worldwide. Moreover, they represent a source of metabolites with applications in medicine and biotechnology. Within the Orchidaceae family, the Laelia genus is a group of orchid species from the Neotropics and is probably one of the most representative genera of America. Laelia orchids are cultivated by their splendid flowers and are widely used in orchid breeding. Here, we revise the use of the Laelia genus in orchid breeding and metabolite bioprospecting. We also analyze the use of plant tissue culture (PTC) as an alternative to conventional propagation and as a strategy for the recovery of those Laelia species threatened with extinction. We summarize and discuss the recent advances in the application of different PTC techniques for mass multiplication based on asymbiotic germination, organogenesis, protocorm-like bodies development, and somatic embryogenesis, and the advances of in vitro conservation by cryoconservation and the use of slow-growth promoting hormones. Finally, we suggest future directions and venues in research for Laelia species.

The methyl jasmonate-responsive transcription factor DobHLH4 promotes DoTPS10, which is involved in linalool biosynthesis in Dendrobium officinale during floral development

Linalool is an aromatic monoterpene produced in the Chinese medicinal plant Dendrobium officinale, but little information is available on the regulation of linalool biosynthesis. Here, a novel basic helix-loop-helix (bHLH) transcription factor, DobHLH4 from D. officinale, was identified and functionally characterized. The expression profile of DobHLH4 was positively correlated with that of DoTPS10 (R² = 0.985, p < 0.01), which encodes linalool synthase that is responsible for linalool production, during floral development. DobHLH4 was highly expressed in petals, and was significantly induced by methyl jasmonate. Analysis of subcellular localization showed that DobHLH4 was located in the nucleus. Yeast one-hybrid and dual-luciferase assays indicated that DobHLH4 bound directly to the DoTPS10 promoter harboring the G-box element, and up-regulated DoTPS10 expression. A yeast two-hybrid screen confirmed that DobHLH4 physically interacted with DoJAZ1, suggesting that DobHLH4 might function in the jasmonic acid-mediated accumulation of linalool. Furthermore, transient overexpression of DobHLH4 in D. officinale petals significantly increased linalool production by triggering linalool biosynthetic pathway genes, especially DoTPS10. We suggest a hypothetical model that depicts how jasmonic acid signaling may regulate DoTPS10 by interacting with DobHLH4 and DoJAZ1. In doing so, the formation of linalool is controlled. Our results indicate that DobHLH4 is a positive regulator of linalool biosynthesis and may be a promising target for in vitro-based metabolic engineering to produce linalool.

Ectopic expression of DoFLS1 from Dendrobium officinale enhances flavonol accumulation and abiotic stress tolerance in Arabidopsis thaliana

Flavonols are important active ingredients that are found in abundance in Dendrobium officinale. Research on flavonol biosynthesis currently focuses on the more ubiquitous kaempferol and quercetin, but little is known on the biosynthesis of myricetin. Notably, flavonol synthase (FLS), which is responsible for the biosynthesis of flavonols, has not yet been identified. In this study, we isolated a flavonol synthase, DoFLS1, from Dendrobium officinale. DoFLS1 harbors conserved 2-oxoglutarate-dependent dioxygenase-specific and FLS-specific motifs. DoFLS1 is a cytoplasmic protein. DoFLS1 was universally expressed in roots, stems, and leaves of juvenile and adult D. officinale plants. DoFLS1 expression was strongly correlated in juvenile and adult D. officinale plants (R² = 0.86 and 0.98, respectively; p < 0.01) with the average of corresponding flavonol levels. Transgenic Arabidopsis thaliana expressing DoFLS1 exhibited a 1.24-fold increase in flavonol content and a 25.78% decrease in anthocyanin content compare to wild-type plants, possibly resulting from a 78.61% increase in myricetin level. Moreover, the loss of anthocyanin was attributed to decreased expression of dihydroflavonol reductase (DFR) and anthocyanidin synthase (ANS) genes in transgenic A. thaliana that expressed DoFLS1. DoFLS1 also complemented the deficiency in flavonol of the A. thaliana fls1-3 mutant, which had reduced anthocyanin but increased flavonol content relative to the fls1-3 mutant. In addition, DoFLS1 was significantly upregulated after treatment with cold, drought or salicylic acid. These findings provide genetic evidence for the involvement of DoFLS1 in the biosynthesis of flavonol and in response to abiotic stresses.

Functional Characterization of a Dendrobium officinale Geraniol Synthase DoGES1 Involved in Floral Scent Formation

Floral scent is a key ornamental trait that determines the quality and commercial value of orchids. Geraniol, an important volatile monoterpene in orchids that attracts pollinators, is also involved in responses to stresses but the geraniol synthase (GES) responsible for its synthesis in the medicinal orchid Dendrobium officinale has not yet been identified. In this study, three potential geraniol synthases were mined from the D. officinale genome. DoGES1, which was localized in chloroplasts, was characterized as a geraniol synthase. DoGES1 was highly expressed in flowers, especially in petals. DoGES1 transcript levels were high in the budding stage of D. officinale flowers at 11:00 a.m. DoGES1 catalyzed geraniol in vitro, and transient expression of DoGES1 in Nicotiana benthamiana leaves resulted in the accumulation of geraniol in vivo. These findings on DoGES1 advance our understanding of geraniol biosynthesis in orchids, and lay the basis for genetic modification of floral scent in D. officinale or in other ornamental orchids.

Genome-Wide Identification and Expression Profile of TPS Gene Family in Dendrobium officinale and the Role of DoTPS10 in Linalool Biosynthesis

Terpene synthase (TPS) is a critical enzyme responsible for the biosynthesis of terpenes, which possess diverse roles in plant growth and development. Although many terpenes have been reported in orchids, limited information is available regarding the genome-wide identification and characterization of the TPS family in the orchid, Dendrobium officinale. By integrating the D. officinale genome and transcriptional data, 34 TPS genes were found in D. officinale. These were divided into four subfamilies (TPS-a, TPS-b, TPS-c, and TPS-e/f). Distinct tempospatial expression profiles of DoTPS genes were observed in 10 organs of D. officinale. Most DoTPS genes were predominantly expressed in flowers, followed by roots and stems. Expression of the majority of DoTPS genes was enhanced following exposure to cold and osmotic stresses. Recombinant DoTPS10 protein, located in chloroplasts, uniquely converted geranyl diphosphate to linalool in vitro. The DoTPS10 gene, which resulted in linalool formation, was highly expressed during all flower developmental stages. Methyl jasmonate significantly up-regulated DoTPS10 expression and linalool accumulation. These results simultaneously provide valuable insight into understanding the roles of the TPS family and lay a basis for further studies on the regulation of terpenoid biosynthesis by DoTPS in D. officinale.