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Flavonoids are important bioactive components in Dendrobium officinale, a medicinal orchid. They are involved in many biological activities, including protecting plants against biotic and abiotic stresses. Research on the key genes related to flavonoid biosynthesis in D. officinale is limited. In this study, one of the key flavonoid biosynthesis genes, flavanone 3-hydroxylase (F3H), was characterized from D. officinale. The open reading frame of DoF3H was 1 134 bp long and it encoded a 377-amino acid protein. The DoF3H protein showed considerably high homology with F3H proteins from other plant species and shared a common evolutionary ancestor with other F3Hs. DoF3H transcripts were detected in different organs of adult plants and mainly accumulated in flowers, followed by roots, stems and leaves, a pattern that was similar to the content of flavonoids. Recombinant DoF3H protein, which was localized in the cytosol, could convert naringenin to dihydrokaempferol. The mRNA levels of DoF3H were significantly induced by salt and cold stresses. Furthermore, the heterologous expression of DoF3H in Escherichia coli conferred it higher tolerance to salt and cold stresses. These results provide insight into the molecular function of DoF3H in the biosynthesis of flavonoids, and provide a new application for improvement of abiotic tolerance in D. officinale.

Dendrobium officinale Kimura et Migo is a precious traditional Chinese medicine. Despite D. officinale displaying a good salt-tolerance level, the yield and growth of D. officinale were impaired drastically by the increasing soil secondary salinization. The molecular mechanisms of D. officinale plants’ adaptation to salt stress are not well documented. Therefore, in the present study, D. officinale plants were treated with 250 mM NaCl. Transcriptome analysis showed that salt stress significantly altered various metabolic pathways, including phenylalanine metabolism, flavonoid biosynthesis, and α-linolenic acid metabolism, and significantly upregulated the mRNA expression levels of DoAOC, DoAOS, DoLOX2S, DoMFP, and DoOPR involved in the jasmonic acid (JA) biosynthesis pathway, as well as rutin synthesis genes involved in the flavonoid synthesis pathway. In addition, metabolomics analysis showed that salt stress induced the accumulation of some compounds in D. officinale leaves, especially flavonoids, sugars, and alkaloids, which may play an important role in salt-stress responses of leaf tissues from D. officinale. Moreover, salt stress could trigger JA biosynthesis, and JA may act as a signal molecule that promotes flavonoid biosynthesis in D. officinale leaves. To sum up, D. officinale plants adapted to salt stress by enhancing the biosynthesis of secondary metabolites.