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Abstract Background The auxin indole-3-acetic acid (IAA) is a vital phytohormone that influences plant growth and development. Our previous work showed that IAA content decreased during flower development in the medicinally important orchid Dendrobium officinale, while Aux/IAA genes were downregulated. However, little information about auxin-responsive genes and their roles in D. officinale flower development exists. Results This study validated 14 DoIAA and 26 DoARF early auxin-responsive genes in the D. officinale genome. A phylogenetic analysis classified the DoIAA genes into two subgroups. An analysis of cis-regulatory elements indicated that they were related by phytohormones and abiotic stresses. Gene expression profiles were tissue-specific. Most DoIAA genes (except for DoIAA7) were sensitive to IAA (10 μmol/L) and were downregulated during flower development. Four DoIAA proteins (DoIAA1, DoIAA6, DoIAA10 and DoIAA13) were mainly localized in the nucleus. A yeast two-hybrid assay showed that these four DoIAA proteins interacted with three DoARF proteins (DoARF2, DoARF17, DoARF23). Conclusions The structure and molecular functions of early auxin-responsive genes in D. officinale were investigated. The DoIAA-DoARF interaction may play an important role in flower development via the auxin signaling pathway.

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.

Abstract Background Gibberellins (GAs) are widely involved in plant growth and development. DELLA proteins are key regulators of plant development and a negative regulatory factor of GA. Dendrobium officinale is a valuable traditional Chinese medicine, but little is known about D. officinale DELLA proteins. Assessing the function of D. officinale DELLA proteins would provide an understanding of their roles in this orchid’s development. Results In this study, the D. officinale DELLA gene family was identified. The function of DoDELLA1 was analyzed in detail. qRT-PCR analysis showed that the expression levels of all DoDELLA genes were significantly up-regulated in multiple shoots and GA3-treated leaves. DoDELLA1 and DoDELLA3 were significantly up-regulated in response to salt stress but were significantly down-regulated under drought stress. DoDELLA1 was localized in the nucleus. A strong interaction was observed between DoDELLA1 and DoMYB39 or DoMYB308, but a weak interaction with DoWAT1. Conclusions In D. officinale, a developmental regulatory network involves a close link between DELLA and other key proteins in this orchid’s life cycle. DELLA plays a crucial role in D. officinale development.

The APETALA2 (AP2) transcription factors (TFs) play crucial roles in regulating development in plants. However, a comprehensive analysis of the AP2 family members in a valuable Chinese herbal orchid, Dendrobium officinale, or in other orchids, is limited. In this study, the 14 DoAP2 TFs that were identified from the D. officinale genome and named DoAP2-1 to DoAP2-14 were divided into three clades: euAP2, euANT, and basalANT. The promoters of all DoAP2 genes contained cis-regulatory elements related to plant development and also responsive to plant hormones and stress. qRT-PCR analysis showed the abundant expression of DoAP2-2, DoAP2-5, DoAP2-7, DoAP2-8 and DoAP2-12 genes in protocorm-like bodies (PLBs), while DoAP2-3, DoAP2-4, DoAP2-6, DoAP2-9, DoAP2-10 and DoAP2-11 expression was strong in plantlets. In addition, the expression of some DoAP2 genes was down-regulated during flower development. These results suggest that DoAP2 genes may play roles in plant regeneration and flower development in D. officinale. Four DoAP2 genes (DoAP2-1 from euAP2, DoAP2-2 from euANT, and DoAP2-6 and DoAP2-11 from basal ANT) were selected for further analyses. The transcriptional activation of DoAP2-1, DoAP2-2, DoAP2-6 and DoAP2-11 proteins, which were localized in the nucleus of Arabidopsis thaliana mesophyll protoplasts, was further analyzed by a dual-luciferase reporter gene system in Nicotiana benthamiana leaves. Our data showed that pBD-DoAP2-1, pBD-DoAP2-2, pBD-DoAP2-6 and pBD-DoAP2-11 significantly repressed the expression of the LUC reporter compared with the negative control (pBD), suggesting that these DoAP2 proteins may act as transcriptional repressors in the nucleus of plant cells. Our findings on AP2 genes in D. officinale shed light on the function of AP2 genes in this orchid and other plant species.

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.