Physiological and transcriptomic analyses of cadmium stress response in Dendrobium officinale seedling

Dendrobium officinale is an economically important Chinese herb with ornamental and medicinal values. However, the mechanisms by which D. officinale adapts to cadmium (Cd) stress is unknown. Here, physiological changes in D. officinale roots and leaves exposed to increasing levels of Cd stress (CdSO₄ concentration of 2, 5, 9, 14 mg L^-1) were analyzed at 7, 15, 30, and 45 days after treatment. The Cd stress of 14 mg L^-1 significantly increased the levels of antioxidants and induced malondialdehyde and proline accumulation (P < 0.05). Cd subcellular distribution showed that Cd sequestration into soluble fraction is the major detoxification mechanism in D. officinale roots. Subsequently, the transcriptome profile of D. officinale roots treated with 14 mg L^-1 Cd for 15 and 30 days was analyzed. Compared to control, 2,469 differentially expressed genes (DEGs) were identified, comprising 1,486 up-regulated genes and 983 down-regulated genes. The DEGs associated with metabolic pathways for Cd uptake, transportation and detoxification were analyzed. Several processes such as metal transporter, sulfate glutathione metabolism, cell wall metabolism, phenylpropanoid metabolism were identified to be important for Cd stress adaptation. More genes were expressed at 15 days after treatment compared to 30 days. WRKY, Trihelix, NF-YC, MYB, bZIP and bHLH transcription factors were over-expressed at both time points. Furthermore, candidate genes from the glutathione metabolism pathway were identified, and qRT-PCR analysis of ten DEGs indicated a high coorelation with RNA-seq expression profiles. Our findings provide significant information for further research of Cd stress responsive genes functions in D. officinale, especially the genes from the glutathione metabolism pathway.