Rhizosphere characteristics combined with physiology and transcriptomics reveal key metabolic pathway responses in Dendrobium officinale upon exposure to calcium-rich karst environments

Dendrobium officinale is a calcicolous herb that adapts to calcium-rich karst environments. However, the mechanism through which D. officinale copes with high calcium stress in karst environments is unclear. In this study, limestone was used as the primary bedrock for cultivating D. officinale in karst areas. The relative calcium content in calcite, which makes up limestone, was 97.46 %, and the total calcium content in black limestone soil was 81.66 mg g⁻¹. Total calcium accumulation in black limestone soil showed a positive correlation with organic matter content, pH, and the dominant microbial groups Firmicutes and Fungi_phy_Incertae_sedis in black limestone soil. Long-term calcium-rich environments induced calcium accumulation and mannose synthesis in the stems of D. officinale grown in karst areas. High calcium stress upregulated the genes implicated in calcium signalling, abiotic stress signalling, mannan degradation, ascorbate biosynthesis, and oxalate transport, including calmodulin-like protein, ascorbate peroxidase 4, and mannan endo-1,4-beta-mannosidase 2 genes, in D. officinale stems. Additionally, high-concentration water-soluble calcium ion stress increased the mannose, ascorbic acid, and calcium oxalate content in the stems of D. officinale. These findings highlight the influence of microbial communities and the physicochemical properties of black limestone soil on high calcium content, as well as the value of calcium oxalate accumulation and the d-mannose pathway of ascorbate biosynthesis in revealing strategies for D. officinale to alleviate calcium-rich soil in karst environments.