Mechanism analysis of calcium nitrate application to induce gibberellin biosynthesis and signal transduction promoting stem elongation of Dendrobium officinale

Dendrobium officinale Kimura et Migo is a precious medicinal plant which has high medicinal and commercial value. The length of the stem of D. officinale is an important factor affecting the yield because of its bioactive polysaccharides found in the stem. Nitrate can promote the elongation of young stems, but the mechanisms of calcium nitrate (CN) mediated stem length growth remain largely unknown. In this study, the transcriptome, main organic components, mineral elements and microstructure of D. officinale sprayed with CN were analyzed. 659 differentially expressed genes (DEGs) were detected in the Control and CN treatment. The concentrations of the calcium and nitrate increased significantly, while the concentrations of two kinds of the aldohexoses decreased significantly. Most of the twenty key genes involved in calcium and nitrate signal transduction and carbohydrate metabolism were up-regulated. In the two pathways of gibberellin (GA) biosynthesis and signal transduction, it was found that the expression of four differential genes related to GA biosynthesis was up-regulated, and the contents of nine GAs were also significantly increased, especially the contents of bioactive GA(1) and GA(4). The expression levels of most genes related to GA signal transduction were also up-regulated after CN treatment. After CN treatment, the expression levels of downstream genes related to stem elongation and tillering were also up-regulated. In addition, the microstructure showed that CN treatment made cells elongate longitudinally, made cell walls and collenchyma thicken, made obvious lignification and the increased the number of vascular bundles, indicating that CN can promote stem elongation by enhancing ductility and mechanical strength. These results provide a theoretical and practical basis for studying how nitrate treatment can improve the stem length and yield of D. officinale.