Glucomannan synthase genes DoCSLA6,7 promote polysaccharides accumulation in Dendrobium officinale under low phosphorus stress

Dendrobium officinale Kimura et Migo (D. officinale) is a precious Chinese herbal medicine. Polysaccharides are one of the most important active components of D. officinale, and they exert a profound impact on the quality of D. officinale. Our previous study found that polysaccharides synthesis can be enhanced under low phosphorus condition, and this enhancement is positively correlated with the expression of glucomannan synthase genes DoCSLAs. However, the mechanism by which DoCSLAs regulate polysaccharides synthesis, as well as the regulatory pathway of DoCSLAs in response to low phosphorus condition, remain unclear. Here, we report that two glucomannan synthase genes, DoCSLA6 and DoCSLA7, promote polysaccharides accumulation in D. officinale. The proteins of DoCSLA6 and DoCSLA7 are subcellularly localized in the cell membrane. Transient overexpression of DoCSLA6 and DoCSLA7 in D. officinale protocorm-like bodies (PLBs) not only increased the content of polysaccharides but also promoting the ratio of mannose to glucose. There were multiple W-box binding sites in the promoter of DoCSLA6/DoCSLA7, and low phosphorus related WRKY family transcription factors were screened. Yeast one-hybrid and Dual-luciferase experiments showed that DoWRKY71 and DoWRKY75 regulate DoCSLA6, while DoWRKY70 and DoWRKY75 regulate DoCSLA7. Collectively, our findings reveal that DoWRKY70,75 respond to low-phosphorus signals, and then regulate DoCSLA6 and DoCSLA7 to promote the accumulation of polysaccharides, thereby enhancing the quality of D. officinale.