Suitable light intensity stimulated polysaccharide biosynthesis in Bletilla striata pseudobulbs through regulating starch and sucrose metabolism

• Low light (LP) produces highest starch and sucrose contents • Middle light (MP) exhibits highest total polysaccharide contents • Transcriptome and metabolome enrich in starch and sucrose metabolism in MP vs LP • Starch and sucrose metabolism could mediate polysaccharide biosynthesis pathway Bletilla striata is a famous medicinal plant, which medicinal function on polysaccharide. However, the effect of light intensity on polysaccharide accumulation in B. striata pseudobulbs is largely unknown. B. striata plants were exposed to three different light intensities: low light (5–20 μmol·m −2 ·s −1 ; LP), middle light (200 μmol·m −2 ·s −1 ; MP), and high light (400 μmol·m −2 ·s −1 ; HP). The results indicated that polysaccharide content of new pseudobulbs significantly increased under MP, and enrichment in starch and sucrose metabolism with metabolomics and transcriptomics. More sucrose was produced in the leaves through photosynthesis and was efficiently distributed to the pseudobulbs, where it was hydrolyzed into key metabolites for polysaccharide biosynthesis function by SUS and CSLA genes. The key metabolites in the polysaccharide biosynthetic pathway, such as sucrose-6-phosphate, fructose, fructose-6-phosphate, glucose-6-phosphate, GDP-mannose, and uridine 5’-diphospho-D-glucose, were upregulated under MP. Furthermore, the expression of the UGP2 gene involved in polysaccharide biosynthesis was significantly higher under MP than under LP and HP. Conversely, sucrose in leaves was minimally transported into pseudobulbs due to insufficient photosynthesis under LP and energy metabolism related to carbohydrate degradation and oxidation was hindered under HP. Thus, suitable light intensity effectively stimulated polysaccharide formation in B. striata pseudobulbs through starch and sucrose metabolism–mediated regulation.