Objective Effects of elevated temperatures on the carbon metabolisms and major medicinal components of Anoectochilus roxburghii were studied.

Method Gene expressions and carbohydrate metabolites of the herbal plant A. roxburghii under 25℃ as control and 30℃ and 35℃ for treatments were analyzed by RNA sequencing and liquid chromatography-tandem mass spectrometry (LC-MS/MS).

Result A transcriptome analysis annotated 4 232 differentially expressed genes (DEGs) in A. roxburghii under the heat treatment. The genes were enriched mainly in the pathways related to metabolism, genetic information processing, environmental information processing, cellular processing, and organismal systems. The most numerous ones were mostly downregulated and associated with metabolism (i.e., ko01100), secondary metabolite biosynthesis (i.e., ko01110), and starch and sucrose metabolism (i.e., ko00500) pathways. The elevated temperatures inhibited the expression of photosystem II light-harvesting complex (LHC) proteins impeding the absorption and transfer of light energy. As shown by the targeted metabolomic analysis, the high temperatures significantly increased the contents of monosaccharides and disaccharides, including fructose, trehalose, maltose, melezitose, glucose, lactose, cellobiose, and sucrose, in the plant. And the analysis of omics revealed that multiple genes, such as MYB102, CYP71BE54.8, LHW1, and TPX2.2, were closely related to the metabolisms of sucrose, fructose, glucose, etc. under heat stress. Heat also significantly reduced the content of the functional biochemical, kinsenoside, in A. roxburghii.

Conclusion The heat-induced soluble sugar accumulation might boost the stress resistance through osmotic adjustment and energy provision in A. roxburghii. The generally downregulated LHC proteins of photosystem II, on the other hand, might involve in the molecular mechanism that caused the decline of photosynthetic efficiency under heat stress. Meanwhile, due to secondary metabolic pathway regulation, an elevated temperature could retard kinsenoside synthesis resulting in quality waning of the medicinal herb.