The mechanistic study of adding polyunsaturated fatty acid to promote triterpenoids production in submerged fermentation of Sanghuangporus baumii

•Linolenic acid promoted triterpenoids in the submerged fermentation process.•Linolenic acid affected cell membrane composition, permeability and morphology.•Linolenic acid affected gene expression levels related to triterpenoids biosynthesis.•Linolenic acid affected enzyme activities related to triterpenoids biosynthesis.•Linolenic acid led to mycelial lipid peroxidation.