The exogenous salicylic acid-induced changes of metabolomic profile and insect resistance in Monotropa uniflora

Monotropa uniflora , a medicinal and edible herb, exhibits antitussive activity ‌and tonifying efficacy on pulmonary deficiency‌‌. In the present study, we aimed to assess the impact of exogenous salicylic acid (SA) on the feeding behaviour of Spodoptera exigua and on the metabolomic profile of M. uniflora . The results showed that the application of exogenous SA on M. uniflora plants significantly reduced the feeding activities of S. exigua on M. uniflora (MuT), when comparing with those on the M. uniflora controls (MuCK) ( P  < 0.05). Based on metabolomic analysis using liquid chromatography-tandem mass spectrometry (LC-MS/MS), a total of 1,644 metabolites in M. uniflora were identified and annotated in the Human Metabolome Database (HMDB). Principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA) revealed significant differences between MuCK and MuT samples. Following SA induction, 321 compounds were upregulated, while 137 compounds were downregulated in M. uniflora . Secondary metabolic classification indicated that terpenes, phenolic acids and their derivatives, and flavonoids were the major categories. Among these compounds, the upregulation of caffeic acid, gibberellins A86, and gibberellins A7, as well as the downregulation of β -ionone, linalool, malvidin, and peonidin 3-rhamnoside 5-glucoside, were suggested to play roles in enhancing the resistance of M. uniflora to diseases and herbivores. The enrichment of metabolic pathways such as monoterpenoid biosynthesis, limonene and pinene degradation, and phenylalanine metabolism was determined to be critical for the improvement of M. uniflora resistance. Furthermore, the 321 upregulated metabolites were highly enriched in pathways including linoleic acid metabolism, while the 137 downregulated metabolites were significantly enriched in pathways such as nucleotide metabolism, cyanoamino acid metabolism, and arginine biosynthesis. These enrichments are also suggested to contribute to the enhanced resistance of M. uniflora .