Transcriptome Analysis Revealed that Oxalic Acid in Exogenous Licorice Root Exudates Can Stimulate the Phenylpropane Metabolic Pathway

Licorice roots can release low-molecular-weight oxalic acid (OA). However, the allelopathic effects of OA on licorice are unknown. This study presents an investigation of the effects of exogenous OA treatment on licorice roots through transcriptomic and physiological analyses. Transcriptomic analysis demonstrated that following OA treatment, differentially expressed genes (DEGs) were primarily involved in the phenylpropanoid metabolism pathway. At 6 h, the expression levels of genes associated with this pathway, such as PAL, CHS, POD, cellulase, CPY, MYB, and bHLH, were upregulated. The metabolism of related metabolites, such as flavonoids and lignin, also increased. These findings suggest that the phenylpropanoid metabolism pathway may play a significant role in the response to OA treatment. In addition, the activities of GST, CAT, and POD markedly increased, whereas the content of H₂O₂ gradually decreased, indicating that OA can activate the antioxidant protective capabilities of licorice roots. These findings suggest that OA may enhance the antioxidant defense of licorice root, as well as the effectiveness of its flavonoid constituents (to a certain degree). Our findings provide valuable theoretical insight into the allelopathic effects of licorice roots.