Characterization of p-Coumaric acid-induced soluble and cell wall-bound phenolic metabolites in relation to disease resistance to Xanthomonas campestris pv. campestris in Chinese cabbage

To characterize the p-coumaric acid (pCA)-induced phenolic metabolites in relation to the disease resistance against Xanthomonas campestris pv. campestris (Xcc.), the responses of soluble- and cell wall-bound flavonoid and hydroxycinnamic acids compounds to the pretreatment of pCA or the inhibitor of the 4-coumarate-CoA ligase, 3,4-(methylenedioxy) cinnamic acid (MDCA), following Xcc inoculation were assessed, and the resulting data were interpreted with regard to susceptibility to Xcc in Chinese cabbage (Brassica rapa var. pekinensis). At 12 days post-inoculation (DPI) with Xcc, disease symptom development could be distinguished by necrotic lesions, and characterized by an enhanced lipid peroxidation. Overall, pCA acts as a positive stimulus for an accumulation of hydroxycinnamic acids and flavonoids, while MDCA acts as a negative regulator. Pretreatment with pCA resulted in an accumulation of specific hydroxycinnamic acids, pCA, ferulic acid (FA), and sinapic acid (SiA) in both soluble and cell wall-bound forms in Xcc-inoculated leaves, while MDCA pretreatment decreased accumulation in a dose-dependent manner. Two flavonoid compounds, epigallocatechin (EGC) and epigallocatechin gallate (EGCG), showed a similar response to pCA and MDCA pretreatments. These results indicate that a lower disease symptom development in pCA-pretreated leaves was associated with a higher accumulation of hydroxycinnamic acids and flavonoids, and vice-versa in MDCA- and non-pretreated (control) leaves.