Semiliquidambar chingii is a highly potent antibacterial plant resource against Xanthomonas citri subsp. citri: Insights into the possible mechanisms of action, chemical basis, and synergistic effect of bioactive compounds

Citrus canker disease is a major setback for citrus cultivation and processing industries since this disease poses a serious threat to the fruit quality and quantity. Semiliquidambar chingii is an important traditional Chinese medicinal plant having high industrial and medicinal potential. The present study evaluated the potential of S. chingii for the management of citrus canker disease induced by Xanthomonas citri subsp. citri (Xcc) for the first time. The ethyl acetate extract of S. chingii twigs (EtOAcE) was screened for the antibacterial effect against Xcc bacteria and it exhibited good antibacterial activity with a MIC value of 0.25 mg/mL. A preliminary study of the mechanisms of action revealed that EtOAcE significantly inhibited the biofilm formation of Xcc and bacterial growth by damaging the cell membrane and inducing the production of ROS (reactive oxygen species). Twenty-one compounds isolated from the S. chingii were then investigated for the antibacterial effect against Xcc, among which four compounds involving resveratrol (1), 3-oxo-oleanoic acid (8), lantanolic acid (10), and eriodictyol (21) were found to exhibit more pronounced antibacterial effects (MIC = 0.063 mg/mL for all these four compounds) than the positive control CuSO4. Structure-activity relationship (SAR) study revealed some important functional groups responsible for the antibacterial activity. The combination of compounds 1/8 or 8/21 showed a synergistic effect against Xcc. Furthermore, the chemical profile of EtOAcE was established by HPLC (high pressure liquid chromatography), indicating it contained resveratrol (1), piceid (2), catechin (12), and piceid-6"-O-gallate (20) as major components. The present results demonstrate that S. chingii represents an excellent plant resource for the development of naturally occurring antibacterial agents against Xcc.