New Insights into the Antibacterial Activity of Hydroxycoumarins against Ralstonia solanacearum

Coumarins are important plant-derived natural products with wide-ranging bioactivities and extensive applications. In this study, we evaluated for the first time the antibacterial activity and mechanisms of action of coumarins against the phytopathogen Ralstonia solanacearum, and investigated the effect of functional group substitution. We first tested the antibacterial activity of 18 plant-derived coumarins with different substitution patterns, and found that daphnetin, esculetin, xanthotol, and umbelliferone significantly inhibited the growth of R. solanacearum. Daphnetin showed the strongest antibacterial activity, followed by esculetin and umbelliferone, with MICs of 64, 192, and 256 mg/L, respectively, better than the archetypal coumarin with 384 mg/L. We further demonstrated that the hydroxylation of coumarins at the C-6, C-7 or C-8 position significantly enhanced the antibacterial activity against R. solanacearum. Transmission electron microscope (TEM) and fluorescence microscopy images showed that hydroxycoumarins may interact with the pathogen by mechanically destroying the cell membrane and inhibiting biofilm formation. The antibiofilm effect of hydroxycoumarins may relate to the repression of flagellar genes fliA and flhC. These physiological changes in R. solanacearum caused by hydroxycoumarins can provide information for integral pathogen control. The present findings demonstrated that hydroxycoumarins have superior antibacterial activity against the phytopathogen R. solanacearum, and thus have the potential to be applied for controlling plant bacterial wilt.