| Summary: | Isolation for antibacterial compounds from natural plants is a promising approach to develop new pesticides. In this study, two compounds were obtained from the Chinese endemic plant <i>Piper austrosinense</i> using bioassay-guided fractionation. Based on analyses of <sup>1</sup>H-NMR, <sup>13</sup>C-NMR, and mass spectral data, the isolated compounds were identified as 4-allylbenzene-1,2-diol and (S)-4-allyl-5-(1-(3,4-dihydroxyphenyl)allyl)benzene-1,2-diol. 4-Allylbenzene-1,2-diol was shown to have strong antibacterial activity against four plant pathogens, including <i>Xanthomonas oryzae</i> pathovar <i>oryzae</i> (<i>Xoo</i>), <i>X. axonopodis</i> pv. <i>citri</i> (<i>Xac</i>), <i>X. oryzae</i> pv. <i>oryzicola</i> (<i>Xoc</i>) and <i>X. campestris</i> pv. <i>mangiferaeindicae</i> (<i>Xcm</i>). Further bioassay results exhibited that 4-allylbenzene-1,2-diol had a broad antibacterial spectrum, including <i>Xoo</i>, <i>Xac</i>, <i>Xoc</i>, <i>Xcm</i>, <i>X. fragariae</i> (<i>Xf</i>), <i>X. campestris</i> pv. <i>campestris</i> (<i>Xcc</i>), <i>Pectobacterium carotovorum</i> subspecies <i>brasiliense (Pcb</i>) and <i>P. carotovorum</i> subsp. <i>carotovorum</i> (<i>Pcc</i>), with minimum inhibitory concentration (MIC) values ranging from 333.75 to 1335 μmol/L. The pot experiment showed that 4-allylbenzene-1,2-diol exerted an excellent protective effect against <i>Xoo</i>, with a controlled efficacy reaching 72.73% at 4 MIC, which was superior to the positive control kasugamycin (53.03%) at 4 MIC. Further results demonstrated that the 4-allylbenzene-1,2-diol damaged the integrity of the cell membrane and increased cell membrane permeability. In addition, 4-allylbenzene-1,2-diol also prevented the pathogenicity-related biofilm formation in <i>Xoo</i>, thus limiting the movement of <i>Xoo</i> and reducing the production of extracellular polysaccharides (EPS) in <i>Xoo</i>. These findings suggest the value of 4-allylbenzene-1,2-diol and <i>P. austrosinense</i> could be as promising resources for developing novel antibacterial agents.
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