New Steroids Obtained from Ailanthus altissima Leaves Inhibit the Invasive Bacteria Xanthomonas oryzae pv. oryzae and Pseudomonas syringae pv. maculicola

Invasive bacteria have caused tremendous losses to global ecosystems and agricultural production, yet effective control measures remain elusive. Plant specialized metabolites are being investigated as an important source of antimicrobial active substances. And <i>Ailanthus altissima</i> is an abundant tree widespread throughout Northeast China. In this study, we identified 21 compounds from <i>A. altissima</i> leaves, including steroids, terpenes, phenolics, and coumarins. Two new steroidal compounds, ailanstigol A (<b>1</b>) and ailanstigol B (<b>2</b>), and one new coumarin (2'<i>R</i>,3'<i>R</i>)-7-(2',3',6'-trihydroxy-3'-methylhexyloxy)-6,8-dimethoxycoumarin (<b>3</b>) were isolated. Antibacterial screening revealed that compounds <b>1</b> and <b>2</b> exhibited inhibitory activity against two invasive bacteria, <i>Xanthomonas oryzae</i> pv. <i>oryzae</i> PXO 71A and PXO 86A and <i>Pseudomonas syringae</i> pv. <i>maculicola</i> ES4326. Further mechanistic screening unveiled that the steroidal compounds <b>1</b> and <b>2</b> may inhibit bacterial growth and reproduction by reducing cell viability, disrupting the cell membrane and increasing protein leakage, and inhibiting biofilm formation. In summary, our results enriched the known chemical diversity of <i>A. altissima</i> and provided a foundation for investigating the mechanisms by which steroidal compounds inhibit invasive bacterial growth.