Discovery of simple β-carboline alkaloids as promising ROS inducer for combating crop bacterial diseases

Gram-negative phytopathogenic bacteria pose a severe threat to global agriculture and food security, urgently requiring innovative bactericides. Although β-carboline alkaloids (βCs) have recently emerged as promising antimicrobial candidates, their structure-activity relationship (SAR) and mechanism of action remain poorly understood. To address these issues, 28 commercially available βCs and their analogues were evaluated for antibacterial activity. In vitro bioassays revealed that fully aromatic β-carboline (FAβC) derivatives exhibited broad-spectrum antibacterial activity, with optimal EC₅₀ values of 3.17 (compound 15), 2.91 (compound 15), and 7.61 (compound 13) μg mL^-1 against Xanthomonas oryzae pv. oryzae (Xoo), Xanthomonas axonopodis pv. citri (Xac), and Pseudomonas syringae pv. actinidiae (Psa), respectively, demonstrating over 13-fold greater potency than bismerthiazol and thiodiazole‑copper. SAR analysis indicated that C-ring aromatization significantly increased the antibacterial effect of βCs, with the activity order of FAβC > 3,4-dihydro-β-carboline (DHβC) > 1,2,3,4-tetrahydro-β-carboline (THβC). Pot experiments showed that selected FAβCs (13 and 15) provided notable in vivo control efficiency toward rice bacterial leaf blight and kiwifruit bacterial canker, while maintaining favorable safety profiles. Fluorescence tracking and histopathological analysis demonstrated that compound 15 significantly suppressed bacterial migration, motility, and colonization in infected plants. Notably, mechanistic studies revealed that FAβCs exerted antibacterial effects through disruption of bacterial membrane integrity and induction of ROS-mediated cell apoptosis. Overall, these findings not only identify natural harmine (15) as a novel bactericide candidate but also advance SAR understanding and application potential of βCs for managing crop bacterial diseases.