Kojic acid attenuates tomato soft rot caused by Pectobacterium carotovorum subsp. carotovorum through quorum quenching.

Pectobacterium carotovorum subsp. carotovorum (Pcc) is the causal agent of bacterial soft rot disease, which triggers rapid decay of infected tomato fruit. Kojic acid, primarily a fermentation metabolite of Aspergillus oryzae, exhibits biocontrol activity against postharvest soft rot of tomato. In this work, we investigated the interference of kojic acid with quorum sensing (QS) in Pcc and elucidated the relevant mechanisms to provide novel theoretical insights into the management of tomato postharvest soft rot. The results demonstrated that kojic acid at a sub-inhibitory concentration (1/4 MIC) markedly suppressed the synthesis of QS signal molecule 3-oxo-C6-HSL in Pcc, as verified by violacein production assays using C. violaceum CV026 and quantitative HPLC assays. Kojic acid downregulated the expression of QS-related genes expI and expR in Pcc. This effect decreased the biosynthesis of 3-oxo-C6-HSL and further inhibited QS signal transduction. Furthermore, molecular docking simulations predicted that kojic acid may share the key residue Ser101 on ExpR with 3-oxo-C6-HSL. Such predicted binding, together with an additional hydrogen bond potentially formed between kojic acid and Asp67 on ExpR, may interfere with QS signal transduction. By interfering with the QS system, kojic acid modulated the expression of the key downstream virulence-related gene pel, impaired bacterial motility, and reduced extracellular polysaccharide production by 41.54%. Meanwhile, kojic acid inhibited biofilm formation of Pcc both in vitro and in tomato wounds. The maximum inhibition rate of biofilm formation reached 36.78% in vitro, while the maximum inhibition rate of biofilm-associated viable cells was 24.10% in tomato wounds. Collectively, these effects attenuated Pcc infectivity for tomato fruit. This study provides a novel approach for mitigating Pcc pathogenicity by targeting its QS system, offering practical value for the postharvest preservation of fruits and vegetables.