Bacillus spp. are known for their ability to control plant diseases; however, the mechanism of disease control by Bacillus spp. is still unclear. Previously, bacterial organic acids have been implicated in the process of disease suppression. We extracted the total organic acid from Bacillus cereus AR156 culture filtrate and identified oxalic acid (OA) as the programmed cell death-inducing factor. OA strongly suppressed the lesion caused by Botrytis cinerea without significant antagonism against the fungus. Low concentration of OA produced by Bacillus spp. inhibited cell death caused by high concentrations of OA in a concentration- and time-dependent manner. Pretreatment with a low concentration of OA led to higher accumulation of active oxygen-scavenging enzymes in tomato leaves and provoked the expression of defense-related genes. The activation of gene expression relied on the jasmonic acid (JA) signaling pathway but not the salicylic acid (SA) pathway. The disease suppression capacity of OA was confirmed on wild-type tomato and its SA accumulation-deficient line, while the control effect was diminished in JA synthesis-deficient mutant, suggesting that the OA-triggered resistance relied on JA and ethylene (ET) signaling transduction. OA secretion ability was widely distributed among the tested Bacillus strains and the final environmental OA concentration was under strict regulation by a pH-sensitive degradation mechanism. This study provides the first systematic analysis on the role of low-concentration OA secreted and maintained by Bacillus spp. in suppression of gray mold disease and determines the dependence of OA-mediated resistance on the JA/ET signaling pathway. [Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 "No Rights Reserved" license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2022.
Keywords: Bacillus cereus; JA/ET signaling pathway; gray mold disease; induced systemic resistance (ISR); oxalic acid (OA).