Metabolomic Profiling of Tomato Root Exudates Induced by <i>Ralstonia solanacearum</i> Strains of Different Pathogenicity: Screening for Metabolites Conferring Bacterial Wilt Resistance

Tomato is one of the most widely cultivated and consumed vegetables in the world, and its production is severely threatened by bacterial wilt. Bacterial wilt caused by <i>Ralstonia solanacearum</i>, is one of the most devastating plant diseases. <i>R. solanacearum</i> is a complex species with both virulent and avirulent strains. The avirulent strains show high biocontrol activity against bacterial wilt. A metabolomics study was conducted on tomato root exudates induced by <i>R. solanacearum</i> of different pathogenicity, and the potential bacterial wilt resistance metabolites were screened. Principal component analysis revealed a clear separation of the metabolic profile between the virulent strain-induced group, the avirulent strain-induced group, and the CK group. Based on the altered abundance in root exudates after <i>R. solanacearum</i> induction, the most differential metabolites were selected for further investigation, including citramalic acid, glucuronic acid, alpha-ketoglutaric acid, and malic acid, etc. The plate inhibition assay showed that alpha-ketoglutaric acid (AKG) and malic acid (MA) had a dose-dependent inhibitory effect on <i>R. solanacearum</i> (5-20 mg/ml, inhibition circle diameter 14.69-25.08 mm). Crystal violet staining showed that 1-2.5 mg/ml of MA and AKG could significantly inhibit <i>R. solanacearum</i> biofilm formation at 24 h (<i>P</i> < 0.0001, inhibition rate 66.93-70.43%). In tomato pot experiments against bacterial wilt, the AKG group had 75.36% biocontrol efficacy, and the MA group had 57.97% efficacy 25 days after inoculation. We conclude that AKG and MA play an important role in resistance to bacterial wilt in tomato.