Plant metabolomics integrated with transcriptomics and rhizospheric bacterial community indicates the mitigation effects of Klebsiella oxytoca P620 on p-hydroxybenzoic acid stress in cucumber

Accumulation of p-hydroxybenzoic acid (PHBA) in soil causes autotoxicity stress in cucumber. When the stress is mitigated by PHBA-degrading bacteria, plant metabolites have not been detected. To explore mechanisms underlining the mitigation, plant metabolites have not been combined with rhizospheric microbes, antioxidant and soil enzymes. In this study, a strain P620 of Klebsiella decomposed PHBA to acetyl CoA. Cucumber was sown into soil supplemented with P620 and/or PHBA. After addition with P620, P620 colonization and the enriched bacterial genera were observed in rhizosphere. Compared to PHBA stress alone, the combination of P620 application and PHBA stress improved plant growth, decreased PHBA concentration in soil, and increased the activities of five soil enzymes and eight antioxidant enzymes in leaves. Metabolomic and transcriptomic analysis highlighted that P620 application decreased the intensities of MAG(18:3) isomer 4, MAG(18:3) isomer 2, lysoPC 18:3 (2n isomer), 2'-deoxyadenosine-5'-monophosphate, pyridoxine, and glucarate O-phosphoric acid in PHBA-stressed leaves and down-regulated the expression of genes related to these metabolites. We propose a mechanism that P620 application alters microbial communities in PHBA-contaminated soil. Thus, the application reduces PHBA concentration in soil, activates antioxidant and soil enzymes, and also influences metabolites in leaves by affecting plant transcriptome, mitigating PHBA stress in cucumber.