Different toxic effects of ferulic and p-hydroxybenzoic acids on cucumber seedling growth were related to their different influences on rhizosphere microbial composition

Rhizosphere microbial communities of cucumber seedlings treated with ferulic (FA) and p-hydroxybenzoic acids (PHBA) were analyzed by quantitative PCR and Illumina sequencing. Then, growth responses of cucumber seedlings to changes in the composition of rhizosphere microbial communities were assessed. Compared with PHBA, FA had higher inhibitory effects on cucumber seedling growth and higher stimulating effects on the pathogen Fusarium oxysporum f.sp. cucumerinum, which led to severer Fusarium wilting. Both FA and PHBA increased the abundances and changed the compositions of bacterial and fungal communities. However, bacterial and fungal community compositions differed between the treatments of FA and PHBA. Both FA- and PHBA-treated cucumber rhizosphere biota inhibited cucumber seedling growth with FA having stronger inhibitory effects. F. oxysporum f.sp. cucumerinum can utilize FA and PHBA in vitro. Overall, the higher phytotoxic effect of FA than PHBA on cucumber seedling growth was linked to their different influences on the composition of rhizosphere microbial communities, especially the stronger stimulating effect of FA than PHBA on F. oxysporum f.sp. cucumerinum.