Changes in rhizosphere microbial communities in potted cucumber seedlings treated with syringic acid

Phytotoxic effects of phenolic compounds have been extensively studied, but less attention has been given to the effects of these compounds on soil microbial communities, which are crucial to the productivity of agricultural systems. Responses of cucumber rhizosphere bacterial and fungal communities to syringic acid (SA), a phenolic compound with autotoxicity to cucumber, were analyzed by high-throughput sequencing of 16S rRNA gene and internal transcribed spacer amplicons. SA at the concentration of 0.1 μmol g-1 soil changed rhizosphere bacterial and fungal community compositions, decreased bacterial community diversity but increased fungal community richness and diversity (P<0.05). Moreover, SA increased the relative abundances of bacterial phylum Proteobacteria and fungal classes Leotiomycetes, Pezizomycetes, Tremellomycetes and Eurotiomycetes, but decreased the relative abundances of bacterial phylum Firmicutes and fungal class Sordariomycetes (P<0.05). At the genus level, SA decreased the relative abundances of microbial taxa with pathogen-antagonistic and/or plant growth promoting potentials, such as Pseudomonas spp. (P<0.05). Real-time PCR validated that SA decreased cucumber rhizosphere Pseudomonas spp. abundance (P<0.05). In vitro study showed that SA (0.01 to 10 mM) inhibited the growth of a strain of Pseudomonas spp. with pathogen-antagonistic activities to cucumber pathogen Fusarium oxysporum f.sp. cucumerinum Owen (P<0.05). Overall, SA changed cucumber rhizosphere bacterial and fungal community compositions, which may exert negative effects on cucumber seedling growth through inhibiting plant-beneficial microorganisms.