Responses of soil microbiome to phenolic acid secreted from tobacco rhizosphere

Objectives To investigate the effect of phenolic acids, a kind of allelopathic substance exudated by roots, on the rhizosphere functional microbiota number, in order to present a theoretical basis on overcoming the obstacle caused by continuous tobacco mono-cropping.

Methods Benzoic acid and 3-phenylpropanoic acid were the two main phenolic acids isolated and identified from tobacco root exudates in previous studies, which were inoculated into soils to simulate the accumulation of tobacco root exudates after many years continuous mono-cropping. The culture experiment had 4 treatments: adding sterilized deionized water (T0), 3 μg/kg soil of benzoic acid (T1), 8 μg/kg soil of 3-phenylpropanoic acid (T2) and both benzoic acid 3 μg/kg soil and 3-phenylpropanoic acid 8 μg/kg soil (T3). High-throughput sequencing using MiSeq system was conducted to evaluate the effect of main phenolic acids on soil bacterial microbes. Real-time PCR was employed to detect the population of Rastonia solanacearum, Brevibacillus brevis, N₂-fixing bacteria, phosphate solubilizing bacteria, potassium release bacteria, bacteria and fungi.

Results Compared with T0, the soil microbial operational taxonomic units(OTUs) in T1 and T2 were decreased by 21.5 % or 17.0 %, respectively, and even lower OTU was in T3. In T2 treatment, the dominant soil microorganisms increased, and structure balance was declined, the phylum and community structures were significantly different from those in T0. Both the principal component (PCoA) and cluster analyses showed that the microbe profiles in treatment T2 and T3 were nearer, and both were relatively far from that in T0. In treatment T3, the population of pathogen was significantly increased while those of antagonist B. Brevis, N₂-fixing bacteria, phosphate solubilizing bacteria, potassium release bacteria, bacteria and fungi were significantly reduced, and the extent of reduction was greater than that in treatment T1 or T2.

Conclusions The accumulation of tobacco root exudates benzoic acid and 3-phenylpropanoic acid could affect rhizosphere soil microbiota significantly by decreasing microbial community diversity, increasing pathogen population, and simultaneously reducing beneficial microbial number. The co-existence of the two phenolic acids would lead even worse impaction.