Identification of phenolic acids in tobacco root exudates and their role in the growth of rhizosphere microorganisms

【Objectives】 Tobacco continuous mono-cropping has caused very serious problems, including soil-borne disease outbreak, tobacco growth suppression, yield reduction and quality decline. Allelopathy comes from tobacco and soil microorganisms may be one of the most important reasons leading to mono-cropping obstacle. Tobacco root exudates (TRE) play a key role in the plant-microorganism interactions in the rhizosphere. It is of great importance to explore effect of tobacco root exudates on rhizosphere microorganisms. 【Methods】 In this study, the main phenolic acids were screened and identified by the UPLC-Q-TOF/MS method, and their contents in tobacco root exudates were evaluated. Effect of identified phenolic acids from TRE in liqiud medium on pathogen and its antagonist was investigated by applying exogenous phenolic acids. Besides, the soil was added with the identified phenolic acids and cultured for 3 d. After that, the rhizosphere microbial diversities and counts, especially the population of pathogen Ralstonia solanacearum and its antagonist Brevibacillus brevis in the soil were measured. 【Results】 1)The tobacco root exudates promote the growth of pathogen by 16.8％ and suppress the growth of antagonist by 29.4％. 2)Two phenolic acids are screened and identified by UPLC-Q-TOF/MS with the concentrations of 0.25 μg/g and 1.15 μg/g dry root, respectively. 3)When the exogenous phenolic acids were added to the culture media, low concentrations of benzoic acid (≤2 μg/L) and 3-phenylpropanoic acid (≤3 μg/L) promote the growth of the pathogen and antagonist, and the 4 μg/L benzoic acid does not significantly affect R. solanacearum, whereas the population of antagonist is decreased by 90.2％. The 6 μg/L 3-phenylpropanoic acid promotes the growth of the pathogen while inhibits the growth of antagonist by 81.1％. High concentrations of benzoic acid (≥ 4 μg/L) and 3-phenylpropanoic acid (≥ 7 μg/L) significantly suppress the pathogen and antagonist. 4)When the soil is applied with the 3 μg/kg benzoic acid, the pathogen count was increased by 12.3％ while the antagonist population is decreased by 21.0％. Besides, the populations of soil bacteria, actinomycetes and fungi are declined by 37.5％,41.9％ and 55.6％, respectively. The 8 μg/kg 3-phenylpropanoic acid significantly suppresses the antagonist growth by 14.5％, and does not significantly affect the pathogen. Soil bacteria, actinomycetes and fungi are decreased by 69.9％, 57.2％ and 80.7％, respectively under this concentration of 3-phenylpropanoic acid. 5)After the 4 μg/kg benzoic acid or 7 μg/kg 3-phenylpropanoic acid application to the soil, the Shannon index, Simpson index and McIntosh index are significantly declined to 57.7％, 88.1％, 73.3％ and 94.1％, 97.6％, 80.0％ of CK. 【Conclusions】 The tobacco root exudates promote the pathogen growth whereas suppress the antagonist growth. The main phenolic acids in tobacco root exudates are benzoic acid and 3-phenylpropanoic acid. The 4 μg/L exogenous benzoic acid or 6 μg/L exogenous 3-phenylpropanoic acid are the boundary concentrations, which do not significantly suppress the growth of pathogen but inhibits the antagonist. When benzoic acid and 3-phenylpropanoic acid are applied to the soil, the growth of pathogen is promoted, while the antagonist population and the rhizo-microbial functional diversity are significantly reduced under the 3 μg/kg benzoic acid or 8 μg/kg 3-phenylpropanoic acid. Pathogen could survive under the condition of tobacco root exudates, rather than antagonist. This may be an explanation of tobacco bacterial wilt outbreak resulted from mono-cropping obstacle.