Abstract A three-year field monoculture trial of Radix pseudostellariae and complementary laboratory studies were conducted to further elucidate the underlying mechanism responsible for significant decreases in the biomass yield and quality of R. pseudostellariae under continuous monoculture regimes. HPLC analysis indicated that continuous monoculture soil was rich in organic acids, which had cumulative effects over time. Further analysis suggested that the application of a mixture of organic acids significantly promoted growth of pathogenic fungi, and increased the expression of chemotaxis-related gene (cheA) and biofilm formation of the specific pathogenic Kosakonia sacchari. However, opposite reactions were observed in the case of Bacillus megaterium and Bacillus pumilus. Concurrently, the present results revealed that the mixed organic acids stimulated the production of toxins, as well as H2O2 in the pathogenic fungi. Furthermore, the presence of organic acids reflecting environmental conditions under monocropping had negative effects on the expression of the biocontrol-related genes, which resulted in attenuated antagonistic activities of plant growth-promoting rhizobacteria (PGPR) to suppress mycelial growth of the pathogenic fungi. These results help to unveil the mechanisms associated with how accumulated organic acids differentially mediate deterioration of soil microbial composition and structure in monocropping system.
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