Abstract
As potent allelochemicals, phenolic acids are believed to be associated with replanting disease and cause microflora shift and structural disorder in the rhizosphere soil of continuously monocultured Radix pseudostellariae. The transcriptome sequencing was used to reveal the mechanisms underlying the differential response of pathogenic bacterium Kosakonia sacchari and beneficial bacterium Bacillus pumilus on their interactions with phenolic acids, the main allelochemicals in root exudates of R. pseudostellariae in the monoculture system. The microbes were inoculated in the pots containing soil and the medicinal plant in this study. The results showed that the addition of beneficial B. pumilus to the 2-year planted soil significantly decreased the activity of soil urease, catalase, sucrase, and cellulase and increased the activity of chitinase compared with those in the 2nd-year monocropping rhizosphere soil without any treatment. However, opposite results were obtained when K. sacchari was added. Transcriptome analysis showed that vanillin enhanced glycolysis/gluconeogenesis, fatty acid biosynthesis, pentose phosphate, bacterial chemotaxis, flagellar assembly, and phosphotransferase system pathway in K. sacchari. However, protocatechuic acid, a metabolite produced by K. sacchari from vanillin, had negative effects on the citrate cycle and biosynthesis of novobiocin, phenylalanine, tyrosine, and tryptophan in B. pumilus. Concurrently, the protocatechuic acid decreased the biofilm formation of B. pumilus. These results unveiled the mechanisms how phenolic acids differentially mediate the shifts of microbial flora in rhizosphere soil, leading to the proliferation of pathogenic bacteria (i.e., K. sacchari) and the attenuation of beneficial bacteria (i.e., B. pumilus) under the monocropping system of R. pseudostellariae.
Highlights
Radix pseudostellariae L. belongs to the Caryophyllaceae family
Replanting disease has become a prevalent problem recently in the production of many annual crops, which are being subjected to intensive consecutive monoculture, such as Rehmannia glutinosa, cucumber, peanut, and tobacco (Zhou et al, 2012; Li et al, 2014; Santhanam et al, 2015; Wu et al, FIGURE 9 | Verification of differentially expressed genes between sample treatment (BP) and control (BPCK) in B. pumilus. (A) Represents the RPKM value; (B) represents the differentially expressed genes determined by qRT-Polymerase Chain Reaction (PCR)
With more research focused on this field, many studies have revealed that root exudates produced by plants have a propensity to shape the rhizosphere microbiome directly or indirectly and have some influences on the growth of plants (Li et al, 2014; Venturelli et al, 2015; Wu et al, 2016)
Summary
Radix pseudostellariae L. belongs to the Caryophyllaceae family. It is a common and popular medicine in China. The typical average annual yield of R. pseudostellariae is about 5000 tons, accounting for more than 22 million dollars per year. This medicinal plant is affected by the replanting disease, in which the consecutive monoculture of R. pseudostellariae leads to a serious decline in the biomass and quality of underground tubers. Consecutive monoculture of R. pseudostellariae caused the yield reduction by 33.3%, the polysaccharide content and ginseng saponins Rb1 of the tuberous root were reduced by 88.08 and 44.33%, respectively (Zeng et al, 2012). It has become a top priority to elucidate the underlying mechanisms of replanting disease, especially in the case of medicinal plant production
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