p-Coumaric Acid Influenced Cucumber Rhizosphere Soil Microbial Communities and the Growth of Fusarium oxysporum f.sp. cucumerinum Owen

Wu F (2012) p-Coumaric Acid Influenced Cucumber Rhizosphere Soil Microbial Communities and the Growth of Fusarium oxysporum f.sp. cucumerinum Owen. PLoS ONE 7(10): e48288. doi:10.1371/journal.pone.0048288 p-Coumaric Acid Influenced Cucumber Rhizosphere Soil Microbial Communities and the Growth of Fusarium oxysporum f.sp. cucumerinum Owen Xingang Zhou 0 Fengzhi Wu 0 Jack Anthony Gilbert, Argonne National Laboratory, United States of America 0 Department of Horticulture, Northeast Agricultural University , Xiangfang, Harbin , People's Republic of China Background: Autotoxicity of cucumber root exudates or decaying residues may be the cause of the soil sickness of cucumber. However, how autotoxins affect soil microbial communities is not yet fully understood. Methodology/Principal Findings: The aims of this study were to study the effects of an artificially applied autotoxin of cucumber, p-coumaric acid, on cucumber seedling growth, rhizosphere soil microbial communities, and Fusarium oxysporum f.sp. cucumerinum Owen (a soil-borne pathogen of cucumber) growth. Abundance, structure and composition of rhizosphere bacterial and fungal communities were analyzed with real-time PCR, PCR-denaturing gradient gel electrophoresis (DGGE) and clone library methods. Soil dehydrogenase activity and microbial biomass C (MBC) were determined to indicate the activity and size of the soil microflora. Results showed that p-coumaric acid (0.1-1.0 mmol/g soil) decreased cucumber leaf area, and increased soil dehydrogenase activity, MBC and rhizosphere bacterial and fungal community abundances. p-Coumaric acid also changed the structure and composition of rhizosphere bacterial and fungal communities, with increases in the relative abundances of bacterial taxa Firmicutes, Betaproteobacteria, Gammaproteobacteria and fungal taxa Sordariomycete, Zygomycota, and decreases in the relative abundances of bacterial taxa Bacteroidetes, Deltaproteobacteria, Planctomycetes, Verrucomicrobia and fungal taxon Pezizomycete. In addition, p-coumaric acid increased Fusarium oxysporum population densities in soil. Conclusions/Significance: These results indicate that p-coumaric acid may play a role in the autotoxicity of cucumber via influencing soil microbial communities. - Soil sickness is a reduction in both crop yield and quality caused by continuous mono-cropping in the same land. It is one of the major problems in agricultural production, especially for greenhouse crops [1]. Cucumber (Cucumis sativus L.), a crop of high economic importance in many countries, is vulnerable to soil sickness [1]. Recently, cultivation of cucumber under greenhouse conditions has greatly expanded in China, but significant agricultural loss is observed each year because continuous monocropping practice is becoming more and more popular. The accumulation of autotoxins is probably responsible for the soil sickness of cucumber [1]. Autotoxicity is an intraspecific allelopathy, where a plant species inhibits the growth of plants of the same species through releasing toxic chemicals into the environment [2]. Cucumber root exudates and plant debris were shown to have autotoxicity potential [1]. Autotoxins, including some phenols, have been identified in cucumber root exudates [3]. Some phenols from living and decomposing plant tissues can be active allelochemicals and they can accumulate in soil and have detrimental effects on the growth of associated and next-season plants [4]. Soil microorganisms may influence the persistence, availability and biological activities of allelochemicals in soil [5,6] and root exudates or allelochemicals can affect soil microbial communities [7]. Thus, it is suggested that allelopathy can be better understood in terms of soil microbial ecology [5,8]. Recently, a great research effort has addressed the effects of plant root exudates, such as low molecular carbohydrate, organic acids, amino acids, and plant secondary metabolites (e.g. flavonoids and glucosinolate) on soil microbial communities [913]. Phenols were shown to affect the growth of microorganisms in vitro [14], but little information is available on how soil microbial communities respond to putative allelochemicals in natural soils [15,16]. Our knowledge about effects of autotoxins, such as phenols, on soil microorganism populations has been mainly achieved by traditional cultivation-dependent methods, which is limited in that only a small fraction of the microorganisms are accessible to study [1719]. Analysis of soil microbial communities with molecular techniques, such as real-time PCR, PCR-denaturing gradient gel electrophoresis (DGGE) and phylogenetic analysis, can improve our understanding of how autotoxins affect abundance, structure and composition of soil microbial communities [20,21]. Several studies have reported that soil may become suppressive in the case of long-term monoculture, for example, Gaeumannomyces graminis var. tritici (take-all disease) of wheat [22], Rhizoctonia solani of sugar beet [23], Thielaviopsis basicola of tobacco [24] and Fusarium wilt of watermelon [25]. However, it is also suggested that the accumulation of soil-borne pathogens is responsible for the soil sickness [2628]. The Fusarium (Ascomycota, Fungi) community size was found to be linked to the soil sickness associated with cucumber cultivation [28]. Autotoxins would accumulate under continuous mono-cropping conditions; therefore, how autotoxins, such as phenols, affect soil-borne pathogens in soil needs to be further clarified. p-Coumaric acid (p-hydroxycinnamic acid) has been identified in plant root exudates or residues [14] and in soils under many plant species, including cucumber [29,30]. We hypothesized that p-coumaric acid could influence cucumber growth and soil microbial communities. The primary aims of this research were to study: 1) the phytotoxic effects of p-coumaric acid on cucumber radicle elongation and seedling growth; 2) the effects of p-coumaric acid on structure, composition, abundance, activity and size of cucumber rhizosphere microbial communities; 3) the effects of pcoumaric acid on the growth of F. oxysporum f.sp. cucumerinum Owen (a soil-borne pathogen of cucumber) both in vitro and in soil. Abundance, structure and composition of bacterial and fungal communities were analyzed by real-time PCR, PCR-DGGE and clone library methods. Soil dehydrogenase activity and microbial biomass C (MBC) were determined to indicate the activity and size of soil microflora. Materials and Methods Cucumber Radicle Elongation Experiment Ten germinated cucumber seeds (cv. Jinlv 3) with radicles of 1 mm length were separately placed in a Petri dish (9 cm diameter), which contained two layers of sterilized filter papers. Five milliliter of different concentrations of p-coumaric acid solutions (0.1, 0.25, 0.5 or 1.0 mM) with pH adjusted to 7.0 with 0.1 M NaOH solution were added in the Petri dish. Cucumber seeds treated with distilled water (p (...truncated)