| The infestation by soil-borne pathogens and autotoxicity are the main restrictions in worldwide muskmelon (Cucumis melo L.) production. Fusarium oxysporum f. sp. melonis (Leach&Currence) Snyd.&Hans., causes Fusarium wilt of muskmelon, which is an economically important disease universally. Root exudates are the largest direct inputs of plant chemicals into the rhizosphere environment which initiate and manipulate biological interactions between roots and soil micro-organisms, and thus play an active role in root-microbe communication. Autotoxicity, a form of intraspecific allelopathy is important in agricultural applications, such as the replant failure of horticultural crops and growth reduction in some fruit vegetables during fruit enlargement. Among all allelochemicals, pheolic compounds are the most abundant under field conditions and are known to affect seed germination, seedling growth, sell division and fungal activity, could accumulate in soil Autotoxicity is a complicated problem in agricultural production, which needs more studies such as the identification of the phytotoxic substances in the root extracts and exudates, their fluctuation in rhizosphere and the elucidation of the possible role of micro-organisms in the phytotoxicity.The purposes of this research are to evaluate the influences of root exudates produced by muskmelon and identify the compounds from muskmelon root exudates that are responsible for muskmelon autotoxicity, to determine the phytotoxicity of autotoxic compounds to muskmelon seed germination and seedling growth, the effects on diversity of soil microbial community, the growth and pathogenicity of F. oxysporum f. sp. melonis and the gene expression of Fochs V using a CFX96Real-time PCR Detection System with the attempt to explain how allelopathic compounds stimulate the grow of the pathogen.1. The allelopathy of root exudates of muskmelonExperiments were conducted to test the influence of allelochemicals from root exudates of muskmelon plants on muskmelon seed germination and seedling growth and Fusarium oxysporum f. sp. melonis. It was observed that the root exudates of muskmelon significantly reduced muskmelon seed germination and seedling growth and at higher concentration (1.0g·mL-1and1.0g·mL-1) and significantly promoted mycelial growth and spore germination of F. oxysporum f. sp. melonis and incidence of Fusarium wilt as determined in bioactivity test and pot experiments. The results of field experiments indicated that the disease incidence and the quantity of the fungi were higher in sole replant cropping.2. Identification and biological activities of phenolic compounds from muskmelon root exudatesThe pheolic compounds from muskmelon root exudates were identified by HPLC analysis and the biological activities were determined. Seven phenolic compounds were identified resulted in peaks with retention time:gallic acid, phthalic acid, syringic acid, salicylic acid, ferulic acid, benzoic acid and cinnamic acid, all of which in the laboratory significantly reduced muskmelon seed germination and seedling growth and at higher concentration (0.5mmol·L-1) when compared with the control. The maximum inhibition in seed germination was found in treatment with cinnamic acid, followed by benzoic acid, ferulic acid. Phthalic acid, ferulic acid, benzoic acid and cinnamic acid affected the physiological characteristics (POD, SOD, CAT, MDA content) at0.5mmol·L-1concentration. Experiments were conducted to test the influence of the seven pheolic compounds on the growth of F. oxysporum f. sp. melonis. It was observed ferulic acid significantly promoted mycelial growth and ferulic acid, benzoic acid and cinnamic acid showed stimulatory effects on spore germination of Fusarium oxysporum f. sp. melonis at higher concentration (0.1and0.25mmol·L-1). Pot experiments results indicated that cinnamic acid, benzoic acid and ferulic acid enhanced the disease incidence of muskmelon at0.05,0.1and0.5mmol·L-concentrations compared with the control.3. Effects of autotoxic compounds on diversity of soil microbial communityCommunity structure and diversity of soil microorganism in treatments of muskmelon root exudates and three autotoxic compounds (cinnamic acid, benzoic acid and ferulic acid) were studied with PCR-DGGE technology. Results showed that soil bacterial community structure and diversity were significantly influenced by root exudates and three autotoxic compounds. Electrophoresis band number and brightness were reduced and richness index and Shannon-Wiener index were decreased significantly.The number of some fungi was significantly decrease, meanwhile, the number of some other fungi was significantly increased and most dominant fungi populations were enriched in soil with treatments of muskmelon root exudates and three autotoxic compounds. The maximum richness index and Shannon-Wiener index were found in treatments of cinnamic acid and benzoic acid at1.0mmol·L-1concentration.4. Effects of autotoxic compounds on pathogenicity of F. oxysporum f. sp. melonis were studied.Fusaric acid in the crude toxin was extracted from liquid medium with ethylacetate and the content of fusaric acid was determined. It was observed ferulic acid, benzoic acid and cinnamic acid promoted the content of fusaric acid, and the stimulatory effects increased with the concentration. The maximum stimulatory was found in benzoic acid at1.0mmol·L-1concentration with3folds of the control. Ferulic acid, benzoic acid and cinnamic acid had different effects on the activity of the four cell wall-degrading enzymes (protease, pectinase, cellulose, β-glucosidase). Ferulic acid significantly promoted the activities of protease and pectinase. Benzoic acid showed significantly stimulatory on protease, cellulose and β-glucosidase, and the stimulatory effects increased with the increase of the concentration. Cinnamic acid showed stimulatory on activities of the cell wall-degrading enzymes except pectinase.5. Effects of autotoxic compounds on the pathogenic gene expression of F. oxysporum f. sp. melonisThe relative expression quantity of chs V of F. oxysporum f. sp. melonis (Fochs V.) was detected using a Real-time PCR Detection System. The result of quantitative PCR analysis showed that ferulic acid, benzoic acid and cinnamic acid promoted Fochs V gene expression and accelerated expression time. The maximum the relative quantity expression of Fochs V were found in the treatment cinnamic acid (20.65-fold increase at72h post inoculation), followed by benzoic acid (17.74-fold increase at48h post inoculation) and ferulic acid (8.47-fold increase at72h post inoculation). These results are consistent with the higher disease infection of Fusarium wilt promoted by autotoxic compounds, which may thus contribute to improving the pathogenicity of F. oxysporum f. sp. melonis that lead to further damage. 6. Mitigation mechanism of autotoxic compounds-degrading Trichoderma strainsAccording to the SA plant culture, eighteen strains were found growth advantage from forty Trichoderma strains. Trichoderma strains (T8, T58and T120) was chosen by second screening which has the ability to decompose the seven pheolic compounds in muskmelon root exudate, especially to ferulic acid, benzoic acid and cinnamic acid. The result of degradation test showed the mixture of spore suspension of Trichoderma strains (T8, T58and T120) had the degradation function to ferulic acid, benzoic acid and cinnamic acid at100,300μg·mL-1concentrations, and the maximum degration was found in treatment with benzoic acid, followed by ferulic acid and cinnamic acid. The influence of Trichoderma strains (T8, T58and T120) and ferulic acid, benzoic acid and cinnamic acid on defense enzymes of muskmelon seedling were compared and analyzed. Results suggest that the activities of defense enzymes (PAL, POD, PPO) were much more indused by inoculation of Trichoderma strains (T8, T58and T120) than by ferulic acid, benzoic acid and cinnamic acid. |
PDF Full Text Request