| Tomato(Solanum Lycopersicum L.)is an important vegetable crop cultivated in different parts of China.Verticillium wilt is an important soil-borne disease affecting the growth and quality of tomato.One of the main reasons of decrease in tomato production is severe incidence of Verticillium wilt caused by continuous cropping of tomato.Previous studies have shown that intercropping with potato onion can promote the tomato resistance against Verticillium wilt,however,the mechanism is still unclear.In this study,we analyzed the mechanism of improved tomato resistance against Verticillium wilt by intercropping with potato onion fr om the perspective of rhizosphere microbiology.In this study,we used tomato and potato onion as test materials and used quantitative PCR and metagenomic sequencing to investigate the effects of intercropping with potato onion on changes in microbial community composition in tomato rhizosphere soil.Soil sterilization experiment,plant-soil feedback experiment and adding bactericide and fungicide experiment were used to investigate the effects on soil microorganisms,changes in soil microorganism induced by intercropping with potato onion and changes in soil bacterial and fungul communities in tomato resistance to Verticillium wilt,respectively.HPLC was used to investigate the effects of intercropping with potato onion on tomato root exudation and its co mposition.Finally,we assessed the effects of tomato root exudates and altered root exudation profiles on the chemotaxis,biofilm formation and expression of antibiotic-producing genes of PGPR(Bacillus P1,Bacillus P2 and Pseudomonas P8).The series of experiments were aimed to reveal the soil microbiological mechanism underlying improved resistance of tomato against Verticillium Wilt during intercropping with potato onion.The main results of this study are as follows:(1)Intercropping with potato onion increased the Pseudomonas,Bacillus,2,4-diacetylphloroglucinol,pyoluteorin,fengycin and surfactin producter abundance,and the relative abundances of potential plant-growth-promoting microorganisms and potential antagonistic microorganisms,such as Streptomyces,Nocardiodies,Masilia and Devosia spp.Intercropping with potato onion changed the tomato rhizosphere microbial community composition,and divided the tomato rhizosphere bacterial community network into three independent parts and reduced the complexity of co-occurrence network.(2)Changes in the functions of tomato rhizosphere microorganisms caused by intercropping with potato onion were mainly focused on metabolism and genetic information processing.Intercropping increased the relative abundances of functions of ABC transporter,Two-component system,Bacterial secretion system,Bacterial chemotaxis and bioflim formation.Metagenomic data analysis identified the taxa involved in these five functions,and showed that intercrop ping increased the relative abundances of Pseudomonadaceae,Enterobacteriaceae,Xanthomonadaceae,Streptomycetaceae,Micromonosporaceae,Nocardioidaceae,Hyphomicrobiaceae and Alcaligenaceae.Intercropping increase Shannon index of tomato microbial communi ty function while PCo A showed that samples from intercropping and monoculture systems were separated from each other.These results indicated that intercropping with potato onion altered the structure,composition and eventually the functioning of tomato rhizosphere microbial community.The dominant two contributors to the differential function of intercropping and monoculture system were Arthrobacter and Pseudomonas spp.,and the functional contribution of Pseudomonas spp.to intercropping was greater than that of monoculture system.(3)Soil sterilization decreased the biomass of tomato plants,increased the disease index and incidence of Verticillium wilt,and delayed the expression of defense-related genes of tomato root in the intercropping system,indicating that soil microorganisms played an important role in improving the resistance of tomato against Verticillium wilt in the intercropping system.Manually application of certain soil microbial communities from intercropping system increased the biomass of tomato plants,decreased the disease index and incidence of Verticillium wilt,and promoted the expression of defense-related genes in tomato root when Verticillium dahliae attacked,indicating that changes in tomato soil microbial community induced by intercropping with potato onion had a positive feedback in promoting tomato growth and improving the resistance of tomato against Verticillium wilt.Addition of bactericide deceased the biomass of tomato plants,increased the disease index and incidence of Verticillium wilt,and delayed the expression of defense-related genes of tomato root in the intercropping system,indicated that soil bactericide application eliminated the positive feedback.(4)Intercropping with potato onion increased the concentration of malic acid,citric acid,fumaric acid,aspartic acid,phenylalanine,arginine,valine,isoleucine,leucine,tryptophan,fructose,total organic acids and total amino acids in tomato root ex udates,and decrease that of glutamate and serine.Aspartic acid and phenylalanine were detected only in the tomato root exudates of intercropping system.(5)The tomato root exudates of intercropping system were more attractive to Pseudomonas P8,Bacillus P1 and Bacillus P2 and induced their biofilm formation than those of monoculture system.The higher concentrations of altered root exudation profile,the more they were to promote the biofilm formation and chemotaxis of Pseudomonas P8,Bacillus P1 and Bacillus P2.These results indicated that changes in tomato root exudates caused by intercropping with potato onion were beneficial in improvement of rhizosphere colonization of Pseudomonas P8,Bacillus P1 and Bacillus P2.(6)Tomato root exudates from intercropping system stimulated the expression of 2,4-diacetylphloroglucinol and pyoluteorin-producing genes of Pseudomonas P8 and the expression of fengycin and surfactin-producing genes of Bacillus P1 and P2.Higher concentrations of altered root exudation profiles were able to increase the expression of these genes.These results indicate that changes in tomato root exudates induced by intercropping with potato onion were beneficial to stimulate the production of antibiotics by Pseudomonas P8,Bacillus P1 and Bacillus P2.Overall,intercropping with potato onion improved the PGPR colonization ability and stimulated the ability of PGPR to produce antibiotics by changing the composition and concentration of tomato root exudates,and changes in tomato rhizosphere microbial community caused by intercropping trigger the tomato induced systemic resistance,and thus enhanced the resistance of tomato against Verticillium wilt. |
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