Mechanisms Of Enhanced Disease Resistance Of Tomato To Verticillium Dahliae Induced By Potato Onion As A Companion Crop

Verticillium Wilt is a severe soil-borne disease for tomato(Solanum lycopersicum), caused by Verticillium dahliae. Recent years it was more severe because of the continuous monocropping of tomato, thus seriously influenced the yield and quality of tomato. The pesticides were excessively used in tomato production for controlling tomato disease, which brought some problems such as food safty, soil eviroment safty, and threatened people’s health and soil ecological environment. Previous studies showed companion cropping with potato onion could alleviate tomato disease, however, the mechanisms are still unclear up to now. The objectives of this study were to 1) evaluate the effects of companion cropping with potato onion on tomato Verticillium Wilt incidence and disease index, 2) investigate the effects of tomato and potato onion root exudates on growth and spore germination of V. dahlia, 3) explore the resistance response of tomato to V. dahliae by RNA-seq, physiology and biochemistry methods, respectively. The purpose of this study was to reveal the mechanisms of comapnion cropping with potato onion alleviating tomato Verticillium Wilt on the level of physiology and molecule. This study could cast light on the knowledges of alleviating plant soil-borne diseases in tomato production affected by companion cropping with potato onion. It also can provide reference to the application of companion cropping and other studies.The main results of this study were as following:(1) The pot experiments demomstrated that companion with potato onion decreased incidence and disease index of tomato Verticillium Wilt, hence alleviating tomato Verticillium Wilt.(2) The root exudates from tomato and potato onion were collected by dipping roots into water, and its antifungal effects were detected in vitro. The results demonstrated the root exudates of onion from both monocropped and companioned potato onion, and the root exudates from monocropped tomato(TM) were no effects on the growth and spore germination of V. dahliae in vitro. But, the root exudates from companioned tomato by potato onion(TC) had significant suppressive effects on the two parameters at the concentration of 1g FW·10mL-1. These results suggested that companion cropping with potato onion induced the antifungal active of tomato root exudates on V. dahliae,hence enhancing tomato denfense to V. dahliae.(3) RNA-Seq was employed to explore the gene expression response of tomato to V. dahliae. Differencial gene expression analysis indicated that 369 Differentially Expressed Genes(DEGs) were screened between TC and TM. Compared to TM, there were 307 DEGs were up-regulated and 62 down-regulated in TC, with the percentage of 83% and 13%, respectively. Furthermore, total 5 condition specific expression genes were detected, 3 in TM and 2 in TC, respectively. Gene Ontology Analysis of DEGs showed the DEGs wth the molecular functions of “catalytic activity”, “binding activity”, “transport activity” and “antioxidant activity” were enriched and up-pregulated. Among them the DEGs relate to “catalytic activity” and “binding activity” were most enriched and occupied the percentage of 40.6% and 24.4% in total DEGs, respectively. Pathway Enrichment Analysis showed the DEGs involved in “Metabolic pathways”, “Biosynthesis of secondary metabolites”, “Phenylpropanoid biosynthesis”, “Glutathione metabolism”, “Plant hormone signal transduction”, “Phenylalanine metabolism”, “Plant-pathogen interaction”, “Flavone and flavonol biosynthesis”, “Flavonoid biosynthesis”, “Cysteine and methionine metabolism” were enriched and up-regulated. Among them the DEGs relate to “Metabolic pathways” and “Biosynthesis of secondary metabolites” were most enriched and occupied the percentage of 33.2% and 25.6% in total DEGs, respectively. The DEGs involved in lignin biosynthesis(18 genes), phytohormone metabolism and signal transduction(55 genes), proteins(enzymes) related to disease resistance(26 genes), and metabolism of sulfur-containing compounds(33 genes) were all up-regulated in TC than those in TM. The percent of the DEGs related to disease resistance was 40% in total up-regulated DEGs. The DEGs which were down-regulated in TC compared to those in TM were almost irrelevant to disease defense, and the the percentage of the DEGs with unknown functions was 46.7% in all down-regulated DEGs.(4) The physiological responses of tomato to V. dahliae were evaluated at different time after inoculating with V.dahliae. The results showed companion cropping with potato increased the contents of lignin, total soluble phenolics, glutathione and ethylene, and decreased the malonaldehyde content and superoxyde dismutase activity. The enzyme activities of peroxidase(POD), polyphenol oxidase(PPO) and phenylalnine ammonialyase(PAL) all increased in tomato plants companioned with potato onion at different stages after inoculation with V. dahliae. These results suggested companioned with potato onion enhanced the resistance to V. dahlia through increasing the defensive substances content and improving the defence related enzyme activity.(5) Sulfur played an important role in potato onion-tomato-V. dahliae interaction in potato onion/tomato companion cropping. Neither companion cropping nor inoculation with V. dahliae induced the up-regulation of High affinity sulfate transporter gene 3(ST3), but the ST3 gene was up-regulated when tomato was companioned by potato onion and inoculated with V. dahliae simultaneously. Also, the content of total sulfur, glutathione, and ethylene in tomato roots were all increased when tomto was companioned by potato onion and inoculated with V. dahliae simultaneously, while the available sulfur contents in tomato rhizosphere did not. The results suggested the up-regultion of ST3 may be regulted via the positive feedback caused by enhanced sulfur metabolism activity in tomato, rather than the change of rhizosphere soil sulfur content. Potato onion root exudate has no significant effect on soil available sulfur content.Conclusion:The study demonstrated that companion cropping with potato onion alleviated tomato Verticillium Wilt, and enhanced the resistance of tomato to V.dahliae. The mechanisms of companion cropping with potato enhacing the resistance of tomato to V.dahliae was as follows: Companion with potato onion induced the antimicrobial activity of tomato root exudates, by regulating the expressions of genes related to secondary metabolites biosynthesis, hence enhancing the defense ability of tomato to V.dahliae. The genes involved in lignin biosynthesis, phytohormone metabolism and signal transduction, proteins(enzymes) related to disease resistance, and metabolism of sulfur-containing compounds were all up-regulated in tomato companioned by potato onion. Additionally, the content of lignin, total soluble phenolics, glutathione and ethylene, as well as defense enzyme(such as POD, PPO, PAL) activities all increased in tomato companioned by potato onion, thus hencing resistance of tomato to V.dahliae. This would contribute to alleviating tomato Verticillium Wilt.