Research On The Mechanism Of Resistance To Fusarium Oxysporum F.sp.niveum Race 1 In Tetraploid Watermelon

Fusarium wilt diease of watermelon is caused by a soilborne fungus Fusarium oxysporum f.sp.Niveum in worldwide.In recent years,with the increasing of continuous cropping obstacle of watermelon,Fusarium wilt disease has become one of the main factors limiting the production of watermelon.Tetraploid watermelons were found to be more resistant to Fusarium wilt disease than their diploid plants in the field.In order to reveal the resistance mechanism of tetraploid watermelon to Fusarium wilt,a diploid watermelon inbred line Zhengzhou No.3 and its autotetraploid watermelon seedlings were used as the experiment materials in this research.After inoculation with Fusarium oxysporum f.sp race 1,the disease resistance difference,the infection process of the pathogens,the content change of metabolites,the changes of DNA methylation and gene expression difference were compared between diploid and tetraploid watermelon seedlings roots.The resistance mechanism of tetraploid watermelon to Fusarium wilt disease was revealed according to our results,which could provide a reliable scientific basis and technical support for the breeding of polyploid watermelon and disease resistance.The main results were as follows:The resistance identification of Fusarium wilt disease: After inoculation with Fon race 1,diploid and tetraploid watermelon seedlings of Zhengzhou No.3 were susceptible to Fusarium oxysporum.The infection in tetraploid watermelon was significantly slower than diploid plants.The wilt symptom delayed 3 days in the tetraploid seedings.The results showed that tetraploid watermelon was more resistance to Fusarium wilt than diploid ones.A green fluorescent protein(GFP)expressing strain to Fusarium oxysporum was used to visualize the infection process by a laser confocal microscopy,our results showed that the Fusarium oxysporum mainly infected from the wounds,natural cracks between the main root and lateral root,root hairs and so on,then quickly spread in the root.The colonization rate in tetraploid watermelon seedlings was observed lower than that in diploid plants,it was found a significant difference in the seedling roots at 4 days and 7 days after inoculation,extremely significant difference was found in the leaf stalk.Taken together,tetraploid watermelon delayed the colonization of Fusarium oxysporum.The activities of peroxidase(POD)and phenylalanine ammonialyase(PAL),the contents of malondialdehyde(MDA),total phenol and flavonoid in watermelon roots after 0d,1d,4d,7d and 10 d inoculation were analized.The activities of POD and PAL,and the contents of total phenols and flavonoids in tetraploid root were higher than those in diploid ones,while the MDA content was lower in tetraploid seedlings.After the infection of Fusarium oxysporum,the cell membrane damage in tetraploid watermelon root was little,the activies of disease-resisting enzymes and contents of secondary metabolites were higher,which was more favorable to resist the infection and colonization of Fusarium oxysporum.DNA methylation changes: There was no significant difference between different ploidies in DNA methylation levels before inoculation,however,after the infection of Fusarium oxysporum,the total methylated level and full methylated level of the roots were decreased,while the hemimethylated level remained unchanged.There were more demethylation and hypermethylation changes in diploid watermelon,the methylation rate of tetraploid watermelon was slightly higher than that of diploid plants.After inoculation with Fusarium oxysporum,the expression levels of MYB,MPK7,PAL and chitinase in tetraploid watermelon roots were 80-fold,3-fold,6-fold,and 10-fold higher than those of diploid ones,respectively.The molecular biology reasults showed that tetraploid watermelon seedlings were more resistant to Fusarium wilt disease than their diploid ones.