Virulence Variation Of Fusarium Oxysporum F.sp.cucumerinum In Continuously Cropped Soil Of Cucumber And Transcriptome Analysis Of The Highly And Weakly Virulent Strains

Fusarium oxysporum f.sp.cucumerinum(Foc)is the causal pathogen of cucumber Fusarium wilt resulting in losses to cucumber production.In China,the continuous cropping system of cucumber is commonly adopted in greenhouses,which leads to aggravated diseases.Many factors such as accumulation of the pathogens,deterioration of soil physicochemical properties,shift of soil microflora,and secretion of autotoxic substances are considered to contribute to the consecutive monoculture problems.Moreover,the virulence variation of the pathogens may also play an important role during the long term interaction between fungal and hosts.To understand the virulence variation of Foc in continuous cropping system,resistant and susceptible cucumber cultivars were successively planted for 5 generations,and rotation of cucumber with white,mize and clover was used as control.Virulence of Foc in soil under monoculture and rotation system of cucumber was assessment.Based on the sequence of TEF-1??histone H3 and IGS genes,the population genetic diversity was analyzed.To better understand the virulence variation mechanisms of Foc,a low virulence strain foc-3b and a virulence-enhanced variant Ra-4 induced by resistant cultivar were selected and their transcriptome profiles during infection were analyzed.The main research results are as follows:(1)Virulence variation of Foc in the cucumber continuously cropped soil The pathogenicity comparison among 205 strains from different crops indicated that after continuous cropping and rotation system of cucumber,the virulence of foc-3b diverged.After continuous cropping of cucumber,percentages of Foc with weakly virulent strains(Disease index,DI ? 40)significantly decreased and percentages of Foc with high virulence(DI > 60)increased from the third generation.Virulence of Foc strains showed increasing trend.What‘s more,the percentages of high virulence strains in soil under continuous cropping of resistant cultivar were higher than the strains in the soil continuously planted susceptible variety.However,the percentages of strains with different types of virulence in rotation soil had on significant change.(2)Population Genetic Diversity Analysis of Foc in continuously cropped soil The population genetic analysis based on the three genes showed the genetic differentiation of Foc population was obvious after continuously cropped cucumber.The genetic diversity of strains isolated from resistant cultivar was higher than that from susuceptible cucumber,but there is no relevance with its virulence.(3)Transcriptome Analysis of the Highly and Weakly virulent strains of Foc The transcriptome analysis of the two strains showed compared to vegetative growth in vitro,2,858 and 2,595 genes were differentially expressed in Ra-4 and foc-3b,respectively.Among them,540 differentially expressed genes(DEGs)commonly expressed in both strains,which principally involved in metabolic,transport,oxidation-reduction and plant cell wall degradation and remodeling.And 1,146 DEGs uniquely expressed in the highly virulent isolate Ra-4,which mainly related to energy metabolism,transport,oxidation-reduction processes,regulation of cellular process,macromolecules modification and stress and defence.Those common and unique DEGs might be critical to Foc pathogenicity and virulence variation.This study will greatly improve the current understanding of monoculture problems and lay foundation on further exploring the mechanisms underlying virulence differentiation of Foc.