Differential Production Of The Phytotoxin Thaxtomin A By Potato Common Scab-causing Streptomyces Spp. In Inner Mongolia

Inner Mongolia is a major province that provides seed and ware potato to the rest potato planting areas in China.The increasing occurrence of potato common scab in Inner Mongolia threatens potato productions in this country.In this study,we selected 6 species of Streptomyces spp.that were reported to cause potato common scab in Inner Mongolia,including 3 strains of the species that were most widely distributed within the province,and one strain for each of the rest 5 species,then confirmed their identities by Multilocus Sequence Analysis(MLSA).Pathogenicity of each strain was evaluated by calculating the growth inhibition ratio of radish seedlings by each stain,and identifying their Pathogenetic Island type.To explain differential pathogenicity between different strains,we measured the quantity of Thaxtomin A that were produced by each strain,analyzed the correlation of Thaxtomin A's production with radish seedlings' length,and measured the expression level of the genes that are involved in the biosynthesis pathway of Thaxtomin A.Our research shows that different species of common scab pathogens have different productions of Thaxtomin A,which will provide a theoretical basis for the formulation of control strategies for common scab caused by different pathogens.Our major findings are followings:(1)MLSA confirmed 8 strains' species,3 were Streptomyces galilaeus,and the rest 5 strains belongs to S.scabiei,S.europaeiscabiei,S.turgidiscabies,S.diastachromogene and S.bobili,respectively.(2)All 8 strains were pathogenic bacterium.All of them were able to cause brown corky lesion on potato slices,and inhibit the growth of radish seedlings.(3)All 8 strains have the pathogenic genes txt AB,nec1 and tom A.(4)The pathogenic islands type of S.turgidiscabies is PAISt,and the others have the typical PAI.(5)The production capacity of Thaxtomin A was different in different strains.The results of ANOVA show that there is no significant difference in the Thaxtomin A's production among 3 strains of S.galilaeus.The production of Thaxtomin A of species S.scabiei,S.turgidiscabies and S.bobili were significantly higher than those of S.europaeiscabiei and S.diastachromogenes.The production of Thaxtomin A is negatively correlated with the radish seedlings' length(P < 0.05).(6)The differences in Thaxtomin A production among strains could not be explained at the transcriptional level.Gene expression analysis(txt AB,txt C,txt D,txt E,txt H,txt R,tom A,nec1,ceb R,ceb E,msi K,and bgl C)results showed that the significant differences(P < 0.05)in the expression level were only found in genes of txt AB,txt C and txt R among 3 strains of S.galilaeus.No significant differences were fund in the expression of any genes when we compared different species.