Effects Of Nitrogen Turnover On The Pathogenicity Of Fusarium Graminearum After Straw Returning To The Field Under Indoor Conditions

In China,straw yield is huge and it is an important biomass resource.At present,returning straw to the field is an important way to deal with crop straw,which can not only solve the problem of excess straw,but also improve soil fertility.However,during the process of returning straw to the field,there are often some pathogenic fungi that enter the soil with the straw.The pathogenic fungi usually do not die after entering the soil,but they would decay on the straw,which may lead to large-scale disease outbreaks in the coming year and affect crop growth.Straw returning to the field leads to the increase of soil carbon content,and the number of soil microorganisms increased hugely,result the soil nitrogen content decreased.Then,due to the decomposition of straw and the supplement of nitrogen fertilizer,the nitrogen content in the soil environment increased.Therefore,after the pathogenic fungi enter the soil,it would undergo a nitrogen turnover process in which nitrogen is deficient and then recovered.Under low nitrogen conditions,pathogenic fungi undergo autophagy to ensure their survival and growth,however the effects of autophagy by pathogenic fungi on their growth and pathogenicity remains to be studied.In this study,Fusarium graminearzum was selected as the research object.The indoor conditions were used to simulate the nitrogen turnover after the straw returned to the field.Among them:5 different nitrogen levels(0%,25%,50%,75%and 100%)were set in the nitrogen starvation test,medium and mycelium were sampled at 0,2,8,24,48,72,120 and 168 hours of the experiment;The nitrogen recovery test was to culture the mycelium at 5 nitrogen levels for 168 h and then transfer to the total nitrogen medium,sampled at 0,2,24,48 and 72 hours.Then the growth,pathogenicity and autophagy of Fusarium graminearum were measured.The main results were as follows:(1)When Fusarium graminearum was cultured in nitrogen starvation medium,the tendency of mycelium weight in all nitrogen level showed increasing first and then decreasing.The growth of mycelium was promoted in the case of partial nitrogen deficiency,the weight of the mycelium reached the maximum value of 0.39 g/mL when cultured in 25%nitrogen level for 72 hours,which was significantly higher than other treatments.When Fusarium graminearum was re-cultured in total nitrogen medium,it didn't grow explosively.With the passage of cultivation time,the weight of mycelium in all nitrogen level gradually tended to be same,and the weight of mycelium per milliliter of medium was about 0.17 g.(2)The germination rate of conidia was promoted in deficiency medium.Under complete nitrogen deficiency,the germination rate of conidia reached 64.5%,which was significantly higher than other treatments,and it was twice of total nitrogen treatment.(3)The toxin production of Fusarium graminearum would promoted in the part of nitrogen deficiency medium,and the toxin content in wheat grains could reach 0.32 ?g/g when cultured in 25%nitrogen level for 72 hours,which was significantly higher than other treatments;Toxicity of Fusarium graminearum was increased significantly after nitrogen recovery.Among them,the toxin production of 25%and 75%group reached 1.36 ?g/g and 1.41 ?g/g when nitrogen recovery for 24 hours and 72 hours,toxin content was 4 to 5 times as much as that of corresponding nitrogen starvation treatments.(4)When Fusarium graminearum was cultured in nitrogen starvation medium,the infectivity of Fusarium graminearum was increased with the passage of the cultivation time,and eventually it reached a stable state.Among them,when cultured in 25%nitrogen level for 72 hours,the infection ability of Fusarium graminearum was strong.The infection length of the wheat seedlings could reach 18.33 mm,which was significantly higher than that of complete nitrogen deficiency treatment;In the initial stage of nitrogen recovery,the infection length was 4.19 mm,and the infection length increased to 7.73 mm after nitrogen recovery for 72 hours,but still significantly lower than the treatment of 25%,50%and 100%nitrogen levels.(5)The autophagy was different in Fusarium graminearum under different nitrogen levels.Through the TEM micro structure observation,it was found that the vacuole in the mycelium become larger after nitrogen starvation,and autophagosomes appeared in the vacuole,confirming that autophagy was occurring.When cultivated for 168 hours in complete nitrogen deficiency medium,a large number of empty vacuoles appeared in the mycelium,indicated that the mycelium were about to die.The roles of growth,pathogenicity and autophagy of Fusarium graminearum after nitrogen turnover studied here would help to analyze the change of pathogenicity of Fusarium gramineae in soil.