Deciphering The Effects Of Fhbl On Kernel Resistance To FHB Infection And On Mycotoxin Accumulation

Fusarium head blight(FHB),mainly caused by Fusarium graminearum(.g.),is one of the most destructive diseases in wheat worldwide.The resistance types can be classified into four categories:resistance to initial infection(Type I),resistance to spread within infected spikes(Type II),kernel resistance to infection(Type III)and resistance to toxin accumulation(Type IV).Among them,the most extensive research focuses on type II.The current method for evaluating Type III is based on the percentage of Fusarium-damaged kernels(FDK),which is an indirect measurement of resistance to spread instead of kernel resistance.However,the kernel resistance to F.g.infection is literally the resistance of the grain itself to the pathogen infection when the grain is exposed to the pathogen under the optimum temperature and humidity conditions.Fhb1 on 3BS is a major and stable QTL for Type II resistance.To understand whether Fhb1 mediates kernel resistance to F.g.infection,we studied the effects of Fhb1 on kernel tolerance in a pair of resistant and susceptible near-isogenic lines(NILs)of Fhbl.Meanwhile,in order to understand the relationship of the tissues and DON production,we studied the effects of the rachis,grain and glume on DON production by Fusarium graminearum and the impacts of the different tissues on the expressions of Tri in vitro.In addition,DON production in 30 varieties differring in FHB resistance were evaluated using two inoculation methods including bilateral floret inoculation(BFI)method and basal rachis internode injection(BRII)method,and the relationship between proportion of symptomatic spikelets(PSS)and DON content under different inoculation methods were also investigated.The varieties both with low disease severity and low DON accumulation were selected.The main results are as follows:The florets became white when each floret of the spike was inoculated at different developmental stages.The change of 1000-grain weight was mainly due to the decrease of grain width.Interestingly,the change of 1000-grain weight of S22V(the susceptible NIL without Fhbl)was smaller than that of R22W(the resistant NIL with Fhbl).At same developmental stage,more amyloid degradation was observed in the kernel of R22W than that in S22V.The result of scanning electron microscopy(SEM)showed that the grains of the resistant variety Ning7840 with Fhb1,the resistant variety HYZ without Fhb1 and the susceptible line S22V without Fhb1 had better resistance to infection of the hyphae than the other varieties despite of Fhb1,and R22W and S22V had no significant difference in promotion of the growth of hyphae.The results both in vivo and in vitro indicated the kernels of different genotypes may differ in tolerance to F.g.infection,but Fhb1 has no effect on kernel resistance to infection.The study developed a comprehensive evaluation system for Type III resistance and enriched our understanding the function or effects of Fhbl.Our experiments in vitro showed that glume of wheat spike could inhibit the growth of hyphae,but kernel and rachis induced the growth of hyphae.The rachis,kernel and glume at the early stage of wheat development did not induce F.g.to produce DON,however,the kernel at the middle and late grain-filling stage significantly induced.g.to produce DON.Unexpectedly,more DON was induced by kernel of R22W than that of S22V.The expressions of Tri genes had temporal and spatial specificity and were regulated by different tissues.These results enriched our understanding of the mechanism of DON production and of the relationship between Fhb1 and DON.Significant positive correlations between DON content in grains and PSS under two inoculation methods was observed,however,there were also exceptions to part of genotypes.Under BRII method,DON accumulation still occured in kernels of the varieties with Fhb1 despite the fact that no diseased spikelets were observed(PSS=0).Therefore,we proposed that the rachis probably induced DON production.The combined analysis based on PSS and DON content under two inoculation methods showed TOKAI66,BSYH,Ning7840,NyuBai,HCD,Chokwang,and Tawanhsiaomai have better overall resistance to FHB than Sumai3.TOKAI66 had the lowest DON content and the lowest PSS among the varieties,this variety could be used to develop cultivars both with low PSS and low DON content,which would be critically important in breeding to ensure both food security and food safety.