| Wheat(Triticum aestivum L.) provides staple food to the human population and constitutes a concern of food security. The disease of sharp eyespot has escalated into a major threat to wheat production in many regions of the world, especially in China where it has been considered by the Ministry of Agriculture as a destructive disease comparable to stripe rust and fusarium head blight. Sharp eyespot is mainly caused by the soil-borne fungus Rhizoctonia cerealis in our country, and no commercially planted wheat cultivar has yet been found to be immune fron the fungus. However, some cultivars exhibit their sharp eyespot intensity much lower than the others, or possess some degree of quantitative resistance to the disease. Wheat resistance to sharp eyespot can be a potential means to reduce the needs for application of fungicides, and thus to save the cost of wheat production and to alleviate environmental pollution. A strategy is to screen the germ plasm resources within wheat species and to map the quantitative trait locus (QTL) for the resistance. Six studies have been reported by several researchers on detection of QTL for sharp eyespot resistance in Chinese journals, while only one report is available in international journal that was reported by the present author in Theoretical and Applied Genetics in2013.In the present study,47wheat cultivars that have been commercially planted widely in the’hotspot’ areas of sharp eyespot in the provinces of Shandong and Shanxi were collected and screened for sharp eyespot resistance, together with some wheat germ plasm resources including Luke and AQ24788-83. The results showed that none of the commercial cultivars were immune from the disease and six of them were moderately resistant to the disease, whereas Luke and AQ24788-83were significantly much less diseased in comparison with all the commercial cultivars. At the same time,55isolates of Rhizoctonia spp. were collected and tested, and it was found that the majority of the isolates were R. cerealis and they showed moderate to weak virulence.The recombinant inbred line (RIL) population of Luke×AQ24788-83was previously constructed in our laboratory consisting of1589F8RILs, from which241ones were sampled by the present author. The241RILs were assessed for sharp eyespot resistance by conducting field and greenhouse trials during the period from2008to2012, and were used to construct a chromosome linkage map containing605simple sequence repeat (SSR) DNA marker loci, the wheat map that has the largest number of SSR loci reported up to date in the world. Analyses based on these phenotype and genotype data found eleven quantitative trait loci (QTLs) to be associated with the sharp eyespot resistance and seven of them expressed consistently across the five trial environments. The seven were designated as Qse.cau-1AS, Qse.cau-2BS, Qse.cau-3BS, Qse.cau-4AL, Qse.cau-5DL, Qse.cau-6BL, and Qse.cau-7BL. Four of these QTLs are unequivocally novel, while it is possible that the other three might also be novel. Plant height and heading date of the241RILs were recorded in the four field trials. All of the seven disease resistant QTLs were independent of plant height and heading time except one that was significantly associated with plant heading time, suggesting that the six QTLs did confer a resistance to the disease instead of an escape from it.A sample of898from the1589Luke×AQ24788-83RILs were evaluated phenotypically for sharp eyespot resistance in four field trials inoculated artificially with a R cerealis isolate of strong virulence or infected with naturally occurring R. cerealis populations. In addition,3%of the898RILs were selected for either extreme resistance (i.e.,27RILs) or extreme susceptibility (another27RILs), and the54RILs were evaluated for sharp eyespot resistance in two greenhouse trials inoculated artificially with a R. cerealis isolate of strong virulenc. These trials showed that the resistance or susceptibility of the RILs was inherited readily across the different experimental environments and that trasgressive inheritance for both resistance and susceptibility occurred. The898RILs were genotyped at the seven detected QTLs, and six of the QTLs were used for examining QTLs accumulation effect. Sixty-four recombinations among the six QTLs (26=64) were compared for the aerea under the disease progress curve (AUDPC). Of the898RILs,17ones had the genotype RRRRRR (i.e., the17RILs had resistant allele at each of the six QTL loci). The mean AUDPC of RRRRRR was significantly lower than or not statistically higher than those of the other63genotypes. Analyses of QTLs accumulation suggested that Luke might harbor some QTLs that have not yet been detected, and that AQ24788-83might harbor some inhibitor or suppresser that might interfere with the expression of the four QTL resistance alleles contributed by AQ24788-83. Seventeen RILs were selected phenotypically for extremely low AUDPC from the898RILs for comparison between the selection based on QTL genotype and the selection based on AUDPC phenotype. The comparison showed a high degree of consistency between the two different selections, and a high degree of consistency between the field trial result and the greenhouse trial result. With the increasement of the number of QTLs accumulated, the AUDPC value decreased steadily. According to these results, I believe that accumulation of QTLs would constitute a strategy for improving sharp eyespot resistance in wheat. |
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