Analysis of wheat (<em>Triticum aestivum </em>L.) resistance to cereal cyst nematode <em>Heterodera filipjevi</em> by genome-wide association mapping

The cereal cyst nematode Heterodera filipjevi is an important plant parasite causing substantial yield loss in wheat. The use of resistant cultivars is the preferable method to manage this nematode, however, only few resistance sources are identified in cereals and no durable resistant cultivar is available for H. filipjevi in wheat. In this study, a collection of 290 winter wheat accessions was screened for resistance against H. filipjevi. The plants were infected with nematode juveniles and the number of developing females and cysts was counted. One percent of the wheat accessions was ranked as resistant, 16% as moderately resistant, 41% as moderately susceptible, 26% as susceptible, and 15% as highly susceptible. To understand the underlying resistance mechanism, nematode invasion, development, and reproduction were analyzed in one resistant accession Nudakota and three moderately resistant accessions Ekonomka, Katea and Lantian 12 and compared with susceptible accession Bezostaya 1. Invasion rate, and number of females and cysts per plant were significantly lower in Nudakota, Ekonomka, Katea and Lantian 12 compared to Bezostaya 1. The different responses of wheat accessions observed in this study suggested genetic variation within the screened population. Therefore, it was decided to identify relevant quantitative trait loci (QTLs) and genes using genome-wide association studies. Association mapping is a powerful approach to detect associations between phenotypic variation and genetic polymorphisms; in this way favorable traits such as nematode resistance can be located in the genome. A mapping panel of 161 wheat accessions was genotyped by 90K iSelect SNP bead chip and was analyzed for QTL identification. These accessions were phenotyped in two nematode infection assays under controlled conditions. High variation was revealed in single nucleotide polymorphisms (SNPs) on wheat genomes A, B, and D. Linkage disequilibrium decayed across wheat genome was H. avenae, while QTL IWB66494 on chromosome 2BL was reported to be linked to resistance against Fusarium head blight. Although the wheat genome is sequenced, annotation of a large number of genes and proteins is still limited. In order to characterize the candidate gene TaAAT we applied a comparative genomics approach and identified the orthologue gene AtAAP6 in Arabidopsis thaliana. Nematode infection assays of an AtAAP6 mutant Arabidopsis line revealed a significant reduction in average number of females, female size, and female-associated syncytia. Different methods showed that AtAAP6 is upregulated in syncytia. We also found a strong expression of TaAAT in infected roots of two susceptible wheat accessions, whereas the expression remained unchanged in infected roots of two resistant accessions. We further analyzed the amino acid sequence of AtAAP6 in 3 lowly susceptible and 3 highly susceptible Arabidopsis accessions and found glycine is substituted by alanine on a specific site in the AtAAP ORF in the lowly susceptible accessions. The molecular mechanism by which the exchange of a single amino acid in AtAAP6 leads to reduced nematode development needs further investigation