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Abstract
Ustilago maydis, a fungal pathogen that causes corn smut disease, infects all aerial parts of maize plants and induces local tumors. The lack of U. maydis-resistant maize cultivars has led to significant corn yield losses. Therefore, it is important to identify new sources of resistance to U. maydis. Two maize-teosinte near-isogenic lines (NILs), resistant to U. maydis were identified from phenotypic evaluation of an introgression population created by crossing a maize inbred line (B73) and a teosinte (Zea mays ssp. parviglumis) inbred line (TIL11). Genotypic analysis identified a 3.9-Mbp teosinte introgressed region on the short arm of chromosome 9 that was present in the two resistant NILs but was absent in the susceptible NILs, suggesting the teosinte introgressed region was responsible for the resistant phenotype. Comparative analysis of the two parental lines and 25 Nested Association Mapping population founder lines identified 69 genes in the 3.9-Mbp region, with 24 genes classified as putative disease resistance, pathogenesis related or defense response genes. RNA sequence analysis of TIL11, B73, and the two NILs identified 24 differentially expressed genes (DEGs) from the 3.9-Mbp region that were also putative disease resistance, pathogenesis related or defense response genes. Four DEGs (1. PK_Tyr_Ser-Thr/Pkinase; 2. LTP_2/Tryp_alpha_amyl; 3. TPR_1; 4. no Pfam) were upregulated in TIL11 and the two NILs and were selected as candidate genes for resistance. To investigate the role of the four genes in response to U. maydis infection, Foxtail Mosaic Virus-induced gene silencing was used to knock down the expression of each gene in TIL11 and the two NILs. The resistance of TIL11 and the two NILs was significantly reduced in the knock down plants and correlated with the reduced relative expression observed for the four genes. These results indicate that the four candidate genes identified in the 3.9-Mbp region may be positively regulating resistance to U. maydis in the resistant TIL11 parent and the two resistant NILs, identifying teosinte as a source of resistance. This work provides new insight into the genes involved in resistance and will open the door for targeted strategies to generate maize lines with enhanced resistance to U. maydis.