Mapping QTL and Identifying Candidate Genes for Resistance to Brown Stripe in Highly Allo-autopolyploid Modern Sugarcane

Disease resistance is one of the most important target traits for sugarcane genetic improvement. Sugarcane brown stripe (SBS) caused by Helminthosporium stenospilum is one of the most destructive foliar diseases, which not only reduces harvest cane yield but also sugar content. This study aimed to identify quantitative trait loci (QTL) and candidate genes associated with SBS resistance. Here, the phenotypic investigation in the field of six habitats showed a continuous normal distribution, revealing that the SBS resistance trait was a quantitative trait. Two high-density linkage maps based on the single dose markers calling from the Axiom Sugarcane100K SNP chip were constructed for the dominant sugarcane cultivars YT93-159 (SBS-resistant) and ROC22 (SBS-susceptible) with a density of 2.53 cM and 2.54 cM per SNP marker, and mapped on 87 linkage groups (LGs) and 80 LGs covering 3,069.45 cM and 1,490.34 cM of genetic distance respectively. A total of 32 QTL associated with SBS resistance were detected by QTL mapping, which explained 3.73%–11.64% of the phenotypic variation, and the total of phenotypic variance explained (PVE) in YT93-159 and ROC22 were 107.44% and 79.09%, respectively. Among these QTL, four repeatedly detected QTL (qSBS-Y38-1, qSBS-Y38-2, qSBS-R8, and qSBS-R46) were considered as stable QTL. Meanwhile, two major QTL, qSBS-Y38 and qSBS-R46, could account for 11.47% and 11.64% of the PVE respectively. Twenty-five disease resistance candidate genes were screened by searching these four stable QTL regions in their corresponding intervals, of which Soffic.01G0010840-3C (PR3) and Soffic.09G0017520-1P (DND2) were significantly up-regulated in YT93-159 by qRT-PCR, while Soffic.01G0040620-1P (EDR2) was significantly up-regulated in ROC22. These results will provide insightful information for future studies of sugarcane breeding in combating this disease.