Abstract
Leaf scald, caused by Xanthomonas albilineans, is a major sugarcane disease worldwide. The disease is managed primarily with resistant cultivars obtained through classical breeding. However, erratic symptom expression hinders the reliability and reproducibility of selection for resistance. The development and use of molecular markers associated with incompatible/compatible reactions could overcome this limitation. The aim of the present work was to find leaf scald resistance-associated molecular markers in sugarcane to facilitate marker-assisted breeding. A genetic linkage map was constructed by selective genotyping of 89 pseudo F2 progenies of a cross between LCP 85-384 (resistant) and L 99-226 (susceptible) using 1,948 single dose (SD) markers generated from SSR, eSSR, and SNPs. Of these, 1,437 SD markers were mapped onto 294 linkage groups, which covered 19,464 cM with 120 and 138 LGs assigned to the resistant and susceptible parent, respectively. Composite interval mapping identified 8 QTLs associated with the disease response with LOD scores ranging from 3.0 to 7.6 and explained 5.23 to 16.93% of the phenotypic variance. Comparative genomics analysis with Sorghum bicolor allowed us to pinpoint three SNP markers that explained 16% phenotypic variance. In addition, representative stress-responsive genes close to the major effect QTLs showed upregulation in their expression in response to the bacterial infection in leaf/meristem tissue.
Highlights
Sugarcane (Saccharum spp. hybrids) is a tropical C4 member of the Poaceae family, which accounts for 70% of the raw sugar produced worldwide (Le Cunff et al, 2008; Aitken et al, 2014)
The phenotypic resistance rating based on the severity of symptom expression in inoculated plants has been the standard method for assessment of the disease resistance response in different sugarcane clones
Erratic symptom expression, the association between environment and symptomatology, the possibility that some sugarcane cultivars can tolerate the pathogen without exhibiting symptoms (Rott et al, 1997), and the occasional systemic infection of inoculated resistant clones (Gutierrez et al, 2016) have made leaf scald resistance evaluation a difficult task
Summary
Sugarcane (Saccharum spp. hybrids) is a tropical C4 member of the Poaceae family, which accounts for 70% of the raw sugar produced worldwide (Le Cunff et al, 2008; Aitken et al, 2014). Cultivated sugarcane is derived from inter-specific hybridizations between two polyploid species Saccharum officinarum (2n = 8× = 80) and S. spontaneum (2n = 10× = 40–120) (Aitken et al, 2014). The hybridization involved the combination of vigorous growth, tolerance to abiotic stresses and disease resistance from S. spontaneum with agronomic characteristics, including high sucrose. Sugarcane Leaf Scald Resistance QTL content from S. officinarum. In the development of modern cultivars, the initial hybrids were backcrossed with S. officinarum to recover high sucrose content. The modern cultivars are complex aneu-polyploids with chromosome numbers of 2n = 100–120 (D’Hont et al, 1998; Aitken et al, 2014) that constitute approximately 80% of S. officinarum, 10–15% of S. spontaneum, and 5–10% recombinant chromosomes (D’Hont et al, 1996). The high ploidy level, the aneuploidy and the cytogenetic complexity have made sugarcane a challenge for breeding, genetics, and gene cloning (D’Hont and Glaszmann, 2001; Rossi et al, 2003)
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