ARS | Publication request: Molecular mapping of the Pl16 downy mildew resistance gene from HA-R4 to facilitate marker-assisted selection in sunflower

Submitted to: Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 4, 2012
Publication Date: June 1, 2012
Citation: Liu, Z., Gulya Jr, T.J., Seiler, G.J., Vick, B.A., Jan, C. 2012. Molecular mapping of the Pl16 downy mildew resistance gene from HA-R4 to facilitate marker-assisted selection in sunflower. Theoretical and Applied Genetics. 125:121-131.

Interpretive Summary: Downy mildew, caused by the fungus Plasmopara halstedii, is one of the more destructive diseases of cultivated sunflower (Helianthus annuus L.). To date, at least 37 races of downy mildew have been reported, including a newly emerged 'hot' race 734. The development of germplasms with diverse disease resistance genes is critical. The major genes controlling sunflower downy mildew resistance have been designated as the Pl genes. At least 23 downy mildew resistance genes have been identified, with ten Pl genes assigned to linkage maps using molecular markers. In this study, we report the molecular mapping of a Pl gene, Pl16, in a sunflower downy mildew differential line, HA-R4. It was mapped on the lower end of LG 1 of the sunflower reference map, with 12 markers covering a distance of 78.9 cM. One dominant SSR marker, ORS1008, co-segregated with Pl16, and another co-dominant EST-SSR marker, HT636, was located 0.3 cM proximal to the Pl16 gene. The HT636 marker was also closely linked to the Pl13 gene in another sunflower differential line, HA-R5. Thus the Pl16 and Pl13 genes were mapped to a similar position on LG 1 that is different from the previously reported Pl14 gene. When the co-segregating and tightly linked markers for the Pl16 gene were applied to other germplasms or hybrids, a unique band pattern for the ORS 1008 marker was detected in HA-R4 and HA-R5 and their F1 hybrids, suggesting that the Pl genes in HA-R4 and HA-R5 are different from other germplasms tested. Allelism tests among different Pl genes are needed to determine the relationship between Pl16 and the other Pl genes. This is the first report to provide two tightly linked markers for both the Pl16 and Pl13 genes, which will facilitate marker-assisted selection in sunflower resistance breeding, and provide a basis for the cloning of these genes. Fine mapping with more molecular markers, such as single-nucleotide polymorphisms and RGC markers, will help saturate the linkage map in these regions. Bacterial artificial chromosome libraries will provide a platform for cloning of other Pl genes, and thus help to understand the mechanism for downy mildew disease resistance and host-pathogen interaction for sunflower.

Technical Abstract: The major genes controlling sunflower downy mildew resistance have been designated as the Pl genes. Ten of the more than 20 Pl genes reported have been mapped. In this study, we report the molecular mapping of a Pl gene, Pl16, in a sunflower downy mildew differential line, HA-R4. It was mapped on the lower end of linkage group (LG) 1 of the sunflower reference map, with 12 markers covering a distance of 78.9 cM. One dominant simple sequence repeat (SSR) marker, ORS1008, co-segregated with Pl16, and another co-dominant expressed sequence tag (EST)-SSR marker, HT636, was located 0.3 cM proximal to the Pl16 gene. The HT636 marker was also closely linked to the Pl13 gene in another sunflower differential line, HA-R5. Thus the Pl16 and Pl13 genes were mapped to a similar position on LG 1 that is different from the previously reported Pl14 gene. When the co-segregating and tightly linked markers for the Pl16 gene were applied to other germplasms or hybrids, a unique band pattern for the ORS 1008 marker was detected in HA-R4 and HA-R5 and their F1 hybrids, suggesting that the Pl genes in HA-R4 and HA-R5 are different from other germplasms tested. This is the first report to provide two tightly linked markers for both the Pl16 and Pl13 genes, which will facilitate marker-assisted selection (MAS) in sunflower resistance breeding, and provide a basis for the cloning of these genes.