A Toolbox to Clone QTLs for Host and Nonhost Resistances in Barley

Cloning genes for nonhost resistance (NHR) and for quantitative host resistance (QHR) to specialized biotroph pathogens is a challenge. To date, only five large-effect resistance QTLs have been cloned for QHR, and none for NHR. We have developed the following tools to map-based clone genes for these two types of resistance in barley.

At seedling stage, barley (Hordeum vulgare) may show some degree of susceptibility to unadapted Puccinia rust fungi and Blumeria graminis powdery mildew fungi. We accumulated such susceptibility in the experimental line SusPtrit (Su), resulting in exceptional susceptibility to at least ten unadapted rust fungi. Standard cultivar Vada (Va) shows nonhost immunity to unadapted rust fungi and high partial resistance to the adapted P. hordei. In the Va/Su mapping population many QTLs for resistance to Puccinia rusts and Blumeriamildews were mapped.

Phenotyping is based on inoculation in a settling tower, and counting infections as flecks or pustules  (rusts) or micro-colonies (mildew).  We find recombinants using EST based markers from a dense integrated linkage map of over 5000 SNP markers, in combination with high throughput genotyping by LightScanner^® technology. We developed non-gridded BAC libraries for Va and for Su. Finally, we developed an experimental line, Golden SusPtrit, which combines the exceptional susceptibility of Su with the amenability for transformation of Golden Promise. This line can be transformed with candidate resistance genes to test their effect on level of infection.

Presently eleven NHR and QHR genes have been fine-mapped, of which four have been physically mapped to BAC clones.