Advances in understanding barley Pyrenophora graminea interactions

Pyrenophora graminea Ito and Kuribayashi (anamorph Drechslera graminea) is a seed-borne pathogen causing barley leaf stripe. The disease is widely distributed in most barley growing areas, where it causes serious damage and yield losses. At present disease control is focused mainly on the application of fungicides, but the risk of naturally occurring resistance within the pathogen populations and the increasing legal limitations to the use of fungicides requires the development of new strategies for disease control. To this aim we are currently applying breeding procedures and studying the molecular bases of P. graminea-barley interaction. The interaction P. graminea-barley provides a good model for understanding the molecular basis of diseases caused by seed-borne pathogens. The expression of defence-related genes occurs in barley tissues inoculated with P. graminea, a response that is regulated by several factors depending on both the fungal isolate and the barley cultivar. The induction of these genes was monitored in barley cultivars inoculated with two highly virulent isolates of the pathogen Dg2 and Dg5 (also called I2 and I5 in previous papers) and in barley tissues infected with a hypovirulent mutant of the isolate Dg2. In the former, it was found that plants challenged by the isolates Dg2 and Dg5 responded with the induction of genes coding for peroxidases, thionins and thaumatin-like proteins. A single genetic factor controlling complete resistance to P. graminea,derived from Hordeum laevigatum via cv "Vada", has been introduced into most resistant North-European two-rowed spring barley cvs. This "Vada resistance" was probably introgressed into the barley genome along with the MlLa (Laevigatum) powdery mildew resistance, because the two factors have been found to be linked; this resistance, named as Rdg 1a, has been mapped on the long arm of barley chromosome 2. It is also known that cvs quantitatively resistant to leaf stripe are widespread in Europe. A major QTL conferring resistance to barley leaf stripe was identified in the cv "Proctor" by means of QTL analysis; this QTL accounted for 58.5% of the variation in the trait and was mapped on the centromeric region of barley chromosome 1. The cv "Thibaut" was identified as highly resistant to the most virulent Italian isolate, isolate Dg2. We have described the characterization of the genetic basis of the cv "Thibaut" resistance, the mapping of the locus harbouring this resistance gene and the development of PCR-based molecular markers associated with the resistance, and useful for marker-assisted selection. Near-isogenic lines (NILs) carrying resistance of the cv "Thibaut" against the highly virulent isolate Dg2 were obtained by introgressing the resistance into the genetic background of the susceptible cv "Mirco". The "Thibaut resistance gene" was mapped and called Rdg2a. A PCR-based marker (MWG2018) associated with the resistance gene has been developed to assess the leaf stripe resistant phenotype in barley breeding programs. Several resistant genotypes were identified and among them four were shown to possess the same allele of the cultivar Thibaut at the marker locus. One of them, the cultivar Rebelle, is being used as a source of leaf stripe resistance in winter barley breeding programs. In several resistant lines obtained from five crosses, in which Rebelle acted directly or indirectly as donor of the resistance, the allelic composition at the MWG2018 locus was verified. The results showed that the resistant phenotype of the lines was always associated with the resistance allele of the molecular marker, thereby demonstrating its reliability for selecting the leaf stripe resistance