Molecular genetics of barley (Hordeum vulgare): Leaf rust (Puccinia hordei) interactions

Abstract

Leaf rust caused by Puccinia hordei is one of the most common and widely distributed rust diseases of barley. Deploying genetic resistance is considered as the most effective and environmentally friendly approach to control this disease. Among 27 designated loci conferring resistance to P. hordei, three genes, namely Rph1, Rph15, and Rph22, have been previously isolated. In this study, the Rph3 gene was isolated using the positional cloning approach. A high-resolution map of the gene constructed using 10,411 F2 plants from six segregating populations delimited the Rph3 locus between two markers covering an interval of 0.024 cM. The physical map revealed that these two closest markers delimited a region of 8.5kb bp in Rph3-carrying cultivars such as cv. Barke and 98kb bp in cultivars lacking Rph3 such as cv. Morex. Two putative genes were determined within the 8.5kb region using FGENESH. The candidate gene for Rph3 was validated by mutational analysis, where four independent point mutations confirmed that ORF2 is Rph3. The resistance gene Rph3 encodes a protein of unknown function that does not belong to any known plant disease resistance protein family. The amino acid sequence showed no sequence homology to any domains/motifs of known function. Predictions using various tools revealed five to seven transmembrane helices in the structure of RPH3. Gene expression was locally induced at the infection site by pathotypes avirulent for Rph3. The current results suggest that the Rph3 gene acts like an executor gene that is regulated by transcriptional activator-like (TAL) effectors. A second study determined that resistance to P. hordei pathotype 5477 P- in the barley line 67:ZBS15 was conferred by two genes interacting in a complementary manner. One of these two genes is Rph3 and the other one was supposed to be Rph5 or an allele of Rph5 or an independent locus in the same genomic region as that of Rph5.