Identification of novel genetic factors underlying the host-pathogen interaction between barley (Hordeum vulgare L.) and powdery mildew (Blumeria graminis f. sp. hordei).

Powdery mildew is an important foliar disease of barley (Hordeum vulgare L.) caused by the biotrophic fungus Blumeria graminis f. sp. hordei (Bgh). The understanding of the resistance mechanism is essential for future resistance breeding. In particular, the identification of race-nonspecific resistance genes is important because of their regarded durability and broad-spectrum activity. We assessed the severity of powdery mildew infection on detached seedling leaves of 267 barley accessions using two poly-virulent isolates and performed a genome-wide association study exploiting 201 of these accessions. Two-hundred and fourteen markers, located on six barley chromosomes are associated with potential race-nonspecific Bgh resistance or susceptibility. Initial steps for the functional validation of four promising candidates were performed based on phenotype and transcription data. Specific candidate alleles were analyzed via transient gene silencing as well as transient overexpression. Microarray data of the four selected candidates indicate differential regulation of the transcription in response to Bgh infection. Based on our results, all four candidate genes seem to be involved in the responses to powdery mildew attack. In particular, the transient overexpression of specific alleles of two candidate genes, a potential arabinogalactan protein and the barley homolog of Arabidopsis thaliana's Light-Response Bric-a-Brac/-Tramtrack/-Broad Complex/-POxvirus and Zinc finger (AtLRB1) or AtLRB2, were top candidates of novel powdery mildew susceptibility genes.

Evolutionarily conserved partial gene duplication in the Triticeae tribe of grasses confers pathogen resistance.

BACKGROUND: The large and highly repetitive genomes of the cultivated species Hordeum vulgare (barley), Triticum aestivum (wheat), and Secale cereale (rye) belonging to the Triticeae tribe of grasses appear to be particularly rich in gene-like sequences including partial duplicates. Most of them have been classified as putative pseudogenes. In this study we employ transient and stable gene silencing- and over-expression systems in barley to study the function of HvARM1 (for H. vulgare Armadillo 1), a partial gene duplicate of the U-box/armadillo-repeat E3 ligase HvPUB15 (for H. vulgare Plant U-Box 15). RESULTS: The partial ARM1 gene is derived from a gene-duplication event in a common ancestor of the Triticeae and contributes to quantitative host as well as nonhost resistance to the biotrophic powdery mildew fungus Blumeria graminis. In barley, allelic variants of HvARM1 but not of HvPUB15 are significantly associated with levels of powdery mildew infection. Both HvPUB15 and HvARM1 proteins interact in yeast and plant cells with the susceptibility-related, plastid-localized barley homologs of THF1 (for Thylakoid formation 1) and of ClpS1 (for Clp-protease adaptor S1) of Arabidopsis thaliana. A genome-wide scan for partial gene duplicates reveals further events in barley resulting in stress-regulated, potentially neo-functionalized, genes. CONCLUSION: The results suggest neo-functionalization of the partial gene copy HvARM1 increases resistance against powdery mildew infection. It further links plastid function with susceptibility to biotrophic pathogen attack. These findings shed new light on a novel mechanism to employ partial duplication of protein-protein interaction domains to facilitate the expansion of immune signaling networks.