Convergent evidence for a role of WIR1 proteins during the interaction of barley with the powdery mildew fungus Blumeria graminis

Pathogen attack triggers a multifaceted defence response in plants that includes the accumulation of pathogenesis-related proteins and their corresponding transcripts. One of these transcripts encodes for WIR1, a small glycine- and proline-rich protein of unknown function that appears to be specific to grass species. Here we describe members of the HvWIR1 multigene family of barley with respect to phylogenetic relationship, transcript regulation, co-localization with quantitative trait loci for resistance to the barley powdery mildew fungus Blumeria graminis (DC.) E.O. Speer f.sp. hordei, the association of single nucleotide polymorphisms or gene haplotypes with resistance, as well as phenotypic effects of gene silencing by RNAi. HvWIR1 is encoded by a multigene family of moderate complexity that splits up into two major clades, one of those being also represented by previously described cDNA sequences from wheat. All analysed WIR1 transcripts accumulated in response to powdery mildew attack in leaves and all mapped WIR1 genes were associated with quantitative trait loci for resistance to B. graminis. Moreover, single nucleotide polymorphisms or haplotypes of WIR1 members were associated with quantitative resistance of barley to B. graminis, and transient WIR1 gene silencing affected the interaction of epidermal cells with the pathogen. The presented data provide convergent evidence for a role of the HvWIR1a gene and possibly other family members, during the interaction of barley with B. gramini

Basal host resistance of barley to powdery mildew: connecting quantitative trait loci and candidate genes

The basal resistance of barley to powdery mildew (Blumeria graminis f. sp. hordei) is a quantitatively inherited trait that is based on nonhypersensitive mechanisms of defense. A functional genomic approach indicates that many plant candidate genes are involved in the defense against formation of fungal haustoria. It is not known which of these candidate genes have allelic variation that contributes to the natural variation in powdery mildew resistance, because many of them may be highly conserved within the barley species and may act downstream of the basal resistance reaction. Twenty-two expressed sequence tag or cDNA clone sequences that are likely to play a role in the barley-Blumeria interaction based on transcriptional profiling, gene silencing, or overexpression data, as well as mlo, Ror1, and Ror2, were mapped and considered candidate genes for contribution to basal resistance. We mapped the quantitative trait loci (QTL) for powdery mildew resistance in six mapping populations of barley at seedling and adult plant stages and developed an improved high-density integrated genetic map containing 6,990 markers for comparing QTL and candidate gene positions over mapping populations. We mapped 12 QTL at seedling stage and 13 QTL at adult plant stage, of which four were in common between the two developmental stages. Six of the candidate genes showed coincidence in their map positions with the QTL identified for basal resistance to powdery mildew. This co-localization justifies giving priority to those six candidate genes to validate them as being responsible for the phenotypic effects of the QTL for basal resistanc

The barley (Hordeum vulgare) cellulose synthase-like D2 gene (HvCslD2) mediates penetration resistance to host-adapted and nonhost isolates of the powdery mildew fungus

Cell walls and cellular turgor pressure shape and suspend the bodies of all vascular plants. In response to attack by fungal and oomycete pathogens, which usually breach their host's cell walls by mechanical force or by secreting lytic enzymes, plants often form local cell wall appositions (papillae) as an important first line of defence. The involvement of cell wall biosynthetic enzymes in the formation of these papillae is still poorly understood, especially in cereal crops. To investigate the role in plant defence of a candidate gene from barley (Hordeum vulgare) encoding cellulose synthase-like D2 (HvCslD2), we generated transgenic barley plants in which HvCslD2 was silenced through RNA interference (RNAi). The transgenic plants showed no growth defects but their papillae were more successfully penetrated by host-adapted, virulent as well as avirulent nonhost isolates of the powdery mildew fungus Blumeria graminis. Papilla penetration was associated with lower contents of cellulose in epidermal cell walls and increased digestion by fungal cell wall degrading enzymes. The results suggest that HvCslD2-mediated cell wall changes in the epidermal layer represent an important defence reaction both for nonhost and for quantitative host resistance against nonadapted wheat and host-adapted barley powdery mildew pathogens, respectively.Dimitar Douchkov, Stefanie Lueck, Goetz Hensel, Jochen Kumlehn, Jeyaraman Rajaraman, Annika Johrde, Monika S. Doblin, Cherie T. Beahan, Michaela Kopischke, Ren, e Fuchs, Volker Lipka, Rients E. Niks, Vincent Bulone, Jamil Chowdhury, Alan Little, Rachel A. Burton, Antony Bacic, Geoffrey B. Fincher and Patrick Schweize

Positively Selected Disease Response Orthologous Gene Sets in the Cereals Identified Using Sorghum bicolor L. Moench Expression Profiles and Comparative Genomics

Disease response genes (DRGs) diverge under recurrent positive selection as a result of a molecular arms race between hosts and pathogens. Most of these studies were conducted in animals, and few defense genes have been shown to evolve adaptively in plants. To test for adaptation in the molecules mediating disease resistance in the cereals, we first combined information from the expression pattern of Sorghum bicolor genes and from divergence to the full genome of rice to identify candidate DRGs. We then used evolutionary analyses of orthologous gene sets from several grass species, to determine whether the DRGs show signals of positive selection and the residues targeted. We found 140 divergent genes upregulated under biotic stress in S. bicolor by evaluating the relative abundance of expressed sequence tags in different libraries and comparing them with rice genes. For 10 of these genes, we found sets of orthologs including sequences from rice and three other cereals; six genes showed a pattern of substitution that was consistent with positive selection. Three of these genes, a thaumatin, a peroxidase, and a barley mlo homolog, are known antifungal proteins. The other three genes with evidence of positive selection were a MCM-1 agamous deficiens SRF- (MADS) box transcription factor, an eIF5 translation initiation factor, and a gene of unknown function but with evidence of expression during stress. Permutation analyses, using different ortholog and paralog sequences, consistently identified five positively selected codons in the peroxidase, a member of a cluster of genes and a large gene family. We mapped the positively selected residues onto the structure of the peroxidase and thaumatin and found that all sites are on the surface of these proteins and several are close to biochemically determined active sites. Identifying new positively selected plant disease resistance genes and the critical amino acid sites provides a basis for functional studies that may increase our understanding of their underlying molecular mechanisms of action. Additionally, it may lead to the identification of individuals having variation at functionally important sites, as well as eventually using this information in the rational design and engineering of proteins involved in plant disease resistance