The utility of NBS-profiling for characterization of yellow rust resistance in an F6 durum wheat population.

Seedling and adult plant (field) resistance to yellow rust in the durum wheat (Triticum turgidum ssp. durum) cross Kunduru-1149 x Cham-1 was characterized using a functionally-targeted DNA marker system, NBS-profiling. Chi-squared analysis indicated a four gene model conferring seedling yellow rust resistance against Puccinia striiformis f. sp. tritici isolate WYR85/22 (virulent on Yr2, Yr6, Yr7 and Yr9). Interval mapping located two QTL for yellow rust resistance on the long arm of chromosome 1B, while Kruskal-Wallis single marker regression identified a number of additional marker loci associated with seedling and/or adult plant, field resistance to yellow rust. These results suggested that much of the yellow rust resistance seen in the field may be due to seedling expressed resistance (R) genes. Characterization of the DNA sequence of three NBS marker loci indicated that all showed significant homology to functionally-characterized R-genes and resistance gene analogues (RGAs), with the greatest homology being NBS-LRR-type R-genes and RGAs from cereal species.

TaWIR1 contributes to post-penetration resistance to Magnaporthe oryzae, but not Blumeria graminis f. sp. tritici, in wheat.

Members of the Wheat-Induced Resistance 1 (TaWIR1) gene family are highly induced in response to a wide range of pathogens. Homologues have been identified in barley, but not in Brachypodium, whereas, in rice, only distant WIR1 candidates are known. Phylogenetic analysis placed TaWIR1a and TaWIR1b within a distinct clade of wheat transcripts, whereas TaWIR1c clustered with HvWIR1 genes. Transcripts of all three TaWIR1 genes were strongly induced by a wheat-adapted isolate of Magnaporthe oryzae. Virus-induced gene silencing of the TaWIR1 gene family had no effect on the initial penetration of epidermal cells by M. oryzae. However, following the establishment of an infection site, the fungus was able to grow more extensively within the leaf tissue, relative to control leaves, indicating a role for the TaWIR1 gene family in the cell-to-cell movement of M. oryzae. In contrast, the silencing of TaWIR1 transcripts had no effect on epidermal cell penetration by a wheat-adapted isolate of Blumeria graminis, or on the subsequent growth of hyphae. Differential transcription of TaWIR1 genes was also seen in epidermal peels, relative to the remaining leaf tissue, following inoculation with M. oryzae.

Genetic variation within and between winter wheat genotypes from Turkey, Kazakhstan, and Europe as determined by nucleotide-binding-site profiling.

The genetic diversity within wheat breeding programs across Turkey and Kazakhstan was compared with a selection of European cultivars that represented the genetic diversity across eight European countries and six decades of wheat breeding. To focus the measure of genetic diversity on that relevant to disease-resistant phenotypes, nucleotide-binding-site (NBS) profiling was used to detect polymorphisms associated with the NBS motifs found within the NBS--leucine-rich repeat (LRR) class of resistance (R) genes. Cereal-specific NBS primers, designed specifically to the conserved NBS motifs found within cereal R-genes, provided distinct NBS profiles. Although the genetic diversity associated with NBS motifs was only slightly higher within the Eastern wheat genotypes, the NBS profiles produced by Eastern and European wheat lines differed considerably. Structure analysis divided the wheat genotypes into four groups, which compared well with the origin of the wheat genotypes. The highest levels of genetic diversity were seen for the wheat genotypes from the Genetic Resource Collection held in Ankara, Turkey, as wheat genotypes within breeding programs were genetically more similar. The wheat genotypes from Kazakhstan were the most similar to the European cultivars, reflecting the significant number of eastern European cultivars used in the breeding program in Kazakhstan. In general, the NBS profiles suggested that NBS-LRR R-gene usage in winter wheat breeding in Turkey and Kazakhstan differed from that deployed in European cultivars.

The Barley stripe mosaic virus system used for virus-induced gene silencing in cereals differentially affects susceptibility to fungal pathogens in wheat.

Barley stripe mosaic virus (BSMV) has emerged as a vector for virus-induced gene silencing (VIGS) in cereals, having been used to study a number of genes involved in resistance in both wheat and barley. However, the effects of the BSMV vector on plant physiology and disease resistance in plants remains unexplored. The BSMV inoculation control vector, BSMV:GFP was shown to cause severe viral symptoms in wheat, displaying chlorosis, leaf curling and growth inhibition typical of the symptoms seen in BSMV-infected barley. These viral symptoms were accompanied by induction of genes implicated in defense against pathogens, namely PR1, PR4, PR5, PR10 and PAL. Subsequent inoculation of BSMV:GFP-infected wheat with a wheat pathotype of Magnaporthe oryzae, the blast pathogen, resulted in decreased susceptibility. Penetration of epidermal cells and subsequent multiple cell colonization by M. oryzae was significantly reduced. This increased restriction of pathogen growth observed for BSMV:GFP infections with and without the viral coat protein gene. However, prior infection with BSMV:GFP had no effect on the development of a compatible isolate of Blumeria graminis f. sp. tritici, the causal agent of powdery mildew.

Cellular and transcriptional responses of wheat during compatible and incompatible race-specific interactions with Puccinia striiformis f. sp. tritici.

The initial stages of Puccinia striiformis f. sp. tritici (the causal agent of yellow rust in wheat) infection triggered a hypersensitive cell death (HCD) response in both compatible and Yr1-mediated incompatible interactions, although the response was earlier and more extensive in the incompatible interaction. Later stages of fungal development were only associated with an HCD response in the incompatible interaction, the HCD response being effectively suppressed in the compatible interaction. Cell autofluorescence was seen in mesophyll cells in direct contact with fungal infection hyphae (primary HCD) and in adjacent mesophyll cells (secondary HCD), indicating the activation of cell-to-cell signalling. Microarray analysis identified a number of defence-related transcripts implicated in Yr1-mediated resistance, including classical pathogenesis-related (PR) transcripts and genes involved in plant cell defence responses, such as the oxidative burst and cell wall fortification. A quantitative reverse transcriptase-polymerase chain reaction time course analysis identified a number of defence-related genes, including PR2, PR4, PR9, PR10 and WIR1 transcripts, associated with the latter stages of Yr1-mediated resistance. A meta-analysis comparison of the Yr1-regulated transcriptome with the resistance transcriptomes of the race-specific resistance gene Yr5 and the race-nonspecific adult plant resistance gene Yr39 indicated limited transcript commonality. Common transcripts were largely confined to classic PR and defence-related genes.