Brachypodium distachyon genotypes vary in resistance to Rhizoctonia solani AG8.

Brachypodium distachyon (L.)P.Beauv. (Bd) has previously been developed as a pathosystem model for the wheat root rot pathogen Rhizoctonia solani Kühn anastomosis group 8 (AG8). Here we explore variation in resistance to R. solani AG8 in Bd, to determine whether genomic tools could be used to find Bd genes involved in the grass defence response, with the aim of using this information for the improvement of Rhizoctonia root rot resistance in wheat. We looked for variation in resistance to R. solani AG8 in a diverse Bd natural accession collection and in Bd T-DNA insertion lines selected based on putative mechanisms reported for tagged genes. All lines were susceptible to the pathogen. Repeatable and significant variation in resistance was measured in both groups, with greater variation in resistance found across the natural accessions than in the T-DNA lines. The widest and most repeatable variation in resistance was between lines Koz-3 and BdTR 13a. The ratio of R. solani AG8-inoculated to uninoculated root length for line Koz-3 was 33% greater than the same ratio for line BdTR 13a. The increased resistance of Koz-3 was associated with nodal root initiation in response to the pathogen. A negative correlation between seedling vigour and resistance was observed, but found not to be the sole source of variation in resistance to R. solani AG8. The only T-DNA line with significantly greater resistance to R. solani AG8 than the reference line had an insertion in a putative galactosyltransferase gene; however, this result needs further confirmation. Genetic resistance to Rhizoctonia root rot is not available in wheat cultivars and only a few instances of quantitative resistance to the pathogen have been described within close relatives of wheat. Brachypodium distachyon offers potential for further investigation to find genes associated with quantitative resistance and mechanisms of tolerance to R. solani AG8.

Brachypodium as an emerging model for cereal-pathogen interactions.

BACKGROUND: Cereal diseases cause tens of billions of dollars of losses annually and have devastating humanitarian consequences in the developing world. Increased understanding of the molecular basis of cereal host-pathogen interactions should facilitate development of novel resistance strategies. However, achieving this in most cereals can be challenging due to large and complex genomes, long generation times and large plant size, as well as quarantine and intellectual property issues that may constrain the development and use of community resources. Brachypodium distachyon (brachypodium) with its small, diploid and sequenced genome, short generation time, high transformability and rapidly expanding community resources is emerging as a tractable cereal model. SCOPE: Recent research reviewed here has demonstrated that brachypodium is either susceptible or partially susceptible to many of the major cereal pathogens. Thus, the study of brachypodium-pathogen interactions appears to hold great potential to improve understanding of cereal disease resistance, and to guide approaches to enhance this resistance. This paper reviews brachypodium experimental pathosystems for the study of fungal, bacterial and viral cereal pathogens; the current status of the use of brachypodium for functional analysis of cereal disease resistance; and comparative genomic approaches undertaken using brachypodium to assist characterization of cereal resistance genes. Additionally, it explores future prospects for brachypodium as a model to study cereal-pathogen interactions. CONCLUSIONS: The study of brachypodium-pathogen interactions appears to be a productive strategy for understanding mechanisms of disease resistance in cereal species. Knowledge obtained from this model interaction has strong potential to be exploited for crop improvement.

The shoot and root growth of Brachypodium and its potential as a model for wheat and other cereal crops.

The grass genetic model Brachypodium (Brachypodium distachyon (L.) Beauv., sequenced line Bd 21) was studied from germination to seed production to assess its potential as a phenotypic model for wheat (Triticum aestivum L.) and other cereal crops. Brachypodium and wheat shoot and root development and anatomy were highly similar. Main stem leaves and tillers (side shoots) emerged at the same time in both grasses in four temperature and light environments. Both developed primary and nodal axile roots at similar leaf stages with the same number and arrangement of vascular xylem tracheary elements (XTEs). Brachypodium, unlike wheat, had an elongated a mesocotyl above the seed and developed only one fine primary axile root from the base of the embryo, while wheat generally has three to five. Roots of both grasses could develop first, second and third order branches that emerged from phloem poles. Both developed up to two nodal axile roots from the coleoptile node at leaf 3, more than eight nodal axile roots from stem nodes after leaf 4, and most (97%) of the deepest roots at flowering were branches. In long days Brachypodium flowered 30 days after emergence, and root systems ceased descent 42 cm from the soil surface, such that mature roots can be studied readily in much smaller soil volumes than wheat. Brachypodium has the overwhelming advantage of a small size, fast life cycle and small genome, and is an excellent model to study cereal root system genetics and function, as well as genes for resource partitioning in whole plants.