Genetic variation in resistance of Brachypodium distachyon to Rhizoctonia solani AG 8
Creators: name: Schneebeli, Katharina
Publisher: name: The Australian National University
Year: 2014
Subjects: subject: Brachypodium Disease and pest resistance Genetic aspects
subject: Rhizoctonia solani
subject: Wheat Root rots
subject: Wheat Roots Diseases and pests Control

description: Wheat is one of the three major cereal crops, alongside rice and maize, which together supply half of the world population's food energy requirement. Wheat production is affected by climatic conditions and biotic factors including weed competition and pathogen attack. Around 18% of yield loss in Australian wheat is attributable to root diseases. Plant defence against root-invading pathogens often involves multiple quantitative resistance genes. Rhizoctonia root rot, caused by 'Rhizoctonia solani' AG 8 (teleomorph 'Thanatephorus cucumeris'), costs Australian farmers around $60 million per annum. The necrotrophic fungus attacks seedling roots, resulting in patches of severely stunted plants. During root invasion R. solani AG 8 secretes a range of enzymes; however specific requirements for pathogenesis are not yet understood. Effective resistance to Rhizoctonia is not available in wheat cultivars, so farmers must rely on management techniques to control the disease. This research aimed to discover genetic resistance to Rhizoctonia root rot in 'Brachypodium distachyon', a grass developed in recent years as a model for cereals. The species has shown strong potential as a model for wheat shoot and crown diseases, as well as cereal root architecture. Two 'B. distachyon' germplasm resources were used in this project: natural accessions collected from Turkey and Spain, and a T DNA insertional mutant collection. 'Rhizoctonia solani' AG 8 produced similar disease phenotypes and severity in 'Brachypodium distachyon' and wheat. A method developed to screen for disease resistance in the 'B. distachyon' collections incorporated toothpick baits to check for the presence of inoculum and contamination in pots. The major indicator of disease severity was reduced root length, with leaf lengths and plant development rate being secondary symptoms. Resistance of different lines was ranked based on the ability of plants grown in infested soil to maintain root and shoot measurements similar to uninfested control plants. Twenty-six genetically diverse natural accessions and 25 selected T-DNA lines of 'B. distachyon' were included in replicated experiments to screen for variation in resistance to 'R. solani' AG 8. Greatest variation in resistance to was identified in the natural accession collection. Root length of the least resistant line was reduced to 19% of the control in 'R. solani' infested soil, while the most resistant line maintained 53% of control root length. This difference is similar to quantitative resistance levels in other grasses and wheat mutant lines described in the literature. Exploring potential resistance mechanisms, nodal root emergence in response to infection correlated with greater resistance in an experiment with natural accessions. Increased endogenous seedling vigour was linked with lower resistance, but this factor alone did not explain all variation in resistance. Further work is required to validate increased resistance associated with a T-DNA tagged gene, Bradi3g14370, that encodes a putative beta-1,3-galactosyltransferase. 'Brachypodium distachyon' was found to be a useful model pathosystem for wheat root diseases. The variation in genetic resistance to 'R. solani' AG 8 described in the natural accession collection provides a basis for further work to discover genes involved in resistance to this pathogen in 'B. distachyon' and subsequently in wheat.