Genetic variation in resistance of Brachypodium distachyon to Rhizoctonia solani AG 8

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

Cascading decrease of the surface snow SSA at Kohnen Station, DML, Antarctica

Grain size of the surface snow is the key parameter for albedo in interior Antarctica, as impurity content is very small. The snow surface at the end of austral winter is characterized by very small grains. The small snow grains consist of broken precipitation particles and partially sublimated or mechanically fractured older ice particles. The albedo is consequently very high. The size of snow grains can be determined quite accurately by measuring its specific surface area (SSA), which is equivalent to the optical grain radius. The SSA as a material property used for albedo estimates typically shows an annual cycle. During the summer it decreases due to grain coarsening caused by snow metamorphism. However the surface layer is affected by a variety of processes including wind driven redistribution, precipitation or surface hoar formation. To discriminate the influence of the different processes on the evolution of the SSA during the summer we measured SSA along a 50m transect on a daily basis at Kohnen Station (75◦00′ S, 00◦00′ E, 2892 m a.s.l) in Dronning Maud Land (DML) indirectly by reflectance at 1310 nm. We found that the SSA was not reducing steadily but showed a cascading decrease. The peaks corresponded to precipitation events and frost formation lasting for several days and inhibiting the general expected decrease during the summer period. Our study indicates that even small amounts of precipitation during the summer period can affect the decrease of SSA, respectively the albedo, in the DML region on the East Antarctic Plateau

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