TRANSCRIPTOME ANALYSIS IN THE INTERACTION BRACHYPODIUM – PUCCINIA BRACHYPODII

The model grass Brachypodium distachyon L (Brachypodium) has recently revealed its potential for studying grass-pathogen interactions. In particular, the identification of genomic regions associated with resistance to the false brome rust fungus Puccinia brachypodii offered perspectives to elucidate the genetic and molecular basis of this trait. In this study, we aimed to: 1) provide an initial whole-genome expression dataset for Brachypodium-P. brachypodii interaction in the two inbred lines Bd3-1 (resistant) and Bd1-1 (susceptible), and 2) fine mapping and cloning Rpbq2 and Rpbq3: to increase the resolution of QTL mapping and to reduce the number of candidate genes underlying QTL LOD curves. For the first, aim the two inbred lines have been characterized macroscopically and by confocal microscopy to follow the development of the fungus and the formation of rust infection structures. The expression of six brachypodium genes, homologous to known wheat and barley defence-related genes, was monitored by qRT-PCR analysis in Bd3-1 and Bd1-1 at three time points (18, 24 and 72 hours post infection, hpi). The 18 hpi time point was selected for transcriptome profiling on the basis of the expression profiles of the defence genes. The Affymetrix Brachypodium Tiling Array (BradiAR1b520742) revealed that expression levels of a set of genes (more than 100 in total) were altered in infected plants, mainly in the resistant line Bd3-1. At 18 hpi a significant re-programming of host metabolism occurred in infected leaves, with a modulation of genes involved in different metabolic networks such as defence, glycolysis, aminoacid and nitrogen metabolism. This study represents the first characterization of the functional genomic basis of resistance to a rust species in the model plant Brachypodium, and could be useful for translational genomics to ‘complex’ cereals. For the second aim, fine mapping Rpbq2 and Rpbq3, a new large segregating RIL population has been developed for each QTL separately. Selection of Bd3-1 x Bd1-1 RILs heterozygous for the QTLs has been completed based on flanking marker haplotypes, with the target QTL in a heterozygous state, while the other two QTLs were selected to be homozygous for the susceptible allele. These marker-selected heterozygous RILs have been selfed to obtain large segregating populations for each QTL. These results represent the first steps of a genetic approach towards the cloning of Rpbq2 and Rpbq3 determinants, and for their possible exploitation in cereals.