MOLECULAR ANALYSIS OF RESPONSES TO BIOTIC AND ABIOTIC STRESS IN RICE

Plants have constantly to face a wide range of stresses in the natural environment, therefore they developed a sophisticated machinery to perceive external signals and optimally respond to constraints. A common facet of plant responses to stresses is the fine tuning of a large number of genes by transcriptional factors and the prompt expression of a battery of responsive genes, such as pathogenesis related proteins (PR proteins) against pathogens and as osmoprotectants implicated in abiotic stress tolerance.. Rice (Oryza sativa L.) is one of the most important crops in the world and Italy is the first rice producer in Europe (58% of European production in 2007) and its growth is mainly affected by Magnaporthe grisea infection (known as blast disease), drought and high salinity. Due to the relevance of blast disease and water limited conditions for rice cultivation and productivity in Italy, Rice Genomics group at Parco Tecnologico Padano is mainly focused on the molecular characterization of rice responses to blast and osmotic stress to supply information and tools to breeders and contribute in the improvement of rice productivity and quality. A high-throughput system for rice infection with Magnaporthe grisea strains, to appraise the resistance/susceptibility of rice varieties was developed in our laboratory. Our system called BLASTETS enables to inoculate rice plantlets of different varieties with M. grisea strains and rapidly and reliably screen macroscopic phenotypes (pathogen lesions, resistance necrosis) on rice leaves after five days incubation in a dedicated growth chamber in controlled conditions. In addition an hydroponic system is available to test in vitro rice tolerance to osmotic stress. The molecular characterization of rice response to biotic and abiotic stress involve the functional characterization of WRKYs transcriptional factors (well known as key factors in biotic and abiotic stress response in many plants) and the expression analysis of many responsive genes (PR proteins, peroxidase, kinase, etc) after blast infection and osmotic stress treatment. In particular, we characterized WRKY genes in response to different Magnaporthe grisea strains, to highlight differences between host and non-host resistance. Expression of thirty OsWRKY candidate genes was quantified by Real Time PCR after infection with FR13 (host strain), BR29 and BR32 (non-host strains) at 12, 24 and 48 hours post infection (hpi). Most of the up-regulated genes were found to be expressed in response to host infection and only a few by both host and non-host strains, whereas only two genes were induced specifically by non-host M. grisea strains. Few WRKY genes showed up-regulation also in presence of osmotic stress, suggesting a role in cross-talk between biotic and abiotic stress. Furthermore, we also analysed the expression of defence genes in a selection of Italian rice cultivars (ranging from highly-susceptible to completely resistant) to investigate a potential correlation between the resistance/susceptibility of the Italian rice cultivars and the expression levels of selected defence genes. We quantified the expression of the five defence genes before and after infection with two host Italian strains of Magnaporthe grisea. In resistant cultivars we observed a strong induction of several defence genes at once after blast infection. In contrast, in the susceptible cultivars we did not observe any variation of expression, confirming the role of these defence genes in resistance to blast. The authors acknowledge the financial support of the Fondazione Cariplo to the project RICEIMMUNITY