Genetic diversity and co-evolution study of avirulence and resistance genes for Italian rice breeding programs

The Ascomycetes Magnaporthe oryzae B. Couch sp. nov. is the causal organism of rice blast, the most serious fungal disease of rice worldwide. Italy is the first European rice producer (52%) with a cultivated area of 235,000 ha, but only a small portion of the Italian rice accessions is known to be resistant to blast disease, highlighting the call for implementation of rice breeding programs. Rice resistance conditioned by a single major R gene is typically effective in preventing infection by races of M. oryzae containing the corresponding avirulence (AVR) gene. However, the pathogen is highly variable and new races can appear and attack resistant varieties breaking down the efforts to introduce resistance genes in rice. Therefore, a comprehensive molecular information on presence and variability of broad range resistant genes in rice along with the corresponding avirulence genes in the Magnaporthe populations is crucial to design successful strategies for rice blast control. In a previous work, a panel of Italian rice genotypes were investigated to identify polymorphisms in resistance genes, commonly used in breeding programs worldwide. Nevertheless, information about the avirulence genes present in Italian Magnaporthe isolates, useful to select the relevant resistance genes to be introgressed into Italian rice genotypes, is still lacking. The main goal of this study was to identify the resistant genes effective in durable resistance towards blast population currently inhabiting Italian rice growing areas for national rice breeding programs. To this aim, the biodiversity of a Magnaporthe collection of more than 350 Italian strains and the presence of AVR-Pita 1, AVR-Pik, AVR-Pia, AVR-Pii and AVR-PizT in these isolates were assessed. The vast majority of the tested isolates contained both AVR-Pita1 and AVR-Pik genes. Thus, the genetic and functional variations of these avirulence genes were investigated. Alongside, the presence and polymorphisms of their cognate resistance genes were analyzed by NGS approach in a set of 96 rice genotypes, representative of the Italian rice germplasm. The correlation between the avirulence and resistance genes diversity will allow us to obtain a preliminary understanding of the co-evolution between rice genotypes and Magnaporthe races in Italian fields to develop new effective blast resistant varieties