Nitrogen fertilization of rice plants affects Magnaporthe oryzae infection process and rice resistance

BGPI : équipe 4International audienceNitrogen Induced Susceptibility (NIS) of plant diseases is a widespread phenomenon. We set an experimental system in which nitrogen supply strongly affects rice blast susceptibility whereas it is only slightly perturbing plant growth. We show that fungal growth is affected before and after penetration in the plant but that the final penetration rate is not affected. Differences in total nitrogen amount and defense gene expression before and after infection are unlikely to be responsible for the observed increase in penetration. On the other hand, the fungus seems to perceive small differences in nitrogen amount after penetration and this may explain enhanced growth under high nitrogen regime. Indeed, exogenous treatment with some free amino acids after inoculation mimicked NIS, further arguing that this phenomenon is mostly due to a trophic relation between the plant and the fungus. We also used our experimental system to show that the capacity of rice to display NIS is highly polymorphic and does not correlate with difference related to indica/japonica sub-groups. However, nitro-gen partially breaks down resistance triggered by the Pi1 gene. Using the CSSL mapping population between Nipponbare and Kasalath, we identified a Kasalath locus on chromosome 1, called NIS1 , which dominantly increases susceptibility under high nitrogen. We discuss the possible relationships between Nitrogen Use Efficiency (NUE), disease resistance regulation and NIS. This work provides evidences that robust forms of partial resistance exist across diversity and can be genetically mapped. This work also suggests that under certain environmental circumstances, complete resistance may breakdow