Nitrogen fertilization increases disease severity to rice blast fungus by affecting fungal pathogenicity but does not dampen plant defense

Abstract

Nitrogen-Induced Susceptibility (NIS) to plant diseases is a widespread phenomenon. In the case of rice blast disease, despite its agronomical importance, the molecular and cellular events underlying this phenomenon called “Koe-Imochi” are poorly understood. In this work, 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; thus a change in penetration is not responsible for increased susceptibility. In contrast, increased fungal growth late during infection of the plant is associated with enhanced lesion number and size. Differences in total nitrogen amount and defense gene expression before infection are unlikely to be responsible for the observed increase in penetration. After penetration, small changes in plant growth, but not modifications of the transcriptional regulation of defense genes, could be responsible for nitrogen-induced susceptibility. 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 Nitrogen-Induced Susceptibility, further arguing that this phenomenon is mostly due to a trophic relation between the plant and the fungus. This experimental system also revealed that nitrogen-induced susceptibility to rice blast is dependent on the plant genetic background.