Genetics of polymorphism in nitrogen-induced-susceptibility of rice to Magnaporthe oryzae.

Nitrogen fertilization is essential for crop yield but can increase susceptibility to rice blast, a phenomenon known as Nitrogen-Induced Susceptibility (NIS). The genetic architecture governing natural variation in NIS remains poorly characterized. To address this, we performed a genome-wide association study (GWAS) using a diverse panel of 268 rice accessions from the rice diversity panel 2, under controlled nitrogen regimes 0N (control), 1N (100 mg/L NH4NO3), 2N (200 mg/L NH4NO3). Our analyses defined a NIS Index (NISI), which revealed extensive phenotypic polymorphism in nitrogen responsiveness across the rice diversity panel. GWAS identified several significant loci, including a novel locus on chromosome 8 (NIS4) harboring F-box protein genes within a stress-responsive gene cluster containing eight F-box genes, a drought resistance gene, and a heat shock protein 70 gene. Physiological dissection showed that a 'negative NIS' phenotype (enhanced resistance under high nitrogen) correlated with a rapid, pathogen-triggered defense response, characterized by immediate induction of peroxidase activity and lignin biosynthesis. Conversely, susceptible lines exhibited delayed defense. Gene expression analysis linked active nitrogen assimilation (GS/GOGAT cycle) to susceptibility. We conclude that NIS polymorphism has a genetic component and may involve the host's ability to redirect nitrogen resources toward early defense activation during pathogen invasion.