Unravelling mechanisms underlying phosphate-induced susceptibility to Bakanae disease in rice

Rice is one of the most important crops in the world and sustains >50 % of the world's population. Rice production is, however, severely threatened by bakanae disease, caused by the fungus Fusarium fujikuroi. Due to low soil phosphorus bioavailability, phosphorus fertilizers are routinely used to optimize rice production, which has led to excessive P accumulation in rice fields. We show that high phosphate fertilization enhances susceptibility to bakanae. Similarly, MIR399 overexpression increases phosphate content and enhances susceptibility to F. fujikuroi infection. In vivo imaging of the infection process using a green fluorescent protein-expressing F. fujikuroi isolate revealed higher fungal colonization in roots of plants grown under high-phosphate supply compared to plants under low-phosphate, which is in agreement with the observed phenotype of bakanae susceptibility in phosphate-accumulating plants. Moreover, a weaker activation of defense-related genes and reduced accumulation of ROS occurs during infection in rice plants grown under high phosphate supply. Histochemical detection of lignin and suberin showed reduced accumulation of lignin and suberin in roots of rice plants grown under high-phosphate fertilization, which was consistent with a weaker induction of lignin biosynthesis genes and suberin-related genes in those plants. Taken together, these results indicate that phosphate accumulation represses host immune responses and promotes susceptibility to bakanae. This information provides a basis to understand mechanisms underlying phosphate-induced susceptibility to pathogen infection in rice, which might be useful to reduce the use of agrochemicals, pesticides and fertilizers, in protecting rice from bakanae.