DataSheet_1_Phosphate accumulation in rice leaves promotes fungal pathogenicity and represses host immune responses during pathogen infection.docx

Rice is one of the most important crops in the world and a staple food for more than half of the world’s population. At present, the blast disease caused by the fungus Magnaporthe oryzae poses a severe threat to food security through reduction of rice yields worldwide. High phosphate fertilization has previously been shown to increase blast susceptibility. At present, however, our knowledge on the mechanisms underpinning phosphate-induced susceptibility to M. oryzae infection in rice is limited. In this work, we conducted live cell imaging on rice sheaths inoculated with a M. oryzae strain expressing two fluorescently-tagged M. oryzae effectors. We show that growing rice under high phosphate fertilization, and subsequent accumulation of phosphate in leaf sheaths, promotes invasive growth of M. oryzae. Consistent with this, stronger expression of M. oryzae effectors and Pathogenicity Mitogen-activated Protein Kinase (PMK1) occurs in leaf sheaths of rice plants grown under high a phosphate regime. Down-regulation of fungal genes encoding suppressors of plant cell death and up-regulation of plant cell death-inducing effectors also occurs in sheaths of phosphate over-accumulating rice plants. Treatment with high Pi causes alterations in the expression of fungal phosphate transporter genes potentially contributing to pathogen virulence. From the perspective of the plant, Pi accumulation in leaf sheaths prevents H2O2 accumulation early during M. oryzae infection which was associated to a weaker activation of Respiratory Burst Oxidase Homologs (RBOHs) genes involved in reactive oxygen species (ROS) production. Further, a weaker activation of defense-related genes occurs during infection in rice plants over-accumulating phosphate. From these results, it can be concluded that phosphate fertilization has an effect on the two interacting partners, pathogen and host. Phosphate-mediated stimulation of fungal effector genes (e.g., potentiation of fungal pathogenicity) in combination with repression of pathogen-inducible immune responses (e.g., ROS accumulation, defense gene expression) explains higher colonization by M. oryzae in rice tissues accumulating phosphate. Phosphate content can therefore be considered as an important factor in determining the outcome of the rice/M. oryzae interaction. As fertilizers and pesticides are commonly used in rice cultivation to maintain optimal yield and to prevent losses caused by pathogens, a better understanding of how phosphate impacts blast susceptibility is crucial for developing strategies to rationally optimize fertilizer and pesticide use in rice production.

水稻是全球最重要的粮食作物之一，亦是全球超过半数人口的主食。当前，由稻瘟病菌（Magnaporthe oryzae）引发的稻瘟病，通过降低全球水稻产量，对粮食安全构成严重威胁。此前已有研究表明，高磷施肥会加重水稻对稻瘟病的易感性。然而，目前学界对水稻中磷诱导易感稻瘟病菌侵染的分子机制仍认知有限。本研究对接种了表达两种荧光标记稻瘟病菌效应蛋白的菌株的水稻叶鞘开展活细胞成像分析。结果显示，在高磷施肥条件下种植的水稻，其叶鞘内磷元素积累会促进稻瘟病菌的侵染扩展。与之相符的是，高磷栽培的水稻叶鞘中，稻瘟病菌效应蛋白及致病性丝裂原活化蛋白激酶（Pathogenicity Mitogen-activated Protein Kinase，PMK1）的表达水平显著上调。在磷过量积累的水稻叶鞘中，编码植物细胞死亡抑制因子的真菌基因表达下调，而诱导植物细胞死亡的效应蛋白编码基因表达上调。高磷处理会改变真菌磷转运蛋白基因（phosphate transporter genes）的表达，这可能与病原菌的毒力调控相关。从宿主植物的视角来看，叶鞘内的磷积累会抑制稻瘟病菌侵染早期的H₂O₂积累，这与活性氧（reactive oxygen species，ROS）产生相关的呼吸爆发氧化酶同源物（Respiratory Burst Oxidase Homologs，RBOHs）基因的激活减弱密切相关。进一步研究发现，磷过量积累的水稻植株在侵染过程中，防御相关基因的激活同样减弱。综上，本研究结果表明，磷素营养对病原菌与宿主这两个互作伴侣均具有调控作用。磷介导的真菌效应蛋白基因表达增强（例如强化真菌致病性），同时抑制病原菌诱导的免疫响应（例如ROS积累、防御基因表达），共同解释了稻瘟病菌在磷积累水稻组织中更易定殖的现象。因此，磷素含量可被视为决定水稻与稻瘟病菌互作结局的关键因素之一。鉴于水稻种植中通常会施用肥料与农药以维持最佳产量、减少病原菌引发的损失，深入理解磷素如何影响稻瘟病易感性，对于合理优化水稻生产中的肥料与农药施用策略至关重要。