Molecular elucidation of tryptamine-pathway mediated hypersensitive cell death in lesion mimic mutants of rice infected with Magnaporthe grisea

水稻病变模拟突变体(cv。Sekiguchi-asahi在光照下对稻瘟病病菌的抗性增强，从而诱导Sekiguchi病变（sl）的形成和色胺的积累。自发性关口病在有叶绿体的绿色组织中发生，而在没有叶绿体的白化组织中没有发生。光合抑制剂3-（3,4-二氯苯基）-1,1-二甲基脲（DCMU）预处理抑制了稻瘟病叶片的关口病形成和色胺积累，抑制了色氨酸脱羧酶基因表达、单胺氧化酶活性、H_2O_2生成和DNA片段化。叶绿体似乎是sl突变体中细胞死亡的关键介质。过氧化氢酶活性在光照条件下受到稻瘟病菌的显著抑制，但在DCMU处理的叶片中保持较高水平。光依赖性关口病的繁殖性质表明，色胺介导的细胞死亡是由过氧化氢等扩散剂引起的。此外，色氨酸在光照条件下在灰霉病菌感染的叶片中积累，而在dcmu预处理的叶片中没有。在外源色氨酸的存在下，这种DCMU对色胺途径介导的细胞死亡的抑制作用消失了，这表明在稻瘟病杆菌感染后，光依赖性色胺的积累与色氨酸生物合成途径的激活之间存在很强的相关性。

A lesion-mimic mutant of rice (cv. Sekiguchi-asahi) showed enhanced resistance to Magnaporthe grisea infection, thereby inducing Sekiguchi lesion (sl) formation and tryptamine accumulation under light. Spontaneous Sekiguchi lesions were induced in green tissues with chloroplasts, but not in albino tissues without chloroplasts. Both Sekiguchi lesion formation and tryptamine accumulation in leaves infected with M. grisea were inhibited by pre-treatment with the photosynthetic inhibitor, 3-(3,4-dicchlorophenyl)-1,1-dimethylurea (DCMU), which suppressed the gene expression of tryptophan decarboxylase, monoamine oxidase activity, H_2O_2 generation and DNA fragmentation. Chloroplasts appear to be the key mediators of cell death in the sl mutant. Catalase activity was significantly inhibited by M. grisea infection under light, but maintained a high level in leaves pretreated with DCMU. The propagative nature of light-dependent Sekiguchi lesions suggests that tryptamine-mediated cell death is caused by a diffusible agent such as hydrogen peroxide. Furthermore, tryptophan accumulated in M. grisea-infected leaves under light, but not in DCMU-pretreated ones. Such DCMU inhibition of tryptamine pathway-mediated cell death disappeared in the presence of exogenous tryptophan, suggesting a strong correlation between light-dependent tryptamine accumulation and the activation of the tryptophan biosynthetic pathway after M. grisea infection.