Thao Nguyen Phuong, Chen Letian, Nakashima Ayako, Hara Shin-ichiro, Umemura Kenji, Takahashi Akira, Shirasu Ken, Kawasaki Tsutomu, Shimamoto Ko
Laboratory of Plant Molecular Genetics, Nara Institute of Science and Technology, Ikoma 630-0101, Japan.
Plant Cell. 2007 Dec;19(12):4035-45. doi: 10.1105/tpc.107.055517. Epub 2007 Dec 21.
A rice (Oryza sativa) Rac/Rop GTPase, Os Rac1, is involved in innate immunity, but its molecular function is largely unknown. RAR1 (for required for Mla12 resistance) and HSP90 (a heat shock protein 90 kD) are important components of R gene-mediated disease resistance, and their function is conserved in several plant species. HSP90 has also recently been shown to be important in mammalian innate immunity. However, their functions at the molecular level are not well understood. In this study, we examined the functional relationships between Os Rac1, RAR1, and HSP90. Os RAR1-RNA interference (RNAi) rice plants had impaired basal resistance to a compatible race of the blast fungus Magnaporthe grisea and the virulent bacterial blight pathogen Xanthomonas oryzae. Constitutively active Os Rac1 complemented the loss of resistance, suggesting that Os Rac1 and RAR1 are functionally linked. Coimmunoprecipitation experiments with rice cell culture extracts indicate that Rac1 forms a complex with RAR1, HSP90, and HSP70 in vivo. Studies with Os RAR1-RNAi and treatment with geldanamycin, an HSP90-specific inhibitor, showed that RAR1 and HSP90 are essential for the Rac1-mediated enhancement of pathogen-associated molecular pattern-triggered immune responses in rice cell cultures. Furthermore, the function of HSP90, but not RAR1, may be essential for their association with the Rac1 complex. Os Rac1 also regulates RAR1 expression at both the mRNA and protein levels. Together, our results indicate that Rac1, RAR1, HSP90, and HSP70 form one or more protein complexes in rice cells and suggest that these proteins play important roles in innate immunity in rice.
一种水稻（Oryza sativa）Rac/Rop GTP酶，即Os Rac1，参与植物先天免疫，但对其分子功能却知之甚少。RAR1（Mla12抗性所需）和HSP90（一种90 kD热休克蛋白）是R基因介导的抗病性的重要组成部分，其功能在几种植物物种中保守。最近研究还表明，HSP90在哺乳动物先天免疫中也很重要。然而，它们在分子水平上的功能尚未完全清楚。在本研究中，我们研究了Os Rac1、RAR1和HSP90之间的功能关系。Os RAR1-RNA干扰（RNAi）水稻植株对稻瘟病菌Magnaporthe grisea的亲和小种和致病白叶枯病菌Xanthomonas oryzae的基础抗性受损。组成型激活的Os Rac1弥补了抗性损失，表明Os Rac1和RAR1在功能上相联系。水稻细胞培养提取物的免疫共沉淀实验表明，Rac1在体内与RAR1、HSP90和HSP70形成复合物。对Os RAR1-RNAi植株的研究以及使用HSP90特异性抑制剂格尔德霉素处理表明，RAR1和HSP90对于水稻细胞培养中Rac1介导的病原体相关分子模式触发的免疫反应增强至关重要。此外，HSP90而非RAR1的功能对于它们与Rac1复合物的结合可能必不可少。Os Rac1还在mRNA和蛋白质水平上调节RAR1表达。总之，我们的结果表明，Rac1、RAR1、HSP90和HSP70在水稻细胞中形成一个或多个蛋白质复合物，并表明这些蛋白质在水稻先天免疫中发挥重要作用。
