長浜バイオ大学

蔡　晃植（さい・こうしょく）
Fang-Sik Che

職位：教授
学位：農学博士（東京大学）
●朝鮮大学理学部卒業
●理化学研究所基礎科学特別研究員、奈良先端技術大学院大学助手を経て本学へ

専門分野

細胞間情報学

研究テーマ

（1）植物における病原菌認識の分子解析

（2）植物免疫反応誘導の機構解析

（3）植物における環境認識機構の分子解析

（4）植物工場を用いた高機能野菜作出

　自発的移動手段をもたない植物は、自然界において病原菌の侵入や、温度、光、湿度の変動などにさらされている。しかし、植物は自己のおかれている環境情報を素早く読み取り、環境に適応することでこのような環境変化から自己を防御している。これまで、植物の外部環境情報認識の中でも特に、植物による病原菌認識と認識情報によって誘導される植物免役システムに注目し、植物がどのように病原菌を認識し、どのようにその情報を細胞内に伝え、どのように植物の免疫システムを誘導しているのかについて分子レベルで解析を行ってきた。これまでの研究で、植物が植物病原細菌の侵入を受けた際、この菌の鞭毛を構成するフラジェリンタンパク質を特異的に認識して免疫反応を誘導することを明らかにすると同時に、この認識特異性がフラジェリンに存在する糖鎖によって決定されていることを明らかにした。
　 さらに、植物の免疫反応の一つである過敏感細胞死誘導の機構についても研究を行い、イネの過敏感細胞死誘導においてOsNAC4という転写因子が細胞死誘導の分子スイッチとして機能していることを明らかにした。病原菌認識情報によって発現誘導されたOsNAC4はリン酸化されることによって細胞質から核に移行し、OsHSP90やヌクレアーゼであるIRENをコードする遺伝子の発現を誘導する。OsHSP90は小胞体に移行し、細胞膜透過性の喪失を引き起こし、IRENは核に移行し、核DNAの断片化を行うことで植物の過敏感細胞死を引き起こしていることを明らかにした（図1）。今後、植物の免疫反応機構解析だけにとどまらず、植物の外部環境認識の分子機構についても研究すると共に、これらの知見を生かした耐病性を有する機能性植物の作成などについても検討を行いたい。

Topics of research

Plants are continuously confronted with diverse potential pathogens and, as a results, they have evolved intricate immune mechanisms to recognize and defend themselves against a wide array of these disease-causing agents by inducing a set of immune responses that can defeat the invading pathogens. One of the most efficient and immediate immune responses is hypersensitive response (HR) cell death. HR is characterized by rapid, localized death of plant cells at the site of pathogen infection. HR cell death requires active plant metabolism and intact transcriptional and translational machinery in the host plant. The induction of HR is associated with signaling events, such as protein phosphorylation and the generation of reactive oxygen species. Spontaneous activation of HR cell death in the absence of pathogens has been reported in transgenic plants expressing foreign genes, suggesting that HR cell death is a form of programmed cell death (PCD). Acidovorax avenae rice-avirulent strains caused HR cell death in rice accompanied with clear 180-bp nucleosomal DNA laddering and typical morphological change such as plasma membrane shrinkage and nuclear condensation, while rice-virulent strains did not. To clarify the induction mechanism of HR cell death, we identified several genes that are upregulated during HR cell death using PCR subtraction analysis and microarray analysis. Among the identified genes, the OsNAC4 gene encoding a plant-specific transcription factor was identified in both analyses. Overexpression of OsNAC4 leads to HR cell death accompanied by the loss of plasma membrane integrity, nuclear DNA fragmentation and typical morphological changes. In OsNAC4 knock-down lines, HR cell death is markedly decreased in response to avirulent bacterial strains. After induction by an avirulent pathogen recognition signal, OsNAC4 is translocated into the nucleus in a phosphorylation-dependent manner. To understand the molecular mechanisms by which OsNAC4 mediates cell death, we compared global gene expression in the OsNAC4-RNAi cell line with a control line using a 22,000 elements oligonucleotide microarray. A microarray analysis demonstrated that 139 genes including OsHSP90 and IREN, encoding a Ca²⁺-dependent nuclease, were expressed differently between the OsNAC4-RNAi and control lines during HR cell death. When OsHSP90 was transiently expressed in rice cells, loss of plasma membrane integrity was rapidly induced, while no nuclear DNA fragmentation was observed. We next cloned the full-length IREN cDNA into a vector under the control of the ubiquitin promoter, and introduced this construct into rice cells. After TUNEL staining, all of the IREN-expressing cultured rice cells exhibited fluorescein-derived bright-green fluorescence of their nuclei, suggesting that IREN overexpression causes rapid DNA fragmentation in nuclei. While no cell death induction was detected when IREN was transiently expressed in rice protoplasts. Overall, our results indicate that two important events occurring during HR cell death are regulated by independent pathways.

主な業績論文等

1. Kaneda, T., Taga, Y., Takai, R., Iwano, M., Matsui, H., Takayama, S., Isogai, A., Che F. S. The transcription factor OsNAC4 is a key positive regulator of plant hypersensitive cell death. EMBO J. 28: 926-936. (2009)
2. Taga, Y., Takai, R., Kaneda, T., Matsui, H., Isogai, A., Che, F. S. Role of OsHSP90 and IREN, Ca2+ dependent nuclease, in plant hypersensitive cell death induced by transcription factor OsNAC4. Plant Signaling & Behavior, 4:8 1-3. (2009)
3. Takai, R., Isogai, A., Takayama, S., Che F. S. Analysis of flagellin perception mediated by flg22 receptor OsFLS2 in rice. Mol. Plant Microb. Interact. 21: 1635-1642. (2008)
4. Takai R., Kaneda T., Isogai A., Takayama S., Che F. S. A new method of defense response analysis using a transient expression system in rice protoplast. Biosci. Biotech. Biochem. 71: 590-593. (2007)