Kaori Niimi

Publications of Kaori Niimi

• The integral membrane protein SEN1 is required for symbiotic nitrogen fixation in Lotus japonicus nodules.

  Authors: Tsuneo Hakoyama, Kaori Niimi, Takeshi Yamamoto, Sawa Isobe, Shusei Sato, Yasukazu Nakamura, Satoshi Tabata, Hirotaka Kumagai, Yosuke Umehara, Katja Brossuleit, Thomas R Petersen, Niels Sandal, Jens Stougaard, Michael K Udvardi, Masanori Tamaoki, Masayoshi Kawaguchi, Hiroshi Kouchi, Norio Suganuma

  Plant & cell physiology. 11/2011; 53(1):225-36.

  Legume plants establish a symbiotic association with bacteria called rhizobia, resulting in the formation of nitrogen-fixing root nodules. A Lotus japonicus symbiotic mutant, sen1, forms nodules thatLegume plants establish a symbiotic association with bacteria called rhizobia, resulting in the formation of nitrogen-fixing root nodules. A Lotus japonicus symbiotic mutant, sen1, forms nodules that are infected by rhizobia but that do not fix nitrogen. Here, we report molecular identification of the causal gene, SEN1, by map-based cloning. The SEN1 gene encodes an integral membrane protein homologous to Glycine max nodulin-21, and also to CCC1, a vacuolar iron/manganese transporter of Saccharomyces cerevisiae, and VIT1, a vacuolar iron transporter of Arabidopsis thaliana. Expression of the SEN1 gene was detected exclusively in nodule-infected cells and increased during nodule development. Nif gene expression as well as the presence of nitrogenase proteins was detected in rhizobia from sen1 nodules, although the levels of expression were low compared with those from wild-type nodules. Microscopic observations revealed that symbiosome and/or bacteroid differentiation are impaired in the sen1 nodules even at a very early stage of nodule development. Phylogenetic analysis indicated that SEN1 belongs to a protein clade specific to legumes. These results indicate that SEN1 is essential for nitrogen fixation activity and symbiosome/bacteroid differentiation in legume nodules.

• Host plant genome overcomes the lack of a bacterial gene for symbiotic nitrogen fixation.

  Authors: Tsuneo Hakoyama, Kaori Niimi, Hirokazu Watanabe, Ryohei Tabata, Junichi Matsubara, Shusei Sato, Yasukazu Nakamura, Satoshi Tabata, Li Jichun, Tsuyoshi Matsumoto, Kazuyuki Tatsumi, Mika Nomura, Shigeyuki Tajima, Masumi Ishizaka, Koji Yano, Haruko Imaizumi-Anraku, Masayoshi Kawaguchi, Hiroshi Kouchi, Norio Suganuma

  Nature. 11/2009; 462(7272):514-7.

  Homocitrate is a component of the iron-molybdenum cofactor in nitrogenase, where nitrogen fixation occurs. NifV, which encodes homocitrate synthase (HCS), has been identified from various diazotrophsHomocitrate is a component of the iron-molybdenum cofactor in nitrogenase, where nitrogen fixation occurs. NifV, which encodes homocitrate synthase (HCS), has been identified from various diazotrophs but is not present in most rhizobial species that perform efficient nitrogen fixation only in symbiotic association with legumes. Here we show that the FEN1 gene of a model legume, Lotus japonicus, overcomes the lack of NifV in rhizobia for symbiotic nitrogen fixation. A Fix(-) (non-fixing) plant mutant, fen1, forms morphologically normal but ineffective nodules. The causal gene, FEN1, was shown to encode HCS by its ability to complement a HCS-defective mutant of Saccharomyces cerevisiae. Homocitrate was present abundantly in wild-type nodules but was absent from ineffective fen1 nodules. Inoculation with Mesorhizobium loti carrying FEN1 or Azotobacter vinelandii NifV rescued the defect in nitrogen-fixing activity of the fen1 nodules. Exogenous supply of homocitrate also recovered the nitrogen-fixing activity of the fen1 nodules through de novo nitrogenase synthesis in the rhizobial bacteroids. These results indicate that homocitrate derived from the host plant cells is essential for the efficient and continuing synthesis of the nitrogenase system in endosymbionts, and thus provide a molecular basis for the complementary and indispensable partnership between legumes and rhizobia in symbiotic nitrogen fixation.