“MIDORIKO” in green algal cells: the first molecular identification of rickettsial endosymbiotic bacteria infecting plant cells
Abstract
The endosymbiosis (long-term living of organisms in the cells of different species) is considered to make great contributions to the establishment of mitochondoria in our eukaryotic cells. However, no detailed information about the endosymbiosis has been revealed. Here, we succeed a molecular identification of bacterial endosymbionts, which were firstly found in 1970, within the cells of volvocalean green algae Carteria and Pleodorina. The endosymbionts "MIDORIKO" belong to the family Rickettsiaceae (Rickettsiales, Alphaproteobacteria); this is the first report of rickettsial endosymbionts infecting plant cells. Bacteria belonging to this family are mainly associated with arthropod hosts (e.g. insects or ticks), and some of them are human-pathogenic species that cause dangerous diseases. However, "MIDORIKO" are hosted by the plant cells and grow without harm to their hosts. Because rickettsias are the closest relatives of mitochondria, our discovery of the harmless rickettsial endosymbionts "MIDORIKO" within the cultured green algal cells will contribute much to medical studies of pathogenic rickettsias and the evolutional studies for incipient stages of mitochondrial endosymbiosis.
Paper information
Kaoru Kawafune 1, Yuichi Hongoh 2, Takashi Hamaji 3, Hisayoshi Nozaki 1
- Department of Biological Sciences, Graduate School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, Japan
- Department of Biological Sciences, School of Bioscience and Biotechnology, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan
- Department of Botany, Graduate School of Science, Kyoto University, Oiwake-cho, Kita-shirakawa, Kyoto, Japan
- Molecular Identification of Rickettsial Endosymbionts in the Non-phagotrophic Volvocalean Green Algae.
- PLoS ONE 7(2): e31749.
- doi:10.1371/journal.pone.0031749
- www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0031749
Figure 1. Schematic illustration of evolutionary scenario of origin of eukaryotic cells. The organelles (mitochondria and chloroplasts) are thought to originate from the endosymbiotic bacteria.
Figure 2. Molecular phylogenetic analysis of various endosymbionts within the family Rickettsiaceae based on ribosomal RNA gene sequences. "MIDORIKO" (endosymbionts of two volvocalean algae Carteria and Pleodorina) belong to the non-arthropod host lineage (yellow box) within the family Rickettsiaceae. Based on Kawafune et al. (2012, PLoS ONE).
Figure 3. (A) Illustration showing the cell of the unicellular green alga Carteria cerasiformis with rickettsial endosymbionts "MIDORIKO". (B) Transmission electron microscopy of C. cerasiformis, cell showing "MIDORIKO" ("e", allow) within cytolasm (from Nozaki et al. 1994, Phycologia). (C, D) Two views of a C. cerasiformis cell stained with DAPI (fluorescence dye for DNA). (C) Nomarski differential microscopy. Note that four flagella of the Carteria cell were not observed in this image. (D) Epifluorescence microscopy showing "MIDORIKO" (DAPI-stained DNA, arrowheads). Note red autofluorescence of chloroplast. (E, F) Epifluorescence images of C. cerasiformis cells showing DAPI-stained DNA (E) and fluorescence in situ hybridization by DNA probes that specifically bind to ribosomal RNA of "MIDORIKO" (F). Correspondence of rod-shaped DNA fluorescences of "MIDORIKO" in (E; arrowheads) and the rickettsia-specific signals in (F; arrowheads) represents the existence of rickettsias inside the plant cells. The "n" indicates the cell nuclei of Carteria. Based on Kawafune et al. (2012, PLoS ONE).
Figure 4. Comparison of the cell size of green algae Carteria cerasiformis and the number of bacterial endosymbionts "MIDORIKO". "MIDORIKO" were observed in all Carteria cells, and the numbers of "MIDORIKO" in the host Carteria cells increase as Carteria cells grow. Based on Kawafune et al. (2012, PLoS ONE).