title: Symbiotic leghemoglobins are crucial for nitrogen fixation in legume root nodules but not for general plant growth and development.
abstract: Hemoglobins are ubiquitous in nature and among the best-characterized proteins. Genetics has revealed crucial roles for human hemoglobins, but similar data are lacking for plants. Plants contain symbiotic and nonsymbiotic hemoglobins; the former are thought to be important for symbiotic nitrogen fixation (SNF). In legumes, SNF occurs in specialized organs, called nodules, which contain millions of nitrogen-fixing rhizobia, called bacteroids. The induction of nodule-specific plant genes, including those encoding symbiotic leghemoglobins (Lb), accompanies nodule development. Leghemoglobins accumulate to millimolar concentrations in the cytoplasm of infected plant cells prior to nitrogen fixation and are thought to buffer free oxygen in the nanomolar range, avoiding inactivation of oxygen-labile nitrogenase while maintaining high oxygen flux for respiration. Although widely accepted, this hypothesis has never been tested in planta. Using RNAi, we abolished symbiotic leghemoglobin synthesis in nodules of the model legume Lotus japonicus. This caused an increase in nodule free oxygen, a decrease in the ATP/ADP ratio, loss of bacterial nitrogenase protein, and absence of SNF. However, LbRNAi plants grew normally when fertilized with mineral nitrogen. These data indicate roles for leghemoglobins in oxygen transport and buffering and prove for the first time that plant hemoglobins are crucial for symbiotic nitrogen fixation.
authors.affiliation: Max Planck Institute of Molecular Plant Physiology, Golm, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Ott
authors.forename: Joost T
authors.initials: JT
authors.lastname: van Dongen
authors.forename: Catrin
authors.initials: C
authors.lastname: Günther
authors.forename: Lene
authors.initials: L
authors.lastname: Krusell
authors.forename: Guilhem
authors.initials: G
authors.lastname: Desbrosses
authors.forename: Helene
authors.initials: H
authors.lastname: Vigeolas
authors.forename: Vivien
authors.initials: V
authors.lastname: Bock
authors.forename: Tomasz
authors.initials: T
authors.lastname: Czechowski
authors.forename: Peter
authors.initials: P
authors.lastname: Geigenberger
authors.forename: Michael K
authors.initials: MK
authors.lastname: Udvardi
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D001483:Base Sequence; D001692:Biological Transport / Q000502:physiology; D017931:DNA Primers; D015151:Immunoblotting; D007874:Leghemoglobin / Q000235:genetics / Q000378:metabolism*; D000070116:Lotus / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D008969:Molecular Sequence Data; D009584:Nitrogen / Q000378:metabolism*; D009591:Nitrogenase / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism; D010802:Phylogeny; D018517:Plant Roots / Q000166:cytology / Q000378:metabolism*; D034622:RNA Interference; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012230:Rhizobiaceae / Q000378:metabolism / Q000502:physiology*; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA; D013559:Symbiosis
chemical_list: D017931:DNA Primers; D007874:Leghemoglobin; D000255:Adenosine Triphosphate; D009591:Nitrogenase; D009584:Nitrogen; D010100:Oxygen