title: Nodulation enhances dark CO₂ fixation and recycling in the model legume Lotus japonicus.
abstract: During symbiotic nitrogen fixation (SNF), the nodule becomes a strong sink for photosynthetic carbon. Here, it was studied whether nodule dark CO(2) fixation could participate in a mechanism for CO(2) recycling through C(4)-type photosynthesis. Differences in the natural δ(13)C abundance between Lotus japonicus inoculated or not with the N-fixing Mesorhizobium loti were assessed. (13)C labelling and gene expression of key enzymes of CO(2) metabolism were applied in plants inoculated with wild-type or mutant fix(-) (deficient in N fixation) strains of M. loti, and in non-inoculated plants. Compared with non-inoculated legumes, inoculated legumes had higher natural δ(13)C abundance and total C in their hypergeous organs and nodules. In stems, (13)C accumulation and expression of genes coding for enzymes of malate metabolism were greater in inoculated compared with non-inoculated plants. Malate-oxidizing activity was localized in stem xylem parenchyma, sieve tubes, and photosynthetic outer cortex parenchyma of inoculated plants. In stems of plants inoculated with fix(-) M. loti strains, (13)C accumulation remained high, while accumulation of transcripts coding for malic enzyme isoforms increased. A potential mechanism is proposed for reducing carbon losses during SNF by the direct reincorporation of CO(2) respired by nodules and the transport and metabolism of C-containing metabolites in hypergeous organs.
authors.affiliation: Department of Agricultural Biotechnology, Agricultural University of Athens, Iera Odos 75, 11855 Athens, Greece.
authors.forename: Mariangela N
authors.initials: MN
authors.lastname: Fotelli
authors.forename: Daniela
authors.initials: D
authors.lastname: Tsikou
authors.forename: Anna
authors.initials: A
authors.lastname: Kolliopoulou
authors.forename: Georgios
authors.initials: G
authors.lastname: Aivalakis
authors.forename: Panagiotis
authors.initials: P
authors.lastname: Katinakis
authors.forename: Michael K
authors.initials: MK
authors.lastname: Udvardi
authors.forename: Heinz
authors.initials: H
authors.lastname: Rennenberg
authors.forename: Emmanouil
authors.initials: E
authors.lastname: Flemetakis
mesh_terms: D057486:Carbon Cycle; D002245:Carbon Dioxide / Q000378:metabolism*; D002247:Carbon Isotopes; D003624:Darkness; D020869:Gene Expression Profiling; D018506:Gene Expression Regulation, Plant; D000070116:Lotus / Q000235:genetics / Q000378:metabolism*; D008293:Malates / Q000378:metabolism; D008954:Models, Biological; D009928:Organ Specificity; D010084:Oxidation-Reduction; D010788:Photosynthesis; D055170:Plant Root Nodulation / Q000235:genetics; D018547:Plant Stems / Q000166:cytology / Q000378:metabolism
chemical_list: D002247:Carbon Isotopes; D008293:Malates; D002245:Carbon Dioxide