Nodulation enhances dark CO₂ fixation and recycling in the model legume Lotus japonicus.

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.