Truncated Hemoglobins in Actinorhizal Nodules of Datisca glomerata.

• Three types of hemoglobins exist in higher plants, symbiotic, non-symbiotic, and truncated hemoglobins. Symbiotic (class II) hemoglobins play a role in oxygen supply to intracellular nitrogen-fixing symbionts in legume root nodules, and in one case (Parasponia sp.), a non-symbiotic (class I) hemoglobin has been recruited for this function. Here we report the induction of a host gene, Dgtrhb1, encoding a truncated hemoglobin in Frankia-induced nodules of the actinorhizal plant Datisca glomerata Induction takes place specifically in cells infected by the microsymbiont, prior to the onset of bacterial nitrogen fixation. A bacterial gene (Frankia trHbO) encoding a truncated hemoglobin with O (2)-binding kinetics suitable for the facilitation of O (2) diffusion ( ) is also expressed in symbiosis. Nodule oximetry confirms the presence of a molecule that binds oxygen reversibly in D. glomerata nodules, but indicates a low overall hemoglobin concentration suggesting a local function. Frankia TrHbO is likely to be responsible for this activity. The function of the D. glomerata truncated hemoglobin is unknown; a possible role in nitric oxide detoxification is suggested.

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