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 (Tjepkema et al., 2002) 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.