Oxygen regulation of a nodule-located carbonic anhydrase in alfalfa

Control of the permeability to oxygen is critical for the function of symbi otic nitrogen fixation in legume nodules. The inner cortex (IC) seems to be a primary site for this regulation. In alfalfa (Medicago sativa) nodules, expression of the Msca1 gene encoding a carbonic anhydrase (CA) was previou sly found to be restricted to the IC. We have now raised antibodies against recombinant Msca1 protein and used them, together with antibodies raised a gainst potato leaf CA, to demonstrate the presence of two forms of CA in ma ture nodules. Each antibody recognizes a different CA isoform in nodule tis sues. Immunolocalization revealed that leaf-related CAs were localized prim arily in the nitrogen-fixing zone, whereas the Msca1 protein was restricted exclusively to the IC region, in indeterminate and determinate nodules. In alfalfa nodules grown at various O-2 concentrations, an inverse correlatio n was observed between the external oxygen pressure and Msca1 protein conte nt in the IC, the site of the putative diffusion barrier. Thus Msca1 is a m olecular target of physiological processes occurring: in the IC cells invol ved in gas exchange in the nodule.