Effects of oxygen on nodule physiology and expression of nodulins in alfalfa

Early nodulin 2 (ENOD2) transcripts and protein are specifically found in the inner cortex of legume nodules, a location that coincides with the site of a barrier to O-2, diffusion. The extracellular glycoprotein that binds the monoclonal antibody MAC236 has also been localized to this site. Thus, it has been proposed that these proteins function in the regulation of nodule permeability to O-2 diffusion. It would then be expected that the levels of ENOD2 mRNA/protein and MAC236 antigen would differ in nodules with different permeabilities to O-2. We examined the expression of ENOD2 and other nodule-expressed genes in Rhizobium meliloti-induced alfalfa nodules grown under 8, 20, or 50% O-2. Although there was a change in the amount of MAC236 glycoprotein, the levels of ENOD2 mRNA and protein did not differ significantly among nodules grown at the different [O-2], suggesting that neither ENOD2 transcription nor synthesis is involved in the long-term regulation of nodule permeability. Moreover, although nodules from all treatments reduced their permeability to O-2 as the partial pressure of O-2 (pO(2)) was increased to 100%, the levels of extractable ENOD2 and MAC236 proteins did not differ from those measured at the growth pO(2), further suggesting that if these proteins are involved in a short-term regulation of the diffusion barrier, they must be involved in a way that does not require increased transcription or protein synthesis.