Tissue distribution and subcellular localization of carbonic anhydrase in mature soybean root nodules indicates a role in CO2 diffusion

Tissue distribution of carbonic anhydrase (CA; EC 4.2.1.1) and phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) was examined in developing soybean (Glycine max) nodules using an immunohistological approach. The data obtained indicate that in young nodules both CA and PEPC proteins are present in the parenchymatous cells, and at much lower levels, in the central nodular region. In mature nodules, high levels of CA were exclusively present in 2-3 cell layers of the inner cortical region, whereas high levels of PEPC were present both in infected and uninfected cells. Immunogold localization indicated that, in mature nodules, CA was localized in the cytoplasm of the inner cortical cells and the cell walls of the endodermal cells. These results considered together suggest that in mature nodules, CA may facilitate the diffusion of the excessive CO2, derived from the respired bacteroids, in the rhizosphere. The distribution of CA was examined in mature nodules of soybean, grown hydroponically, in either limiting or non-limiting phosphate concentrations. The data indicated that in plants growing on non-limiting phosphate concentrations, an additional strong signal was found in cortical cells surrounding the nodule vascular bundles.