The Distribution of Aquaporin Subtypes (PIP1, PIP2 and (gamma)-TIP) is Tissue Dependent in Soybean (Glycine max) Root Nodules

Background and Aims The inner cortical cells (IC-cells) of legume root nodules have been previously shown to regulate the resistance to nodule O2 diffusion by a rapid contraction/expansion mechanism, which controls the volume of intercellular spaces and their occlusion by a liquid phase. The expression of aquaporins in IC-cells was also found to be involved in this nodule O2 diffusion mechanism. The aim of this study was to compare the expression of plasma membrane intrinsic proteins (PIP) aquaporin isoforms with tonoplast intrinsic protein ((gamma)-TIP) in both IC-cells and adjacent cell types. Methods Using immunogold labelling in ultra-thin sections of Glycine max nodules, the expression of two PIP isoforms was observed and compared with the (gamma)-TIP pattern. Key Results The plasma membrane aquaporins PIP1 and PIP2 were expressed more in IC-cells and endodermis than in pericycle and infected cells. The tonoplast aquaporin (gamma)-TIP has shown a distribution pattern similar to that of the PIPs. Conclusions PIPs and (gamma)-TIP aquaporins are highly expressed in both plasmalemma and tonoplast of nodule IC-cells. This distribution is consistent with the putative role of water fluxes associated with the regulation of nodule conductance to O2 diffusion and the subsequent ATP-dependent nitrogenase activity. In the endodermis, these aquaporins might also be involved in nutrient transport between the infected zone and vascular traces.