Flux of protons released by wild type and ferritin over-expressor tobacco plants : effect of phosphorus and iron nutrition

Tobacco (Nicotiana tabacum) plants over-expressing the iron storage protein ferritin, either in the cytoplasm or in the plastids, were grown under various P and Fe conditions. The crossed effects of both the genotypes and the environmental conditions on iron and chlorophyll concentrations in leaves, ferric reductase (EC 1.6.99.13) and plasmalemma H+-ATPase (EC 3.6.3.6) activities in roots, and fluxes of H+ released by roots were determined. The increase in leaf Fe concentration observed in plants over-expressing ferritin was accompanied by an increase in root ferric reductase and H+-ATPase activities. Iron deficient conditions induced a decrease in Fe and chlorophyll concentrations in leaves, an increase in ferric reductase and H+-ATPase activities in roots and an increase in H+-flux released by roots in all genotypes. Phosphorus abundant conditions induced also an increase in ferric reductase and H+-ATPase activities in roots in all genotypes, and an increase in H+-flux released by roots in the genotype over-expressing ferritin in the cytoplasm. These results suggest that P could regulate the root Fe uptake system, at the gene expression level, as already reported for Zn in barley. Moreover, they show that H+-flux was not always consistent with H+-ATPase activity, revealing that the global measurement of root plasmalemma H+-ATPase activity has some limits compared to the localized determination of the H+-flux. These limits could be due to the fact that part of the H+-flux is not directly linked to H+-ATPase activity in roots.