Dissecting iron deficiency-induced proton extrusion in Arabidopsis roots

Here, we have analysed the H(+)-ATPase-mediated extrusion of protons across the plasma membrane (PM) of rhizodermic cells, a process that is inducible by iron (Fe) deficiency and thought to serve in the mobilization of sparingly soluble Fe sources. The induction and function of Fe-responsive PM H(+)-ATPases in Arabidopsis roots was investigated by gene expression analysis and by using mutants defective in the expression or function of one of the isogenes. In addition, the expression of the most responsive isogenes was investigated in natural Arabidopsis accessions that have been selected for their in vivo proton extrusion activity. Our data suggest that the rhizosphere acidification in response to Fe deficiency is chiefly mediated by AHA2, while AHA1 functions as a housekeeping isoform. The aha7 knock-out mutant plants showed a reduced frequency of root hairs, suggesting an involvement of AHA7 in the differentiation of rhizodermic cells. Acidification capacity varied among Arabidopsis accessions and was associated with a high induction of AHA2 and IRT1, a high relative growth rate and a shoot-root ratio that was unaffected by the external Fe supply. An effective regulation of the Fe-responsive genes and a stable shoot-root ratio may represent important characteristics for the Fe uptake efficiency.