Development of Fe-deficiency responses in cucumber (Cucumis sativus L.) roots : involvement of plasma membrane H+-ATPase activity

One of the mechanisms through which some strategy I plants respond to Fe‐deficiency is an enhanced acidification of the rhizosphere due to proton extrusion. It was previously demonstrated that under Fe‐deficiency, a strong increase in the Hplus;‐ATPase activity of plasma membrane (PM) vesicles isolated from cucumber roots occurred. This result was confirmed in the present work and supported by measurement of ATP‐dependent proton pumping in inside‐out plasma membrane vesicles. There was also an attempt to clarify the regulatory mechanism(s) which lead to the activation of the H+‐ATPase under Fe‐deficiency conditions. Plasma membrane proteins from Fe‐deficient roots submitted to immunoblotting using polyclonal antibodies showed an increased level in the 100 kDa polypeptide. When the plasma membrane proteins were treated with trypsin a 90 kDa band appeared. This effect was accompanied by an increase in the enzyme activity, both in the Fe‐deficient and in the Fe‐sufficient extracts. These results suggest that the increase in the plasma membrane H+‐ATPase activity seen under Fe‐deficiency is due, at least in part, to an increased steady‐state level of the 100 kDa polypeptide.