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-de ficiency is an enhanced acidification of the rhizosphere due to proton extr usion, It was previously demonstrated that under Fe-deficiency, a strong in crease in the H+-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 regulat ory mechanism(s) which lead to the activation of the H+-ATPase under Fe-def iciency conditions, Plasma membrane proteins from Fe-deficient roots submit ted 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 inc rease in the enzyme activity, both in the Fe-deficient and in the Fe-suffic ient extracts. These results suggest that the increase in the plasma membra ne H+-ATPase activity seen under Fe-deficiency is due, at least in part, to an increased steady-state level of the 100 kDa polypeptide.