Localization of the plasma membrane H+-ATPase in Fe-deficient cucumber roots by immunodetection

As in several Strategy I plants, iron-deficiency induces in cucumber (Cucumis sativus L.) roots a strong increase in the PM (plasma membrane) H+-ATPase activity with a consequent acidification of the rhizosphere. This increase is mainly due to enhanced synthesis of the enzyme even though other regulatory mechanisms have not yet been ruled out. As it was found in many other dicot species, in Fe-deficient cucumber roots we found a strong development of short lateral roots and swollen root tips; furthermore an enhanced H+ extrusion was particularly confined to the subapical swollen zone. Analysis of the root tissue structure by optical and electron microscopy showed a strong proliferation of root hairs at the subapical level and some ultrastructural modifications in the external layers. The structural localization of the PM H+-ATPase studied by immunohistochemistry showed that the increase of the PM protein is not simply due to a greater number of root tips in Fe-deficient plants but also to an increased enzyme content in each tip; moreover, the enhanced H+ extrusion seems to be functionally coupled to the root hairs formation. Northern analysis of PM H+-ATPase mRNA extracted from apical root segments using a cDNA probe specific for the PM H+-ATPase of cucumber roots suggests that the modulation of the enzyme synthesis occurs at the transcriptional level.