Studying Membrane Transport Processes by Non-invasive Microelectrodes: Basic Principles and Methods

Non-invasive microelectrode flux measurement is a convenient tool to study membrane transport processes in plants in situ. Its high resolution (a few microns in space and several seconds in time) enables real-time studies of membrane transport processes at various levels of plant structural organization, from intact organs to various tissues, single cells or protoplasts derived from these cells. Being non-invasive, the technique allows flux measurements over several hours or days, even from rapidly moving tissues such as growing or nutating plant axial organs. Importantly, fluxes of several ions and neutral molecules can be measured concurrently, enabling not only quantitative estimates of rates of measured processes, but also providing some valuable information about the stoichiometry between activities of various membrane transporters and their time dependence. In this review, we revise the principles of non-invasive ion and neutral molecule flux measurements using the MIFE™ technique and discuss some methodological aspects of manufacturing, calibrating, and using flux measuring microelectrodes.