Ussing's conundrum and the search for transport mechanisms in plants

Abstract

Plant transport physiologists have developed a range of models describing the movement of ions across cell membranes. However, while substantial progress has been made towards providing precise descriptions of the mechanisms underlying these fluxes, important instances remain in which the prevailing models cannot account for repeated observations, particularly in terms of energy transformations. As we shall show, disagreements with experimental findings may entail a revision of the proposed models, similarly to what has been required in animal transport studies (Ussing, 1994; see below). We present, as a key example, the futile cycling of sodium under toxic conditions (see Britto & Kronzucker, 2006, and discussed later), and show that unidirectional flux magnitudes measured by several groups, including our own, cannot be explained energetically by current models. We attempt to explain these observations by proposing alternative mechanisms of Na + transport across the root-cell plasma membrane. The influx/efflux cycle of Na + in plants has been attributed to the sophisticated activity of distinct transport proteins in