Futile cycling at the plasma membrane: a hallmark of low-affinity nutrient transport

Plant nutrient acquisition from concentrated soil solutions is governed by low-affinity transport systems in the plasma membranes of root cells. In this Opinion article, we illustrate that for six major nutrient ions, in addition to influx mediation by low-affinity transporters, high rates of ion cycling at the plasma membrane are a hallmark of nutrient transport at elevated external concentrations. This phenomenon is characterized by high rates of ion influx and concurrent high efflux of the same ion, resulting in efflux:influx ratios close to 1. Prolonged maintenance of futile cellular ion cycling can be energetically unfavorable and detrimental to plant growth and survival. We discuss how ion cycling can contribute to the toxicities of ions such as Na+ and NH4+ in the low-affinity range of ion provision. We also argue that cellular ion cycling makes the reliable measurement of ion influxes in the low-affinity range a formidable challenge.