Abstract

In plants, the elements Fe, Zn, Mn, Cu, Ni, B, Mo and Cl are considered to be micronutrients essential for plant growth. Micronutrient uptake systems are intrinsically more difficult to investigate than their macronutrient counterparts because of the low fluxes involved. Currently, the mechanism of transport for these micronutrients has not been clearly identified, except for Cl. In the case of the trace metal micronutrients, uptake studies point to the presence of high and low affinity transporters with broad substrate specificity. The kinetics of these transporters is clouded by the failure of many investigators to consider the effects of the electrostatic nature of the plasma membrane on cation uptake. Recent work has helped to clarify the nature of B movement across membranes and there is now evidence of a facilitated transport system for B, in addition to its passive permeation directly through the membrane. The uptake of Mo is known to be induced by NO₃ and inhibited by W, but little further information is available on how Mo enters cells. In recent years, the emphasis has shifted from physiological studies of micronutrient uptake to molecular investigations of transporters cloned in plants and characterized in heterologous expression systems. There is now a substantial catalogue of transporter genes, mostly for trace metals, whose functions in plants have yet to be clearly defined.