High- and low-affinity zinc transport systems and their possible role in zinc efficiency in bread wheat

There is considerable variability among wheat (Triticum aestivum L.) cultiv ars in their ability to grow and yield well in soils that contain very low levels of available Zn. The physiological basis for this tolerance, termed Zn efficiency, is unknown We investigated the possible role of Zn2+ influx across the root cell plasma membrane in conferring Zn efficiency by measuri ng short-term Zn-65(2+) uptake in two contrasting wheat cultivars, Zn-effic ient cv Dagdas and Zn-inefficient cv BDME-10. Plants were grown hydroponica lly under sufficient and deficient Zn levels, and uptake of Zn-65(2+) was m easured over a wide range of Zn activities (0.1nM-80 muM). Under low-Zn con ditions, cv BDME-10 displayed more severe Zn deficiency symptoms than ev Da gdas. Uptake experiments revealed the presence of two separate Zn transport systems mediating high- and low-affinity Zn influx. The low-affinity syste m showed apparent K-m values similar to those previously reported for wheat (2-5 muM). Using chelate buffered solutions to quantify Zn2+ influx in the nanomolar activity range, we uncovered the existence of a second, high-aff inity Zn transport system with apparent K-m values in the range of 0.6 to 2 nM. Because it functions in the range of the low available Zn levels found in most soils, this novel high-affinity uptake system is likely to be the predominant Zn2+ uptake system. Zn2+ uptake was similar for cv Dagdas and c v BDME-10 over both the high- and low-affinity Zn2+ activity ranges, indica ting that root Zn2+ influx does not play a significant role in Zn efficienc y.