High- and Low-Affinity Zinc Transport Systems and Their Possible Role in Zinc Efficiency in Bread Wheat

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Abstract
There is considerable variability among wheat (Triticum aestivum L.) cultivars 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 measuring short-term 65Zn2+ uptake in two contrasting wheat cultivars, Zn-efficient cv Dagdas and Zn-inefficient cv BDME-10. Plants were grown hydroponically under sufficient and deficient Zn levels, and uptake of 65Zn2+ was measured over a wide range of Zn activities (0.1 nm–80 μm). Under low-Zn conditions, cv BDME-10 displayed more severe Zn deficiency symptoms than cv Dagdas. Uptake experiments revealed the presence of two separate Zn transport systems mediating high- and low-affinity Zn influx. The low-affinity system showed apparent K m values similar to those previously reported for wheat (2–5 μm). Using chelate buffered solutions to quantify Zn2+ influx in the nanomolar activity range, we uncovered the existence of a second, high-affinity 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 cv BDME-10 over both the high- and low-affinity Zn2+ activity ranges, indicating that root Zn2+influx does not play a significant role in Zn efficiency.
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