Submitted to: New Phytologist
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/22/2005
Publication Date: 12/8/2005
Citation: Hart, J.J., Welch, R.M., Norvell, W.A., Kochian, L.V. 2005. Zinc effects on cadmium accumulation and partitioning in near-isogenic lines of durum wheat that differ in grain cadmium concentration. New Phytologist. 167:391-401. Interpretive Summary: Durum wheat grown in the Great Plains of North America can accumulate excessive levels of the toxic heavy metal cadmium (Cd), which occurs naturally in some soils of that region. In order to ensure the safety and marketability of this important crop, it is vital to understand the physiological mechanism(s) behind Cd accumulation in durum grains. Cultivars of durum wheat that differ significantly in grain Cd levels have been developed by standard breeding techniques. Two of these near-isogenic lines that are genetically identical except for the differential Cd-accumulation trait, were employed in an effort to identify the function of the gene(s) that confers differential Cd accumulation. Results of these experiments show that there was no difference between isolines in uptake of Cd from soil into roots. There was a significant difference between isolines in the ability to move Cd from roots to shoots, with the low grain Cd accumulating isoline retaining more Cd in roots. Analysis of Cd storage in roots revealed that all root Cd was bound as a complex with phytochelatins (PCs), molecules previously shown to bind heavy metals in plant roots. There was no difference between isolines in the capacity for Cd storage in roots, nor was the ability to synthesize PCs a limiting factor in either isoline. These results suggest that the ability to store Cd is not the basis for differential Cd grain accumulation. Rather, movement of Cd from cell to cell in roots or transport of Cd into the vascular tissue leading to shoots are more likely to be responsible for differential Cd accumulation in durum wheat. The study also provided evidence that PCs may be involved with the ability of root cells to regulate levels of zinc, an essential micronutrient in plants and animals.
Technical Abstract: Here, we examined several physiological properties of two near-isogenic lines of durum wheat (Triticum turgidum L. var durum) that differ in grain cadmium (Cd ) accumulation in an effort to identify the function of the gene that confers differential grain Cd levels. Results show that the two isolines did not differ in either time- or concentration-dependent root uptake of Cd, but did exhibit strong differences in translocation and root/shoot partitioning of Cd, with the low grain Cd accumulating isoline showing decreased movement of Cd from roots to shoots. All buffer-soluble Cd extracted from roots of both isolines grown in low Cd solution was in the form of a low molecular weight phytochelatin (PC)-containing complex; 12-16% of root Cd was insoluble. The data suggest that PC synthesis is not a limiting factor in the storage of Cd in roots, and that intercellular movement of Cd through the root and into the transpiration stream is a more likely candidate for the basis of differential Cd partitioning in the two isolines. Root/shoot partitioning of zinc (Zn) was not different between isolines, and Zn was present in the same gel filtration-derived fractions that contained PCs and Cd, suggesting a possible role of PCs in Zn homeostasis in durum wheat roots.