Abstract
In response to new European Union regulations, studies are underway to mitigate accumulation of toxic cadmium (Cd) in cacao (Theobroma cacao, Tc). This study advances such research with Cd isotope analyses of 19 genetically diverse cacao clones and yeast transformed to express cacao natural resistance-associated macrophage protein (NRAMP5) and heavy metal ATPases (HMAs). The plants were enriched in light Cd isotopes relative to the hydroponic solution with Δ114/110Cdtot-sol = −0.22 ± 0.08‰. Leaves show a systematic enrichment of isotopically heavy Cd relative to total plants, in accord with closed-system isotope fractionation of Δ114/110Cdseq-mob = −0.13‰, by sequestering isotopically light Cd in roots/stems and mobilisation of remaining Cd to leaves. The findings demonstrate that (i) transfer of Cd between roots and leaves is primarily unidirectional; (ii) different clones utilise similar pathways for Cd sequestration, which differ from those of other studied plants; (iii) clones differ in their efficiency of Cd sequestration. Transgenic yeast that expresses TcNRAMP5 (T. cacao natural resistance-associated macrophage gene) had isotopically lighter Cd than did cacao. This suggests that NRAMP5 transporters constitute an important pathway for uptake of Cd by cacao. Cd isotope signatures of transgenic yeast expressing HMA-family proteins suggest that they may contribute to Cd sequestration. The data are the first to record isotope fractionation induced by transporter proteins in vivo.
Highlights
Cadmium (Cd) is highly toxic to humans as it accumulates in the body to cause chronic damage to kidneys and bones, and it is classified as a Group 1 carcinogen by the International Agency for Research on Cancer, with no safe level of exposure[1]
The Cd isotope and concentration data that were obtained for relevant reference and quality control materials are in excellent agreement with certified values and/or published results
Replicate analyses of BAM I012 Cd (n = 16) and NIST Spinach Leaf SRM 1570a (n = 4) provide δ114/110Cd data with a reproducibility of ±0.06‰ (2 s.d.; the reproducibility of all isotopic data is quoted in the same manner in the following)
Summary
Cadmium (Cd) is highly toxic to humans as it accumulates in the body to cause chronic damage to kidneys and bones, and it is classified as a Group 1 carcinogen by the International Agency for Research on Cancer, with no safe level of exposure[1]. Additional analyses were carried out for transgenic yeast cells that were modified to express cacao genes, which encode the NRAMP5- and HMA-family transporter proteins.
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