Kinetic Analysis of Zinc/Cadmium Reciprocal Competitions Suggests a Possible Zn-Insensitive Pathway for Root-to-Shoot Cadmium Translocation in Rice

Abstract

BackgroundAmong cereals, rice has a genetic propensity to accumulate high levels of cadmium (Cd) in grains. Xylem-mediated root-to-shoot translocation rather than root uptake has been suggested as the main physiological factor accounting for the genotypic variation observed in Cd accumulation in shoots and grains. Several evidence indicate OsHMA2 – a putative zinc (Zn) transporter – as the main candidate protein that could be involved in mediating Cd- and Zn-xylem loading in rice. However, the specific interactions between Zn and Cd in rice often appear anomalous if compared to those observed in other staple crops, suggesting that root-to-shoot Cd translocation process could be more complex than previously thought. In this study we performed a complete set of competition experiments with Zn and Cd in order to analyze their possible interactions and reciprocal effects at the root-to-shoot translocation level.ResultsThe competition analysis revealed the lack of a full reciprocity when considering the effect of Cd on Zn accumulation, and vice versa, since the accumulation of Zn in the shoots was progressively inhibited by Cd increases, whereas that of Cd was only partially impaired by Zn. Such behaviors were probably dependent on Cd-xylem loading mechanisms, as suggested by: i) the analysis of Zn and Cd content in the xylem sap performed in relation to the concentration of the two metals in the mobile fractions of the roots; ii) the analysis of the systemic movement of 107Cd in short term experiments performed using a positron-emitting tracer imaging system (PETIS).ConclusionsOur results suggest that at least two pathways may mediate root-to-shoot Cd translocation in rice. The former could involve OsHMA2 as Zn2+/Cd2+ xylem loader, whereas the latter appears to involve a Zn-insensitive system that still needs to be identified.Electronic supplementary materialThe online version of this article (doi:10.1186/s12284-016-0088-3) contains supplementary material, which is available to authorized users.

Cadmium adsorption, chelation and compartmentalization limit root-to-shoot translocation of cadmium in rice (Oryza sativa L.)

“…7d). These results strongly indicate that other mechanisms different from those involved in the sequestration and immobilization of metal ions in roots may be important to systemic allocation of Cd (Fontanili et al 2016), particularly at high levels, leading to differing capacities for Cd root retention in different genotypes. Otherwise, the same level of root retention would have been observed in the roots of two cultivars under all of the Cd concentration treatments.…”

Real‐time whole‐plant dynamics of heavy metal transport inArabidopsis halleriandArabidopsis thalianaby gamma‐ray imaging

“…Several whole plant positron emission tomography (PET) imaging systems have been developed using 11 C (Jahnke et al., ; Kawachi et al., ; Weisenberger et al., ), other groups have developed large scanners for β‐imaging using 32 P (Kanno et al., ), and autoradiography has been used to image radioisotope distribution of whole plants (Page & Feller, ). A scanner (Kawachi et al., ) has been used to perform PET imaging of 65 Zn and 107 Cd in rice plants (Fontanili et al., ; Suzui, Yin, Ishii, Sekimoto, & Kawachi, ). Each of these nuclear imaging methods has drawbacks: for β‐imaging, the range of β‐particles can be too short to escape the plant; for PET, radioisotope lifetimes are short and the range of the positron can be too long for a thin, low density plant, limiting the yield of annihilation gamma‐rays; and autoradiography is invasive and can require exposure times of weeks or months.…”

Visualization of zinc dynamics in intact plants using positron imaging of commercially available 65Zn

“…The radionuclides typically used for positron imaging of plants are limited to 11 C, 13 N, 15 O, and 18 F, which are processed using well-established purification methods developed for medical research [3, 9]. We have previously studied positron imaging of minor essential and toxic elements, such as 64 Cu (half-life: 12.7 h) [10] and 107 Cd (half-life: 6.5 h) [11–16], using the positron-emitting tracer imaging system (PETIS), a two-dimensional positron imaging system (special resolution: approximately 2 mm). However, this approach involves difficulties in purifying the positron-emitting metal radionuclides in other research facilities.…”