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.
Highlights
IntroductionRice has a genetic propensity to accumulate high levels of cadmium (Cd) in grains
Among cereals, rice has a genetic propensity to accumulate high levels of cadmium (Cd) in grains
Several experimental sources of evidence indicate OsHMA2 – a putative Zn transporter belonging to the P1B-type ATPase family – as the main candidate protein that could be involved in mediating Cd- and Zn-xylem loading in rice (Nocito et al 2011; Satoh-Nagasawa et al 2012; Takahashi et al 2012)
Summary
Rice has a genetic propensity to accumulate high levels of cadmium (Cd) in grains. 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. Cd is not essential for plant nutrition, it can be taken up from the soil by plant roots and accumulated in different plant organs. In this way, Cd contamination of soils can result in moderate Cd accumulation in the edible parts of staple crops, which, in turn, represent the major pathway for Cd entry into the agricultural food chain, as well as the Fontanili et al Rice (2016) 9:16 major route for chronic Cd exposure in general populations (Wagner 1993; FAO/WHO 2001). Cd is known to accumulate in the kidneys and shows a long biological half-life in the whole body, ranging from 10 to 33 years (Ellis et al 1979)
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