Summary• The cadmium (Cd) over-accumulating rice (Oryza sativa) cv Cho-Ko-Koku was previously shown to have an enhanced rate of root-to-shoot Cd translocation. This trait is controlled by a single recessive allele located at qCdT7.• In this study, using positional cloning and transgenic strategies, heavy metal ATPase 3 (OsHMA3) was identified as the gene that controls root-to-shoot Cd translocation rates. The subcellular localization and Cd-transporting activity of the gene products were also investigated.• The allele of OsHMA3 that confers high root-to-shoot Cd translocation rates (OsHMA3mc) encodes a defective P 1B -ATPase transporter. OsHMA3 fused to green fluorescent protein was localized to vacuolar membranes in plants and yeast. An OsHMA3 transgene complemented Cd sensitivity in a yeast mutant that lacks the ability to transport Cd into vacuoles. By contrast, OsHMA3mc did not complement the Cd sensitivity of this yeast mutant, indicating that the OsHMA3mc transport function was lost.• We propose that the root cell cytoplasm of Cd-overaccumulating rice plants has more Cd available for loading into the xylem as a result of the lack of OsHMA3-mediated transportation of Cd to the vacuoles. This defect results in Cd translocation to the shoots in higher concentrations. These data demonstrate the importance of vacuolar sequestration for Cd accumulation in rice.

The heavy metal cadmium (Cd) is highly toxic to humans and can enter food chains from contaminated crop fields. Understanding the molecular mechanisms of Cd accumulation in crop species will aid production of safe Cd-free food. Here, we identified a single recessive gene that allowed higher Cd translocation in rice, and also determined the chromosomal location of the gene. The Cd hyperaccumulator rice variety Cho-Ko-Koku showed 3.5-fold greater Cd translocation than the no-accumulating variety Akita 63 under hydroponics. Analysis of an F(2) population derived from these cultivars gave a 1:3 segregation ratio for high:low Cd translocation. This indicates that a single recessive gene controls the high Cd translocation phenotype. A QTL analysis identified a single QTL, qCdT7, located on chromosome 7. On a Cd-contaminated field, Cd accumulation in the F(2) population showed continuous variation with considerable transgression. Three QTLs for Cd accumulation were identified and the peak of the most effective QTL mapped to the same region as qCdT7. Our data indicate that Cd translocation mediated by the gene on qCdT7 plays an important role in Cd accumulation on contaminated soil.

Cadmium (Cd) is a heavy metal that is toxic to humans, and the accumulation of Cd in rice (Oryza sativa L.) grains is a widespread problem in rice agriculture. Phytoremediation is one of the most effective methods for reducing soil Cd levels in paddy fields, and rice is a promising candidate for phytoremediation. In this study, we developed the rice line "Akita 110" for use in Cd phytoremediation, particularly in Akita Prefecture, Japan. "Akita 110" was derived from a cross between "Cho-ko-koku" and "Akita 63." Compared to "Akitakomachi," which is the leading cultivar in Akita Prefecture, Japan, "Akita 110" exhibited a heading date and maturing date that were 16 and 15 d later, respectively. The culm length of "Akita 110" was remarkably shorter than that of "Cho-ko-koku", and "Akita 110" exhibited improved lodging resistance. The shoot dry weight of "Akita 110" was larger than that of "Cho-ko-koku." "Akita 110" exhibited a non-shattering phenotype. The Cd extraction of "Akita 110" was equivalent to that of "Cho-ko-koku." The average of Cd extraction using "Akita 110" in five different Cd-contaminated fields was 20.1 mg m −2 , whereas that using "Cho-ko-koku" was 24.3 mg m −2 . In a large-scale field trial, Cd levels in shoots of "Akita 110" were equivalent to those of "Cho-ko-koku". One year from initial planting, soil Cd concentrations in plots remediated with "Akita 110" reduced by 15.5%, whereas remediation with "Cho-ko-koku" reduced soil Cd levels by 10.1%. The "Akita 110" developed here exhibits improved cultivation traits compared to the "Cho-ko-koku," with comparable Cd extraction performance. Thus, "Akita 110" is a promising candidate for Cd phytoremediation in northern parts of Japan. ARTICLE HISTORY

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