Breeding for low cadmium barley by introgression of a Sukkula-like transposable element.

Barley is the fourth most produced cereal crop in the world and one of the major dietary sources of cadmium (Cd), which poses serious threats to human health. Here, we identify a gene that encodes a P-type heavy metal ATPase 3 (HvHMA3) responsible for grain Cd accumulation in barley. HvHMA3 from the high Cd barley variety Haruna Nijo in Japan and the low Cd variety BCS318 in Afghanistan shared 97% identity at the amino acid level. In addition, the HvHMA3 from both varieties showed similar transport activity for Cd and the same subcellular localization at the tonoplast. However, the expression of HvHMA3 was double in BCS318 than in Haruna Nijo. A 3.3-kilobase Sukkula-like transposable element was found to be inserted upstream of the gene in the low Cd variety, which functioned as a promoter and enhanced the expression of HvHMA3. Introgression of this insertion to an elite barley cultivar through backcrossing resulted in decreased Cd accumulation in the grain grown in Cd-contaminated soil without yield penalty. The decreased Cd accumulation resulting from the insertion was also found in some other barley landraces in the world. Our results indicate that insertion of the Sukkula-like transposable element plays an important role in upregulating HvHMA3 expression.