GmHMA3 sequesters Cd to the root endoplasmic reticulum to limit translocation to the stems in soybean

A single point-mutation in GmHMA3 (Glycine max heavy metal-associated ATPase; a wild type allele cloned from a low Cd-accumulated soybean) is closely associated with seed cadmium (Cd) concentration. It is linked to Cd transportation in yeast, and is primarily expressed in the roots of plants. We hypothesized that the function of GmHMA3w in soybean would be akin to that of OsHMA3 in rice, which expresses in the root tonoplast and sequestrates Cd into the root vacuole to reduce Cd translocation to the shoots and limit its accumulation in the seeds. In this study, the transient expression of the GmHMA3w-GFP fusion protein in rice mesophyll protoplasts indicated that the subcellular localization of GmHAM3w was in the endoplasmic reticulum (ER). Overexpression of GmHMA3w increased the Cd concentration in the roots, decreased the Cd concentration in the stems, and did not affect the Cd concentration in the leaves. Additionally, its overexpression did not alter the Cd concentration across the whole plant. These findings indicated that GmHMA3w does not influence the Cd uptake, but limits the translocation of Cd from the roots to the stems. GmHMA3w thus acts in metal transportation. Assessment of the subcellular distribution of Cd indicated that GmHMA3w facilitated transport of Cd from the cell wall fraction to the organelle fraction, and then sequestrated Cd into the root ER, thus limiting its translocation to the stems. Additionally, the results also suggested that the ER constitutes a site of particularly high Cd sensitively in plants.