Oxalate secretion from the root apex of Sedum alfredii contributes to hyperaccumulation of Cd

Organic acids play an important role in metal detoxification in plant, but accumulation of heavy metals with the help of organic acids secretion in hyperaccumulators has not been well documented. The aim of this study was to investigate the contribution of oxalate secretion to cadmium (Cd) hyperaccumulation in S. alfredii. Hydroponic experiments were conducted to investigate the characteristics of Cd-induced secretion of oxalate in terms of pattern, location and oxalate-induced Cd translocation pathway, using ion channel inhibitors, scanning ion-selective electrode technique (SIET) and fluorescence imaging. Hyperaccumulating ecotype (HE) had nearly 2-fold higher oxalate secretion than non-hyperaccumulating ecotype (NHE). Phenylglyoxal effectively blocked Cd-induced oxalate secretion and decreased Cd concentration in HE while exogenous oxalate supply promoted Cd accumulation efficiently. SIET analysis indicated that Cd2+ influx into roots of HE mainly occurred at zone 0-10 mm from root apex where oxalate secretion was localized. Cd was distributed preferentially to the root stele of the HE but not the NHE, and was significantly increased with the secreted oxalates in HE. Cd hyperaccumulation by HE S. alfredii is partially associated with oxalate secretion from the root apex, which mediates the loading of Cd from the xylem parenchyma cells.