Importance of CAX and HMA transporters in the distribution of Cd in Arabis alpina

Abstract

The membrane transporters of plants are important for the transport and distribution of heavy metals, which is the basis for accumulation of heavy metals in hyperaccumulators. Pot and hydroponic experiments were conducted in order to understand effects of Cd dose (0, 10, 20 mg·kg⁻¹) on the transport pathway of Cd in hyperaccumulator Arabis alpina, the concentrations of transporter CAX (cation/H⁺ reverse transporter) and HMA (heavy metal ATPase), and the response of Cd distribution to the inhibitors DNP (2,4-dinitrophenol), which is oxidative phosphorylation uncoupling agent and reduce HMA activity. The results showed that the concentrations of transporter CAX and HMA in roots under 20 mg∙kg⁻¹ Cd treatment decreased by 31% and 561% compared with under 10 mg∙kg⁻¹ Cd treatment, respectively. The Cd contents of the roots and leaves under 20 mg∙kg⁻¹ Cd treatment were significantly increased by 1.95 and 1.84 times compared with 10 Cd mg∙kg⁻¹ Cd treatment (P < 0.05), the similar increase trend for subcellular components. Cd contents in saps of apoplast, symplasm, phloem, and xylem under 20 mg∙kg⁻¹ Cd treatment significantly increased by 78%, 287%, 238%, and 191% compared with 0 mg∙kg⁻¹ Cd treatment, respectively. The internal flow rates of Cd²⁺ showed in sequence: endodermis > leaf vein > epidermis (root xylem). Cd with DNP treatment was mainly distributed in the cell wall, which accounted for 92%. The Cd contents of A. alpina leaves decreased by 17% and 33% with 10 mg·L⁻¹ Cd + 25 or 50 µmol·L⁻¹ DNP treatments, respectively. The results suggested that the symplast should be the main pathway of Cd transport in A. alpina root related to transporter CAX. Cd loading from endodermis to xylem based on HMA and Cd transport in phloem should be the key for Cd distribution of A. alpina.

Graphical Abstract