Net cadmium flux and gene expression in relation to differences in cadmium accumulation and translocation in four apple rootstocks

• Net Cd2+ fluxes in roots of four apple rootstocks were investigated in the present study.
• The differences of Cd translocation and accumulation in four apple rootstocks were assessed.
• Key genes involved in Cd uptake, transport and detoxification were differentially expressed among different apple rootstocks.

In order to investigate the variations in Cd uptake, translocation and tolerance among four apple rootstocks, seedlings were exposed to either 0 or 50 μM CdCl2 for 50 days. Net Cd2+ influxes in the rhizosphere were greatest at 600 μm in root meristematic zone of all apple rootstocks. There were large variations in net Cd2+ influxes among different apple rootstocks and the lowest Cd2+ influxes into roots and leaf petiole vascular bundles were observed in Malus baccata (Mb). Net Cd2+ influxes into roots of all apple rootstocks were significantly decreased after the treatment with metabolic inhibitors and channel blockers, such as vanadate, verapamil and N-ethylmaleimide. Moreover, net Cd2+ influxes into roots were also affected by exogenous Fe3+ concentrations. Cd exposure caused decline in photosynthesis and biomass increment in all apple rootstocks except in M. baccata (Mb). Among four apple rootstocks, M. baccata (Mb) displayed the lowest Cd concentrations and amounts in tissues, and the smallest bio-concentration factor (BCF) and translocation factor (Tf) under Cd exposure. Gene expression levels regarding Cd uptake, transport and detoxification were in line with the tendencies of Cd absorption, transport and tolerance, respectively. M. baccata (Mb) had lower transcript levels of genes related to Cd uptake and translocation but stronger gene expression for Cd detoxification. On the basis of this experiment, it is concluded that M. baccata (Mb) is superior to other three apple rootstocks to be used as rootstock in Cd contaminated soil.