[1,2]. Conventional remediation technologies for heavy metal contaminated soils include excavation and landll, thermal treatment, acid leaching and electro-reclamation [3]. These techniques, however, are limited for large-scale remediation engineering due to expensive cost and destruction of soil biota and fertility. Alternatively, phytoremediation, an emerging solution which refers to the use of green plants for the removal of contaminants or rendering them harmless, is cost- effective, environmental-friendly and can be applied to large-scale soils [4]. Usually, it is a determining factor for phytoremediation that the plant has the ability to cultivate a large biomass with high contents of toxic metals in its shoots [5,6]. However, low biomass, low bioavailability and limited translocation of some metals to the shoots are major obstacles in the phytoextraction