Abstract:

Owing to serious heavy metal pollution, much attention has been paid to its effects on soil-plant systems. The research of heavy metal tolerance and hyperaccumulation of higher plants has become a hot topic in the field of pollution ecology. With the development of molecular ecology, research on the mechanisms of heavy metal tolerance, detoxification and accumulation in higher plants has made progress in recent years. There are significant differences in the tolerance to and accumulation of heavy metals among higher plant species and genotypes. Root systems are the first entrance of heavy metal pollutants from the soil into plant. Root exudates reduce the availability and toxicity of metal pollutants and play an important role in ability for plants to absorb heavy metals. Almost all heavy metal ions enter root cells with the help of a metal transporter protein that are subsequently transported to the vacuole. The synthesis of PC in response to the stress caused by heavy metals is one of the adaptive responses common in higher plants. Heavy metal tolerant genotypes have higher levels of PC than non-tolerant genotypes under heavy metal stress. GSH is the substrate that synthesizes PC, which chelates the heavy metals. Heavy metal-PC chelatins are subsequently transported from the cytosol to the vacuole and heavy metal detoxification is thus achieved. MTs play the same role and in the same way as PC under heavy metal stress. The article reviews recent advances in understanding the role of root exudates, metal transporter proteins (MTs, PC and GSH), molecular mechanisms of heavy metal tolerance and hyperaccumulation in higher plants at the molecular level. Existing problems and major topics of future research were discussed.

Key words: Higher plant, Heavy metal, Tolerance, Hyperaccumulation, Phytoremediation