Mercury toxicity, molecular response and tolerance in higher plants

Mercury (Hg) contamination in soils has become a great concern as a result of its natural release and anthropogenic activities. This review presents broad aspects of our recent understanding of mercury contamination and toxicology in plants including source of Hg contamination, toxicology, tolerant regulation in plants, and minimization strategy. We first introduced the sources of mercury contamination in soils. Mercury exists in different forms, but ionic mercury (Hg(2+)) is the predominant form in soils and readily absorbed by plants. The second issue to be discussed is the uptake, transport, and localization of Hg(2+) in plants. Mercury accumulated in plants evokes severe phytotoxicity and impairs numerous metabolic processes including nutrient uptake, water status, and photosynthesis. The mechanisms of mercury-induced toxicology, molecular response and gene networks for regulating plant tolerance will be reviewed. In the case of Hg recent much progress has been made in profiling of transcriptome and more importantly, uncovering a group of small RNAs that potentially mediates plant tolerance to Hg. Several newly discovered signaling molecules such as nitric oxide and carbon monoxide have now been described as regulators of plant tolerance to Hg. A recently emerged strategy, namely selection and breeding of plant cultivars to minimize Hg (or other metals) accumulation will be discussed in the last part of the review.