Accumulation of pollutants and plant wellness: bottleneck or key to a successful phytoremediation?

Plants have efficient mechanisms for detoxification and sequestration of xenobiotic chemicals and trace elements, basis of any efficient phytoremediation process. However the amount of pollutants that can be accumulated and detoxified without disrupting the normal plant metabolism and wellness is probably limited and specific to a particular plant species, ecotype or cultivar. For example, overloading a plant with a xenobiotic requiring oxidation by cytochrome P450 monooxygenases may compete with the physiological functions of these enzymes. An increase in their activity may also impose a major demand on cellular pools of oxygen and NAD(P)H, affecting plant redox and energy status, and possibly the complex respiratory chains of plant mitochondria. Molecules involved in conjugation of xenobiotics, like glutathione, also play a major role in normal plant metabolism. The presence of many xenobiotic compounds induces the biosynthesis of glutathione transferases and thus an increased use of glutathione. Plant glutathione level and redox status are thus affected under such conditions, with probable implications on sulphur requirement and assimilation. Finally many plant specific metabolites, often involved in plant interactions with its environment, have a structure similar to xenobiotics, and detoxification of the latter does probably use at least partially the metabolic pathways of the former. For example, most of the natural anthraquinones are glycosylated, whereas glycosyl-transferases are known to be involved in the conjugation of many xenobiotics. Xenobiotics metabolism seems to be integrated into the metabolism involved in the plant interactions with its environment, with probable crosstalks between detoxification and secondary metabolism.