Plant detoxification capacity of xenobiotic compounds as limiting factor for phytoremediation

Plants have efficient mechanisms for detoxification and sequestration of xenobiotic chemicals and trace elements, basis of any phytoremediation process. However the amount of pollutants that can be accumulated and detoxified without disrupting the primary and secondary plant metabolism is probably limited and specific to a particular plant species. Enzymes involved in xenobiotics detoxification are often linked to the redox biochemistry. Overloading a plant with a xenobiotic requiring oxidation by cytochrome P450 monooxygenases may thus 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 the complex respiratory chains of plant mitochondria. Molecules involved in conjugation of xenobiotics, like glutathione, also play a major role in plant metabolism. The presence of many xenobiotic compounds induces the biosynthesis of glutathione transferases and also 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 biotransformation seems integrated into the metabolism involved in the plant interactions with its environment, with probable crosstalks between detoxification and secondary metabolism.