The role of organic acids in heavy metal tolerance in plants

Organic acid metabolism is of fundamental importance at the cellular and at the whole plant level. In recent years there has been increased attention in the role of organic acids in modulating adaptation to the environment, including organic acids participation in the detoxification of heavy metals. The basis of the phenomenon is the ability of acids such as citrate, malate, oxalate, malonate, aconitate and tartrate to form strong bonds with heavy metal ions through metal chelatation with carboxyl groups carrying the function of donor oxygen in metal-ligands. This review deals with aspects of extracellular and intracellular chelation of heavy metal ions with the involvement of organic acids. We consider the role of metal-induced secretion of malate, citrate and oxalate by roots of various plant species in extracellular complexation of heavy metals and in the reduction of their bioavailability for plants. We also review the possible mechanisms of stimulation of metals uptake by plants under the influence of exogenous application of organic acids in the soil. The efficiency of intracellular chelation of heavy metal ions with the participation of organic acids is considered due to the importance of this strategy in hyperaccumulators and non-hyperaccumulators to improve metal tolerance in plants.