Abstract
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.
Highlights
Environmental pollution by heavy metals and its negative effect on living organisms, including plants, is one of the urgent problems of our time
The ability of plants to grow under heavy metal pollution indicates the existence of different mechanisms of their metal resistance (Yang, Feng, He, and Stoffella, 2005; Titov, Kaznina, and Talanova, 2014) that correspond to the two main strategies of plant survival under stress — 1) the strategy of avoidance or limitation of metals uptake by plant cells and 2) the strategy of resistance coupled with intracellular mechanisms of metal detoxification (Chirkova, 2002; Titov, Kaznina, and Talanova, 2014)
The basis of the phenomenon of metal ion detoxification with organic acids 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 (Anjum et al, 2015)
Summary
Environmental pollution by heavy metals and its negative effect on living organisms, including plants, is one of the urgent problems of our time. Organic acids are important metabolites formed in plants mainly in the Krebs cycle and glyoxylate cycle, as well as in the processes of C4 and CAM photosynthesis (Lopez-Bucio, Nieto-Jacobo, Ramirez-Rodriguez, ad Herrera-Estrella, 2000; Igamberdiev and Eprintsev, 2016). They can form both active cytosolic and storage vacuolar pools that can be used to maintain the ionic balance in plant cells (Osmolovskaya, Kuchaeva, and Novak, 2007). The objective of this review is to discuss advances in the study of organic acids secretion by plant roots in response to metal exposure, the role of organic acids in heavy metals uptake, and the investment of organic acids in transport and internal chelation of metals in plants
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