When exposed to toxic heavy metals like nickel, lead, cadmium, and mercury in the soil, plants initiate a complex sequence of steps to counteract the effects of these poisons. Defensive measures prevent metals from getting inside cells, sequester and neutralize metals that do enter the cell, or mitigate damage caused by the metals that overwhelm these defenses.
Two mechanisms keep metal ions out of plant cells. In one case, the plant releases chelating agents into the soil which form molecular "cages" around the metal ions making them too bulky to fit through slim ion channels in the cells' protective outer cell wall. Alternatively, compounds in the cell wall material (hystidyl groups, pectic sites, and certain carbohydrates) bind to the metal ions keeping them away from the cells' internal machinery. Of course, these preventative defensive measures can be overwhelmed by high concentrations of the metals so other means of protection are required. When metal ions do get into the cell, efflux pumps (heavy metal ATPases and HMAs) actively pump metal ions out of the cells and back into the soil or into isolated spaces within the cell (vacuoles) where they are effectively sequestered. If concentrations of the metals are particularly high, the plant cells will pump the metals into its vascular network so that they can be carried to the shoots where they become imprisoned in vacuoles. Metal ions that do get past these defenses can destroy plant cells by producing reactive compounds such as hydrogen peroxide. Presence of these damaging compounds trigger cells to produce defense chemicals that, in turn, scavenge or destroy them.
"The response to heavy metal stress involves a complicated signal transduction network that is activated by sensing the heavy metal, and is characterized by the synthesis of stress-related proteins and signaling molecules, and finally the transcriptional activation of specific metal-responsive genes to counteract the stress. The relevant signal transduction pathways include the Ca-calmodulin system, hormones, ROS signaling, and the mitogen-activated protein kinase (MAPK) phosphorylation cascade, which converge by activating the above- mentioned stress-related genes. Different signaling pathways may be used to respond to different heavy metals...Excess heavy metals modify the stability of Ca channels, thus increasing calcium flux into the cell. Intracellular Ca is a secondary messenger, which interacts with calmodulin to propagate the signal and ultimately to regulate downstream genes involved in heavy metal transport, metabolism, and tolerance." (Manara 2012:28).
"One of the major roles of root exudates is to chelate metals and to prevent their uptake inside the cells. For example, Ni-chelating histidine and citrate are present in root exudates and these reduce the uptake of Ni from soil." (Manara 2012:30)
"The cell wall can play a key role in the immobilization of toxic heavy metal ions by providing pectic sites and hystidyl groups, and extra-cellular carbohydrates such as callose and mucilage, and thus prevents heavy metals uptake into the cytosol." (Manara 2012:31).
"P1B-ATPases pump metal ions out of the cytoplasm against their electrochemical gradient, into either the apoplast or into the vacuole. The eight P1B-type ATPases in A. [Arabidopsis] thaliana and rice were renamed heavy metal ATPases (HMAs). HMAs are divided into two classes, one involved in transport of monovalent cations (Cu/Ag) and the second in the transport of divalent cations (Zn/Co/Cd/Pb)." (Manara 2012:33).
"Once taken up by the roots, metal ions are loaded into the xylem and transported to the shoots as complexes with various chelators. Organic acids, especially citrate, are the major chelators for Fe and Ni in the xylem...Metal ions are also translocated from source to sink tissue via phloem. Therefore, phloem sap contains metals arising from source tissue, like Fe, Cu, Zn, and Mn." (Manara 2012:34).
"The P-type ATPases reclassified as HMAs (see above) function not only as efflux pumps to remove metal ions from the cell, but also as internal transporters to load Cd and Zn metals into the xylem from the surrounding tissues." (Manara 2012:35).
"If the toxic metal concentration exceeds a certain threshold inside the cells, an active metabolic process contributes to the production of chelating compounds. Specific peptides such as PCs and MTs are used to chelate metals in the cytosol and to sequester them in specific subcellular compartments. A large number of small molecules are also involved in metal chelation inside the cells, including organic acids, amino acids, and phosphate derivatives." (Manara 2012:36).
"If the intracellular concentration of metal ions saturates the defense mechanisms discussed above then the plant will begin to suffer oxidative stress caused by the production of ROS and the inhibition of metal-dependent antioxidant enzymes. Under these circumstances, plants activate their antioxidant responses, including the induction of enzymes such as CAT and SOD and the production of non-enzymatic free radical scavengers." (Manara 2012:43).