Copper and Cadmium Signaling in Plants Under Stressed and Non-stressed Conditions

Soil contamination by heavy metals is a prevalent environmental problem caused by factors like urbanization, industrial growth, agriculture, waste disposal, and mining. These contaminants have harmful effects on the environment, reduce agricultural production, and pose health risks to living organisms. Cadmium (Cd) and copper (Cu) are particularly concerning due to their high mobility, toxicity, and non-degradability, affecting both plants and animals. Cadmium, being extremely noxious even at low levels, lacks any physiological or biochemical role in plant metabolism. Metal exposure leads to an excess of reactive oxygen species, causing oxidative stress in plants. To combat metal-induced impairment, plants have developed various approaches, such as compartmentalization, metal exclusion, chelation, and a range of mechanisms to scavenge reactive oxygen species, including antioxidant enzymes and other stress-relieving molecules. Furthermore, plants have evolved miRNA-directed regulation to respond to damage caused by Cd and Cu. miRNAs inhibit target mRNAs, causing post-transcriptional cleavage. Under high Cu stress, miR398 is downregulated, which boosts the expression of Cu, Zn-superoxide dismutase, an enzyme that scavenges superoxide radicals. On the other hand, miR319, miR393, miR166, and miR171 play critical roles in regulating the response to Cd stress.