Diversity and versatile functions of metallothioneins produced by plants: A review

Metal ions are essential for plant growth and development, but in excess, these compounds can become highly toxic. Plants have adopted numerous ways to maintain metal homeostasis while mitigating adverse effects of excess metal ions, including phytochelatin and the metal-chelating proteins metallothioneins (MTs). A family of cysteine (Cys)-rich, intracellular, and low-molecular-weight (4–8 kDa) MTs are proteins found in nearly all phyla including plants, animals, and fungi, and they have the potential to scavenge reactive oxygen species and detoxify toxic metals including copper, cadmium, and zinc. Based on their Cys numbers and residues, MTs have been categorized into three major classes. Class I MTs, which have highly conserved Cys residues, are found in animals, while class II MTs, with less conserved Cys residues, are present in plants and are classified further into four groups. Class III MTs include phytochelatins, a group of enzymatically synthesized Cys-rich proteins. The MTs have been an area of interest for five decades with extensive studies, which have been facilitated by advancements in instrumental techniques, protein science, and molecular biology tools. Here, we reviewed current advances in our understanding of the regulation of MT biosynthesis, their expression, and their potential roles in the alleviation of abiotic stresses (i.e., drought, salinity, and oxidative stresses) and heavy metal detoxification and homeostasis.