Polyamines, metallothioneins, and phytochelatins—Natural defense of plants to mitigate heavy metals

Polyamines are naturally occurring secondary metabolites that are known to possess nonenzymatic antioxidant properties. They are carbon- and nitrogen-containing polycationic compounds (at physiologic pH) and are widely found in plants, animals, and bacteria. The three most predominant polyamines in plants are putrescine, spermidine, and spermine. Putrescine serves as the precursor molecule in the subsequent biosyntheses of spermidine and spermine. Plants under oxidative stress have enhanced endogenous polyamine levels. Polyamines combat oxidative stress by scavenging the free radicals. The exogenous addition of polyamines to growth medium is found to be beneficial for plants as it happened to alleviate the damage caused by oxidative stress. Polyamines are known to bind metals and this can be an implication at their mode of action in mitigating stress. Metallothioneins, on the other hand, are gene-encoded protein products that function as a nonenzymatic, antioxidant system in plants. Metallothioneins are low molecular weight, cysteine-rich polypeptides that are responsive to heavy metal stress, and their gene expression is induced by oxidative stress of different kinds. Metallothioneins have cysteine-rich N- and C-terminal domains that bind heavy metal ions, mostly, Cu, Zn, and Cd. Among the various kinds of oxidative stress, heavy metal-induced stress has become a serious concern in recent years. Plants are regularly being exposed to heavy metal-induced toxicity and this is affecting the food (e.g., Oryza sativa, Triticum aestivum, Zea mays) and the cash (e.g., Brassica juncea, Brassica napus, Hordeum vulgare, Sesamum indicum) crops at large. Heavy metals are defined as those metals and metalloids which have an atomic number greater than 20 and density 5 g cc− 1 and also have properties of metals. Essential and nonessential heavy metals, when present in the soil in an excess concentration, causes reactive oxygen species (ROS) generation. ROS contains unpaired electron(s) which causes oxidative damage to the plants which ultimately leads to the death of the plant. In this review, we discuss the effect of two essential and two nonessential heavy metals—copper, zinc, and cadmium and lead, respectively, and their effects on plants in general. Plantago ovata is a medicinally and commercially vital crop with a repertoire of antioxidant compounds like polyphenols, flavonoids, metallothionein (types 1, 2, and 3), phytochelatins, and polyamines present in it and it is the test system. The biochemical and molecular alterations that occur due to the exposure of P. ovata to copper, zinc, cadmium, and lead are discussed in this review. This review also talks of how metallothionein genes and especially type 2 metallothionein in P. ovata aid in tolerance and homeostasis of the four heavy metals—Zn, Cu, Cd, and Pb; and their differential expressions. This study also demonstrates alteration in metallothionein type 2 (PoMT2) expression of P. ovata in the presence of polyamines putrescine, spermidine, and spermine in addition to ZnSO4·H2O by semiquantitative and quantitative PCR techniques. When P. ovata seedlings are exposed to heavy metals—Cu, Zn, Cd, and Pb, they showed increased expressions of metallothionein type 2. We have observed downregulations in the expression of metallothionein type 2 gene in the presence of polyamines, putrescine, spermidine, and spermine, which implies their concerted protective and antioxidant activities to fight against the zinc-induced oxidative stress. This review also shows phytochelatins, which are naturally produced by plants in vivo, in response to oxidative stress induced by heavy metals. They are also cysteine-rich peptides which aid in heavy metal homeostasis and detoxification—a function very similar to metallothioneins and hence also classified as class III metallothioneins. Concerted action of all the heavy metal chelating entities has been emphasized in this study