Heavy metal uptake and tolerance mechanisms of serpentine flora: Implications for phytoremediation

Serpentine soils derived from ultramafic rocks containing high contents of Ni, Cr, Mn, and Co. However, Ni is the most predominant and bioavailable heavy metal in serpentine soil. Due to high heavy metal content, high Mg, low nutrient contents (N, P, Ca), and low organic matter in this particular extreme environments give an inhospitable environment for plant growth. Therefore, these extreme conditions have led for the formation of serpentine specialized flora, which could thrive under these unfavorable conditions. Hence, serpentine sites are distributed throughout the world as patches leading to serpentine endemics. The genes of these specialized plants do not show an interchange, thereby forming a specialized genetic pool. However, some of the serpentine areas are utilized in agriculture and it has caused accumulation of toxic heavy metals in edible plant parts. Consumption of heavy metal-containing plants causes bioaccumulation in herbivores and humans. Moreover, these sites may cause groundwater contaminations again causing bioaccumulation in living organisms. The plants living in serpentine areas show exclusive mechanisms confined to them for both heavy metal uptake and storage. The rhizosphere acidification is one of the predominant factors for heavy metal uptake. ZIP family proteins are a group of transporter proteins, which are involved in membrane transport of Ni serpentine flora whereas via histidine, nicotianamine, and yellow strip L like family is important in Ni transport through the xylem. Moreover, cation diffusion factor and organic acids mediate vacuolar sequestration of Ni. Though numerous studies are carried on heavy metals such as Cd and As, no sufficient research is carried out on membrane Ni transporters. Hence, more research is necessary to manipulate Ni hyperaccumulators in biotechnological applications.