The fate of heavy metals and metalloids in plant systems – a comprehensive review
Anupama Balagopal, Anushka Alva, Harsha K. Chandrashekar, Sachin Ashok Thorat, Arya Kaniyassery, Apurba Koley, Srinivasan Balachandran, Nisha Govender, Annamalai Muthusamy
Abstract
Heavy metals/metalloids are highly poisonous, non-degradable components that accumulate in the environment, challenging plant growth and biodiversity. They downregulate photosynthesis by disrupting pigment biosynthesis, photosynthetic electron transport, stomatal function, and inhibiting Calvin cycle enzymes, decreasing sugar biosynthesis. Understanding plant stress responses and tolerance, including photosynthetic impact, is fundamental to risk assessment and breeding tolerant cultivars. Plants have evolved coping mechanisms, including expression of defense genes and protective compounds like phytochelatins, glutathione, metallothioneins, and antioxidants, which help in sequestration, transport, compartmentalization and detoxification. Plant chelators and hormones are key markers of oxidative stress tolerance. Genetic and metabolic changes drive detoxification in monocots and dicots. Root exudates play a vital role in dicots, which secrete organic acids like citrate and malate to detoxify metals in the rhizosphere, while monocots secrete phytosiderophores to chelate metals/metalloids. Plants utilize compartmentalization and accumulation to reduce metal ion mobility. Although metal/metalloid toxicity is well-documented, systematic analysis on uptake, compartmentalization, and detoxification in both plant types is lacking. This review provides a complete understanding of these processes, investigates the physio-biochemical, molecular and metabolic responses, and includes bibliometric analysis identifying research trends, publication patterns, country-specific contributions, facilitating phytoremediation strategy development through deeper understanding of plant adaptation to metal stress.