Arsenic Uptake, Toxicity, Detoxification, and Speciation in Plants: Physiological, Biochemical, and Molecular Aspects

Abstract

Environmental contamination with arsenic (As) is a global environmental, agricultural and health issue due to the highly toxic and carcinogenic nature of As. Exposure of plants to As, even at very low concentration, can cause many morphological, physiological, and biochemical changes. The recent research on As in the soil-plant system indicates that As toxicity to plants varies with its speciation in plants (e.g., arsenite, As(III); arsenate, As(V)), with the type of plant species, and with other soil factors controlling As accumulation in plants. Various plant species have different mechanisms of As(III) or As(V) uptake, toxicity, and detoxification. This review briefly describes the sources and global extent of As contamination and As speciation in soil. We discuss different mechanisms responsible for As(III) and As(V) uptake, toxicity, and detoxification in plants, at physiological, biochemical, and molecular levels. This review highlights the importance of the As-induced generation of reactive oxygen species (ROS), as well as their damaging impacts on plants at biochemical, genetic, and molecular levels. The role of different enzymatic (superoxide dismutase, catalase, glutathione reductase, and ascorbate peroxidase) and non-enzymatic (salicylic acid, proline, phytochelatins, glutathione, nitric oxide, and phosphorous) substances under As(III/V) stress have been delineated via conceptual models showing As translocation and toxicity pathways in plant species. Significantly, this review addresses the current, albeit partially understood, emerging aspects on (i) As-induced physiological, biochemical, and genotoxic mechanisms and responses in plants and (ii) the roles of different molecules in modulation of As-induced toxicities in plants. We also provide insight on some important research gaps that need to be filled to advance our scientific understanding in this area of research on As in soil-plant systems.

Keywords: arsenic contamination; bioavailability; oxidative stress; phosphate; plant health; potentially toxic elements; reactive oxygen species.

Figure 1

Diagrammatical representation of different components involved in As uptake, transportation, and detoxification in the plants. Modified with permission from Kumar et al. [128].

Figure 2

Arsenic toxicity in plants: morphological (reduction in leaf number, chlorosis, necrosis leaf senescence, and defoliation), physiological (reduction in shoot and root growth, restricted stomatal conductance and nutrient uptake, chlorophyll degradation, and limited biomass and yield production), and biochemical (overproduction of reactive oxygen species (ROS), leading to carbohydrate, protein, and DNA damage) responses.

Figure 3

Protective functions of proline in plants under arsenic (As) stress. Proline provides tolerance against As mainly by reducing As uptake, providing osmoregulation, enhancing pigment concentrations, stabilizing macromolecules and cell membranes, maintaining redox state of cell, and scavenging reactive oxygen species (ROS) by through stimulating antioxidant enzymes activities and by vacuolar sequestration of As via enhanced synthesis of PCs.

Figure 4

Functions of nitric oxide in plants under arsenic (As) stress. Nitric oxide provides tolerance against As by reducing As uptake through regulating various transporters, reducing chlorosis by increasing iron (Fe) concentrations in shoots, causing vacuolar sequestration of As through enhanced synthesis of PCs, and reducing ROS-mediated oxidative stress by enhanced activities of antioxidant enzymes.

Figure 5

Role of salicylic acid in plants under arsenic (As) stress. Salicylic acid provides tolerance against As by reducing As uptake by regulating transporters, limiting As translocation to shoots, maintaining redox balance of the cell, reducing chlorosis by increasing shoot iron (Fe) concentrations, scavenging reactive oxygen species (ROS), and stabilizing the membrane by enhanced production of NO and antioxidant enzymes.

Figure 6

Impact of phosphate in plants under arsenic (As) stress. Phosphate provides tolerance against As by reducing arsenate (As(V)) uptake and limiting its translocation to shoots, arsenate reductase-mediated conversion of arsenate to arsenite (As(III)), and its subsequent sequestration into vacuoles, scavenges reactive oxygen species (ROS), and reduces oxidative stress by increasing the activities of antioxidant enzymes.