Arsenic-induced metabolic disturbances and their mitigation mechanisms in crop plants: A review

Arsenic (As) is one of the highly toxic metalloids distributed ubiquitously in nature. Two inorganic forms of As are present abundantly: arsenite (Asᴵᴵᴵ) and arsenate (Asⱽ), former being 100 times more toxic than latter. Arsenic is a well known inducer of reactive oxygen species (ROS) in crop plants either directly during conversion of Asⱽ to Asᴵᴵᴵ or indirectly via inactivation of the antioxidants by binding to their thiol groups. Arsenic-mediated oxidative stress causes an array of metabolic dysfunctions in plants. Therefore, in recent years, demonstration of various mechanisms to improve crop productivity and/or alleviation of As toxicity has become a prime concern. Modulation of cellular thiol molecules for protection against ROS-induced damage has been used as a strategy against As. Accrual of proline, polyphenols and exogenous application of salicylic acid, nitric oxide, phosphate and potassium show protection against As-mediated injuries in crop plants. Proline, nitric oxide and salicylic acid display defensive functions by activating antioxidant machinery of crop plants whereas phosphate and potassium reduce As toxicity by controlling As-uptake or maintaining cellular protein and antioxidant enzymes in plants. Likewise, polyphenols serve as antioxidants and reduce activities of ROS synthesizing enzymes, thereby confering As-stress tolerance. In this review we have attempted to collate recent advances on 1) mechanism(s) of As uptake by plants, 2) toxicity responses [physiological, biochemical and molecular] exerted by As, and 3) roles of varied molecules in amelioration of As effects in crop plants. Gaps in the existing knowledge and future research prospects have also been highlighted in this review.