Abscisic Acid: Metabolism, Signaling, and Crosstalk with Other Phytohormones under Heavy Metal Stress

Heavy metal (HM) stress poses a global risk to crops, ecological systems, and human health. It disrupts cellular ionic equilibrium, cell membrane integrity, metabolic balance, and the activities of enzymes and proteins, severely impacting physiological processes, plant development, and agricultural productivity. Although plants naturally activate defense mechanisms to mitigate the adverse effects of HM stress, they cannot completely prevent them. Phytohormones counter HM toxicity, aiding growth. External application and internal regulation via signaling/biosynthesis genes offer defense against HM-induced damage. A pivotal signaling molecule in plant adaptive responses to environmental stressors, including HM toxicity, is abscisic acid (ABA). Despite ABA’s role in abiotic stress responses such as drought and salinity, its function and crosstalk with other phytohormones under HM stress remain poorly understood. Nonetheless, exogenously applied ABA serves as a strategic approach to enhancing plants’ resistance to HM toxicity by promoting osmolyte accumulation and reinforcing antioxidant activity. ABA significantly regulates various plant growth and metabolic activities under diverse environmental conditions. This review highlights the effects of HM stress on plants and explores ABA involvement in production, signaling, catabolism, and transport within plant tissues. The purpose of this paper is to shed light on the complex interplay between the metabolism of ABA, its signaling, and its interactions with other phytohormones (e.g., auxins, gibberellins, and ethylene) during HM exposure. Furthermore, we delve into the function of ABA to mitigate HM stress and elucidate its interactions with other phytohormones.