Understanding cytokinin: Biosynthesis, signal transduction, growth regulation, and phytohormonal crosstalk under heavy metal stress

Anthropogenic activities have gradually led to heavy metal (HM) stress in plants, a growing concern threatening global food security. Hence, this issue requires significant efforts to mitigate the impact of HM stress on plants. Recently, phytohormones have gained attention as an approach to alleviate HM stress by enhancing plant tolerance through exogenous application and strategic modulation of endogenous biosynthesis and signaling pathways. Cytokinins (CKs) have emerged as key regulators in various physiological processes, particularly under HM stress, as they play a vital role in orchestrating adaptive responses by altering antioxidant defense systems, maintaining ion balance, and regulating stress-responsive gene expression. Moreover, CKs interact with other phytohormones, forming complex signaling networks that influence diverse aspects of biological processes. Understanding these interactions offers valuable insights into enhancing plant growth, development, and HM stress tolerance, particularly in contaminated areas. This review explores critical aspects of CK biosynthesis, transport, and signal transduction, as well as their roles in growth, development, and HM stress response. Additionally, it examines the crosstalk between CKs and other phytohormones, highlighting their potential to facilitate plant adaptations and gene expression, thus enhancing resilience under HM stress. This study integrates the mechanisms by which CKs modulate HM tolerance in plants, emphasizing their signaling dynamics under HM stress and novel interactions with other phytohormones. Furthermore, it addresses various aspects of these complex regulatory networks.