Heavy metal signalling in plants: linking cellular and organismic responses

Heavy metals are required in plants as essential micronutrients or act as toxic compounds. How do plants perceive heavy metals and which signalling cascades are triggered leading to plant adaptation or injury? Copper (Cu) and cadmium (Cd) are reviewed as examples for heavy metals with contrasting physicochemical properties and functions in plants. Cu is an essential ligand for the catalytic activity of many enzymes. Its uptake and trafficking are tightly regulated and mediated by specific transporters and chaperones. Cu serves as a signalling intermediate for ethylene reception. Excess Cu is sensed by binding to transcription factors, thereby, activating an arsenal of abiotic stress defences including increased expression of metallothioneins, phytochelatins, and antioxidants which contribute to remove “free” Cu and to re-establish cellular ion and redox homeostasis. In contrast to Cu, no specific uptake systems are known for Cd. Cd enters cells by metal transporters with broad substrate specificities and probably also via Ca channels. It is toxic because of its high reactivity with sulphhydryl groups and causes oxidative stress by depletion of antioxidative systems and stimulation of H2O2-producing enzymes. As a result, Cd triggers stress signalling pathways similar to those activated by Cu including cascades leading to programmed cell death. Important cross-talk exists between heavy metal and other abiotic stress signalling pathways (drought, oxidative stress). Excess heavy metals affect root functions at multiple levels and cause accumulation of abscisic acid (ABA). We propose a model how ABA and Cd signalling may interact at the organismic level to influence plant water status. Cytokinins act as antagonists of Cd indicating that the plant internal hormonal status may critically affect heavy metal tolerance.