The dual role of nitric oxide (NO) in plant responses to cadmium exposure

Anthropogenic activities such as mining, smelting, and overapplication of fertilizers contribute to introducing cadmium (Cd) into the biosphere. Cd accumulation in edible plants leads to phytotoxicity and reduces biomass formation and food production, posing a significant threat to global food security. Nitric oxide (NO) is a highly active gaseous signalling molecule involved in regulating plant responses to Cd stress. These responses include the protective role of NO in enhancing plant resistance to Cd exposure via activating the antioxidant defense system, maintaining intracellular redox homeostasis, and initiating the expression of genes relevant to stress defense. However, NO exacerbates Cd toxicity by promoting Cd uptake and accelerating programmed cell death in plants. These contradictory responses render the role of NO in regulating plant performance under Cd exposure highly controversial. To better understand the mechanisms responsible for the dual role of NO, we summarized the current knowledge on (1) the processes of Cd accumulation and detoxification in plants, (2) the pathways of NO synthesis and metabolism under Cd stress, and (3) the function of NO in regulating plant responses to Cd stress at the physiological and molecular levels. From this literature review, the processes responsible for the dual role of NO in plant responses to Cd exposure were deduced, and topics for future studies on the mechanisms of NO-mediated regulation of Cd detoxification in plants were identified.