Cobalt stress induces photosynthetic and ultrastructural distortion by disrupting cellular redox homeostasis in maize

Heavy metal contamination of the environment is becoming alarmingly serious with the rapid pace of urbanization and unrestricted industrialization. Cobalt (Co) is one of the toxic and non-essential metals for plants; however, it is widely recognized as a beneficial component for numerous enzymes and coenzymes within biological systems. Nevertheless, if its level increases, it leads to severe toxicity and plants are highly susceptible to the toxic effects of elevated cobalt concentration. Herein, we investigated the effect of different levels of Co stress on maize seedlings using physiological, biochemical, ultrastructural, and molecular markers in order to depict a panoramic view of the maize response mechanism. The findings revealed that Co stress inhibited plant growth and development by causing oxidative stress, reduced nutrient uptake, and photosynthetic efficiency. The increasing concentration of Co led to the overproduction of reactive oxygen species (ROS) in root and shoot that principally caused oxidative damage as monitored by high MDA contents and ultrastructural damages. The in vivo detection of ROS stained with ROS fluorescent (H₂DCFDA and DHE) further confirmed the overproduction of ROS under stress conditions. In response to stress, a substantial upward trend was noticed in plant antioxidant defense activities with the increasing Co concentration. However, a sharp decline was observed in antioxidant enzymes under the highest used concentration, while the level of ROS continued to rise, indicating a disruption in cellular redox homeostasis. Furthermore, the qPCR results showed that Co stress differentially expressed antioxidant-related genes. Collectively, we provide the first report tracking down the underlying mechanism of Co stress-induced oxidative damage in maize.