Reactive Oxygen Species (ROS): An Introduction

Reactive oxygen species (ROS) are highly reactive metabolites of oxygen derived in living organisms as a by-product of aerobic activities. ROS is a collective term for a plethora of oxygen-based radicals and non-radicals, generated at various organelles, in particular, chloroplast, mitochondria, plasma membrane, apoplast, cell wall, peroxisomes, cytosol, and endoplasmic reticulum. Production of ROS in plant cell is mediated through a number of cellular pathways and activities of enzymes. Most of the ROS formed are short-lived entities and therefore display their effects in close proximity to the site of production. Nevertheless, certain ROS can travel longer distances and show their impact at other locations in the cell as well. Due to reactive characteristics, ROS demonstrate a high damaging affinity toward biomolecules including lipid, proteins, carbohydrates, and nucleic acids, resulting in cellular damage and even death under severe conditions. Over-production of ROS has been evidently found toxic for living cells; however, their accumulation below damaging level has been recently viewed as boon for plant growth and development. Low concentration of ROS in plant cell can mediate signaling pathways which facilitate tolerance against an array of environmental stress. Therefore, ROS are often known as dual-edged swords. In plant cell, the level of ROS is checked through the recruitment of diverse enzymatic and non-enzymatic antioxidants such as superoxide peroxidase, catalase, peroxidase, carotenoids, ascorbic acid, tocopherols, and many others. Efficient functioning of antioxidant machinery support plants to withstand and perform well under stress conditions.