Reactive oxygen species (ROS): plant perspectives on oxidative signalling and biotic stress response

In plants, the production of reactive oxygen species (ROS) is a predictable outcome of an aerobic lifestyle. In general, ROS is an umbrella term that includes both oxygen radicals and non-radical species. These are formed continuously in different cellular compartments as a normal byproduct of various metabolic processes. ROS are known to perform essential roles in controlling various biological events in plants, such as growth and development, stress tolerance, programmed cell death (PCD), etc. In controlled concentrations, ROS are signalling molecules associated with stimulating variable defence-related pathways in stressed plants. Under stressed conditions, ROS function as secondary messengers which modulate the expression of multiple stress-responsive genes through the activation of various transcription factors. Furthermore, by regulating cytosolic Ca2⁺ levels, ROS also intensify the cellular defensive response through calcium-dependent protein kinase (CDPKs). Similarly, ROS also activate the mitogen-activated protein kinase (MAPK) cascade, which itself phosphorylates various downstream targets required for cellular defence. However, overabundance of these ROS induces several harmful effects on plant systems, including changes in cellular structures, yield loss, degradation of biomolecules, and dysfunction of various vital physiological processes. Therefore, to survive, plants must exert some tight control over ROS accumulation; otherwise, these ROS can damage the essential constituents of plants and ultimately lead towards PCD. Interestingly, plant cells also possess a collection of antioxidants, which not only play a crucial role in ROS scavenging but also protect biomolecules from oxidative damage. Based on these facts, this review is designed to highlight the potential of ROS as molecules linked with ‘oxidative signalling (at lower concentrations)’ and ‘oxidative stress’ (at higher concentrations) in plants. This article deals with ROS generation and their dual function as a cell destructor and a regulator of redox homeostasis. In addition, it also focuses on the chief enzymatic and non-enzymatic regulatory mechanisms in the plants that firmly control the ROS level and maintain oxidative balance in plants under pathogenically stressed conditions.