ROS as Signaling Molecule Under Unfavorable Conditions

ROS generated in plants display hormesis. In addition to oxidative damage, ROS also mediate signaling that facilitates acclimatization or resistance against abiotic and biotic constraints. The intrinsic antioxidants routinely scavenge the ROS produced in different organelles. Under stress conditions, capacity of antioxidants diminishes, resulting in accumulation of ROS. The transient elevation in ROS level triggers signal transduction mediating within cell and/or cell-to-cell communication. The cell-to-cell communication induces systemic signaling via H2O2 formed in apoplast and triggers acclimation responses at the distal sites. The acclimation induced in systemic tissues against local pathogen infection is known as "systemic acquired resistance" (SAR), whereas systemic acclimation induced in response to abiotic stresses such as heat, high light intensity, salinity, or drought is termed "systemic acquired acclimation" (SAA). In a cell, elicitation of response initiates on the perception of stress, which is mediated via ROS from affected or stress-exposed organelle to the nucleus, known as retrograde signaling. The retrograde signaling causes modulation of gene expression that ultimately triggers anterograde signals back from nuclei, facilitating appropriate response. The retrograde signaling pathway involves modulation of signaling components, like Ca2+, mitogen-activated protein kinases (MAPKs), and transcription factors (TFs), to relay messages from organelles to nucleus. Thus, retrograde signals arising from particular organelles show specific pathways, making ROS signaling a complex process. Moreover, ROS-mediated signaling is influenced by the chemical nature of particular reactive species, their site of production in a cell, and rate of generation under multifarious environmental conditions. Therefore, comprehensive study of ROS-mediated signaling pathways induced from particular organelle and under different stress conditions is imperative to understand the complex nature of ROS signaling.