Nitric Oxide: A Key Bioactive Regulator of Plant Tolerance Mechanism Under Metal Induced Oxidative Stress

The redox-related signalling molecule nitric oxide (NO) is considered a crucial regulator of various physiological mechanisms and response to abiotic stress in plants. NO is produced by various pathways, such as oxidative, reductive and non enzymatic, in plants. It has a significant role in plant tolerance mechanisms against heavy metal (HM) stress that is most dominant abiotic stresses that affects growth and production. The common attribute of heavy metal toxicity is the increased production and accumulation of ROS (reactive oxygen species) that causes oxidative stress. These ROS disturb the normal cellular functioning by interacting with and damaging essential components of plant cell, such as lipids, protein, enzymes, and DNA. NO regulates the antioxidant enzymes activity and transcription factors (TFs) of genes regarded to stress tolerance, especially through post-transcriptional modifications (PTMs) such as S-nitrosylation and tyr-nitration. NO activates MAP-kinase and Ca2+-dependent signalling to induce HM stress tolerance mechanisms. Over the past 20 years, nitric oxide has received a lot of attention as a molecule that can reduce the toxicity of heavy metals through both endogenous production and exogenous delivery. This chapter addressed various aspects of NO, such as biosynthesis, interactions with other molecules, signalling, and its participation in the alleviation of heavy metal toxicity.