Nitric oxide: A key player in plant defense against biotic stress.

Nitric oxide (NO), a small yet highly reactive gaseous molecule, has emerged as a crucial signaling component in plant biology, mainly in mediating its defense responses against biotic stress. Apart from mammalian role of NO, it also actively involved in the broad physiological, biochemical, and immunological processes in plants. NO is synthesized through a set of multiple enzymatic and non-enzymatic pathways, whereupon it rapidly accumulates upon pathogen recognition. NO is implicated in plant immunity by working in synergy with reactive oxygen species (ROS) in triggering the hypersensitive response (HR) and activating systemic acquired resistance (SAR). Moreover, it is involved in extensive crosstalk with the defense-related phytohormones salicylic acid (SA), jasmonic acid (JA), and ethylene (ET), often through post-translational modifications such as S-nitrosation of regulatory proteins. Research proves that NO plays a multifaceted role in enhancing disease resistance through activation of MAPKs, cGMP pathways, triggering of PTI and ETI, enzymes related to cell wall components, regulation of PR-proteins, antimicrobial peptides etc. against many pathogens like fungi, bacteria, viruses, and insect vectors. The results indicates both endogenous production of NO and exogenous application of its donors increase resistance. Deciphering the molecular basis of NO-mediated defense responses offers promising avenues for developing broad-spectrum and sustainable disease resistance strategies in crops through genetic and biotechnological interventions. This review provides an insight on NO-mediated plant-pathogen interaction with special emphasis on biosynthesis, signaling mechanisms, and functional integration within the plant immune network to an extent. Also, it indicates its possible application within sustainable agriculture.