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Plant growth and development is significantly influenced by environmental factors. Due to rapid global climate change both biotic and abiotic stresses have intensified and so have the deleterious effects on normal plant growth and productivity. Environmental stress-mediated decline in plant productivity imparts significant pressure on global food security, and therefore threatens the likelihood of serious food crises in the near future for the increasing world population. Stresses result in oxidative damage through excessive generation of reactive oxygen species (ROS), thereby inducing the oxidation of lipids, proteins, and nucleic acids. Plants have evolved key mechanisms to counteract the damaging effects of stresses. Osmolyte and secondary metabolite accumulation, the antioxidant system, phytochelatin production, ion compartmentation and exclusion, etc. are some of the key mechanisms to counteract the stress factors. All these tolerance mechanisms are regulated at the gene and protein level. Plants exhibiting upregulation of the tolerance mechanisms show better performance in terms of photosynthesis, mineral uptake and assimilation, enzyme functioning, and hence yield productivity. However, it should be mentioned here that enhancing the tolerance potential by employing efficient management practices can be very effective in protecting the yield potential of plants. Every tolerance pathway mentioned above is a combination of many components, which could be organic molecules or enzymes or metabolites. From the past decade onward there has been increasing advocacy for exploiting these individual beneficial molecules to improve the tolerance pathways. In this respect exogenous usage of the key components of tolerance pathways means either foliar or through the roots. Nitric oxide (NO) is a gaseous signaling molecule that was considered toxic for plant metabolism; however, advances in research have confirmed its beneficial role in stress tolerance through its role in signaling and regulation of key developmental events including germination and programmed cell death. Fine-tuned mechanisms elicited by NO have been confirmed through metabolomic, transcriptomic, and proteomic studies. Identification of genes and other key molecules interacting in NO-mediated growth and developmental regulation under stressful conditions is being investigated. Modulations in the endogenous NO concentrations, either through stress exposure or by exogenous application of protectants, confirm the role of NO in plant stress management. In addition, physiological and biochemical studies have confirmed the vital role of optimal NO concentrations in regulation of photosynthesis, carbon assimilation, osmolyte synthesis, antioxidant and secondary metabolite metabolism, and nutrient uptake and assimilation. Interactions with phytohormones like abscisic acid, ethylene, salicylic acid, jasmonic acid, and cross-talk with other signaling molecules have been reported to play pivotal roles in NO-mediated stress tolerance in plants. Keeping in view the above mentioned facts this book is compiled with the aim of providing the scientific community with the latest updates and future goals of NO research. Nitric Oxide in Plants: A Molecule with Dual Roles has 13 chapters, with every chapter having updated information about the relevant topic. The book aims to fill the existing knowledge gap in NO and plant metabolism regulation.

Chapter 1 provides an overview of the biosynthesis and regulation of NO synthesis in plants vis-à-vis its regulatory role in plant growth regulation. In addition the interaction of NO with certain key stress molecules is also discussed. Chapter 2 examines the enzymatic and nonenzymatic biosynthesis of NO, and the influence of different stress factors on NO synthesis. Moreover, modulations in NO synthesis due to osmolytes are considered, and a brief discussion about signaling components including transcription factors and phytohormones is also included. Chapter 3 discusses the reductive and oxidative pathways of NO synthesis, and the physiological, biochemical, and molecular modulations resulting from interactions of NO with auxin, gibberellic acid, cytokinin, ethylene, and abscisic acid. NO-mediated regulation of growth, photosynthesis, and tolerance mechanisms under different environmental stresses are also discussed. Chapter 4 reviews the molecular interventions for enhancing NO synthesis for its optimum exploitation for plant growth enhancement. NO-mediated regulation of phytohormones and fatty acids vis-à-vis signaling mechanisms is also the subject of this chapter. In addition, gene expression modulation and stress tolerance are examined. The role of nitrogen in production of NO and the subsequent regulation of major plant cellular pathways are described in Chapter 5. The role of beneficial microbes in NO production under normal and changing environmental conditions are dealt in Chapter 6, with a focus on physiological-, biochemical-, and molecular-level tolerance mechanisms. In addition, the interactive effects of ROS and NO in tolerance to biotic and abiotic stresses are mentioned. There is also a focus on the contribution of nitrifying and denitrifying bacteria in NO production and subsequent alterations in gene expression, as well as the role of polyamines. Chapter 7 discusses the synthesis, metabolism, and transport of ROS and reactive nitrogen species (RNS). The role of reactive oxygen, sulfur, and nitrogen species in stress signaling and the underlying cross-talk for tolerance against abiotic and biotic stresses are explained. Chapter 8 deals with understanding the role of endogenous and exogenous NO in modifying the key antioxidant pathway – the ascorbate–glutathione cycle. The role of NO-induced changes in enzymatic and nonenzymatic components of the ascorbate–glutathione cycle in plant growth regulation is examined. The authors also explain the posttranslational modifications of NO and their role in signaling and stress tolerance. In Chapter 9 the authors take the opportunity to explain the cross-talk mechanisms underlying environmental stress tolerance mediated by NO and phytohormones like auxins, cytokinins, gibberellins, abscisic acid, ethylene, jasmonic acid, salicylic acid, and brassinosteroids. The regulation (positive and negative) of several signaling pathways and the elicitation of tolerance mechanisms for improved plant performance are also discussed. Phytohormone–NO cross-talk for improved nutrient acquisition and photosynthetic regulation is the topic of Chapter 10. In addition the authors also examine the antagonistic and synergistic influence of NO on phytohormone synthesis, photosynthate partitioning, gene expression, and programmed cell death regulation. Chapter 11 covers the role of polyamines in NO synthesis and their interactive effects in alleviating the damaging effects of stresses. Polyamine metabolism-induced ROS production and subsequent triggering of NO accumulation are also discussed. In Chapter 12 the authors consider the NO-mediated modulation of the synthesis and transport of auxins, abscisic acid, and brassinosteroids under normal and adverse environmental conditions. In addition the molecular mechanisms of the signaling events resulting from their interactions are discussed. Finally, Chapter 13 looks at the interactive effects of NO and salicylic acid, jasmonic acid, and ethylene, with a focus on the antioxidant functioning of plants with regard to stress tolerance. Posttranslational modifications by NO and its cross-talk with salicylic acid, jasmonic acid, and ethylene for regulation of several downstream targets are discussed.

This volume presents a wealth of knowledge on the current understandings of NO and growth regulation in plants, and provides insights into the abiotic and biotic stress tolerance mechanisms regulated by NO. The chapters included in this volume are the authors’ own work and necessary editorial modifications were incorporated wherever the need was felt. We have tried our best to gather the most up-to-date information about NO research; however, some areas are inevitably missing and there is always the possibility of errors creeping into the book. Therefore, we seek the readers’ indulgence, and suggestions are welcome for improvements in future editions. We are very thankful to the well-versed contributors who accepted our invitation and contributed chapters despite the hardships created by the pandemic. Moreover, we extend our sincere thanks to the entire team at Wiley, especially Kerry Powell, Rebecca Ralf, and Camille Bramall, for their invaluable suggestions and constant help in accomplishing this project and its publication.

Dr. Mohammad Abass Ahanger

Dr. Parvaiz Ahmad