Nitric oxide signaling and homeostasis in plants: a focus on nitrate reductase and S-nitrosoglutathione reductase in stress-related responses

Studies in the last two decades have firmly established that the gaseous free radical nitric oxide (NO) is an intracellular and intercellular mediator of signal transduction pathways controlling plant growth and development, as well as plant responses to biotic and abiotic stresses. The underlying mechanisms of NO action may rely on its reactivity with different kinds of biomolecules, leading to modulation of enzymatic activities, and of gene transcription, with profound impact on metabolism and signal transduction pathways. NO homeostasis depends on the appropriate coordination of NO synthesis and degradation under different physiological conditions. The mechanisms by which NO is synthesized de novo in plants are still a matter of controversy, although in the last years, the key role of the enzyme nitrate reductase (NR) in plants NO production has been widely accepted. In addition, Snitrosoglutathione (GSNO), which forms by spontaneous reaction of NO with glutathione, is likely a major NO reservoir and NO donor in plant cells. GSNO levels are controlled by the enzyme GSNO reductase that has emerged as the main enzyme responsible for the modulation of S-nitrosothiol pools. The number of plant processes influenced/modulated by NO has dramatically increased in the last years. This review particularly emphasizes the roles of NR and GSNOR enzymes in NO homeostasis and NO-mediated plant responses to environmental challenges.