Figure 1. Schematic representation of NO signalling in plant cells. NO is produced via different enzymatic (NOS, NR, NiNOR) or chemical processes. NO exerts different functions, such as intracellular calcium homeostasis regulation (implying cGMP and/or cADPR as second messengers) or MAPK and other protein kinase activity modulations. NO leads to target protein posttranslational modifications by metal-nitration, S-nitrosylation or tyrosine nitration, due to its high reactivity with transition metal or specific amino acid residues like cysteine or tyrosine. GSNO might serve as a carrier for the short-lived NO and may release NO, which subsequently interacts with specific targets. All these effects of NO mediate signal transduction leading to the appropriate cellular response(s) to a given stimulus that governs major processes of plant physiology. CADPR, cyclic ADPR; cGMP, cyclic GMP; MAPK, mitogen-activated protein kinases; NiNOR, nitrite:NO reductase; NOS, NO synthase; NR, nitrate reductase; Tyr-nitration, tyrosine-nitration.