Nitric oxide (NO) is a small redox molecule that acts as a signal in different physiological and stress-related processes in plants. Recent evidence suggests that the biological activity of NO is also mediated by S-nitrosylation, a well-known redox-based posttranslational protein modification. Here, we show that during programmed cell death (PCD), induced by both heat shock (HS) or hydrogen peroxide (H₂O₂) in tobacco (Nicotiana tabacum) Bright Yellow-2 cells, an increase in S-nitrosylating agents occurred. NO increased in both experimentally induced PCDs, although with different intensities. In H₂O₂-treated cells, the increase in NO was lower than in cells exposed to HS. However, a simultaneous increase in S-nitrosoglutathione (GSNO), another NO source for S-nitrosylation, occurred in H₂O₂-treated cells, while a decrease in this metabolite was evident after HS. Consistently, different levels of activity and expression of GSNO reductase, the enzyme responsible for GSNO removal, were found in cells subjected to the two different PCD-inducing stimuli: low in H₂O₂-treated cells and high in the heat-shocked ones. Irrespective of the type of S-nitrosylating agent, S-nitrosylated proteins formed upon exposure to both of the PCD-inducing stimuli. Interestingly, cytosolic ascorbate peroxidase (cAPX), a key enzyme controlling H₂O₂ levels in plants, was found to be S-nitrosylated at the onset of both PCDs. In vivo and in vitro experiments showed that S-nitrosylation of cAPX was responsible for the rapid decrease in its activity. The possibility that S-nitrosylation induces cAPX ubiquitination and degradation and acts as part of the signaling pathway leading to PCD is discussed.

Retrograde pathways occurring between chloroplasts, mitochondria, and the nucleus involve oxidative and antioxidative signals that, working in a synergistic or antagonistic mode, control the expression of specific patterns of genes following stress perception. Increasing evidence also underlines the relevance of mitochondrion–chloroplast–nucleus crosstalk in modulating the whole cellular redox metabolism by a controlled and integrated flux of information. Plants can maintain the acquired tolerance by a stress memory, also operating at the transgenerational level, via epigenetic and miRNA-based mechanisms controlling gene expression. Data discussed in this review strengthen the idea that ROS, redox signals, and shifts in cellular redox balance permeate the signalling network leading to cross-tolerance. The identification of specific ROS/antioxidative signatures leading a plant to different fates under stress is pivotal for identifying strategies to monitor and increase plant fitness in a changing environment. This review provides an update of the plant redox signalling network implicated in stress responses, in particular in cross-tolerance acquisition. The interplay between reactive oxygen species (ROS), ROS-derived signals, and antioxidative pathways is also discussed in terms of plant acclimation to stress in the short and long term.