S-Nitrosylation Control of ROS and RNS Homeostasis in Plants: The Switching Function of Catalase

Nitric oxide (NO), a short-lived gaseous molecule, plays a key role in many physiological and developmental processes. As a lipophilic molecule that can cross all the barriers of biological membranes, NO is able to react with various intracellular/extracellular targets and form a series of reactive nitrogen species (RNS), such as NO radicals (NO−), nitrosonium ions (NO+), peroxynitrite (ONOO−), S-nitrosothiols (SNOs), higher oxides of nitrogen (NOx) and dinitrosyl-iron complexes among others. As a signal mediator, RNS executes its physiological effects mainly through protein S-nitrosylation: the addition of an NO moiety to the cysteine thiol of a protein forms an S-nitrosothiol (SNO). Many studies have established that the dynamic processes of S-nitrosylation and denitrosylation are predominantly modulated by the intracellular level of S-nitrosoglutathione (GSNO), a major bioactive NO species in organisms. GSNO is irreversibly degraded by the highly conserved GSNO reductase (GSNOR), which is also known as Sensitive to Hot Temperature 5 (HOT5) or Paraquat Resistant 2 (PAR2) in Arabidopsis. Mutations in GSNOR1 cause a dramatically increased intracellular level of NO and proteome-wide SNOs, resulting in a pleiotropic phenotype with severe developmental defects and altered responses to biotic and abiotic stresses (Feechan et al., 2005Feechan A. Kwon E. Yun B. Wang Y. Pallas J. Loake G. A central role for S-nitrosothiols in plant disease resistance.Proc. Natl. Acad. Sci. U S A. 2005; 102: 8054-8059Crossref PubMed Scopus (390) Google Scholar, Lee et al., 2008Lee U. Wie C. Fernandez B.O. Feelisch M. Vierling E. Modulation of nitrosative stress by S-nitrosoglutathione reductase is critical for thermotolerance and plant growth in Arabidopsis.Plant Cell. 2008; 20: 786-802Crossref PubMed Scopus (234) Google Scholar, Chen et al., 2009Chen R. Sun S. Wang C. Li Y. Liang Y. An F. Li C. Dong H. Yang X. Zhang J. et al.The Arabidopsis PARAQUAT RESISTANT2 gene encodes an S-nitrosoglutathione reductase that is a key regulator of cell death.Cell Res. 2009; 19: 1377-1387Crossref PubMed Scopus (119) Google Scholar). Thus, GSNOR1 is a master regulator for intracellular RNS levels, which determines proteome-wide S-nitrosylation. Interestingly, GSNOR1 itself is the target of S-nitrosylation. S-nitrosylation at Cys-10 of GSNOR1 induces local conformational changes, facilitating its binding by Autophagy-related 8 (ATG8), a class of ubiquitin-like proteins of the autophagy machinery, eventually being degraded via autophagy (Zhan et al., 2018Zhan N. Wang C. Chen L. Yang H. Feng J. Gong X. Ren B. Wu R. Mu J. Li Y. et al.S-Nitrosylation targets GSNO reductase for selective autophagy during hypoxia responses in plants.Mol. Cell. 2018; 71: 142-154.6Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar). Therefore, GSNOR1 is regulated by a positive feedback of NO level in plant cells (Figure 1). The absence of GSNOR function in gsnor1-3 confers plant resistance to oxidative stress; consistently, wild-type plants treated with an NO donor display resistance to paraquat (Chen et al., 2009Chen R. Sun S. Wang C. Li Y. Liang Y. An F. Li C. Dong H. Yang X. Zhang J. et al.The Arabidopsis PARAQUAT RESISTANT2 gene encodes an S-nitrosoglutathione reductase that is a key regulator of cell death.Cell Res. 2009; 19: 1377-1387Crossref PubMed Scopus (119) Google Scholar). This indicates that there may be a tight link between RNS and reactive oxygen species (ROS), a class of key signaling molecules involved in various developmental processes and stress responses in plants, including responses to hydrogen peroxide (H2O2), superoxide anion(O2−), hydroxyl radicals (.OH), and singlet oxygen (1O2) etc. Accumulating evidence has shown that S-nitrosylation regulates the activities of key enzymes involved in ROS homeostasis. For example, S-nitrosylation of NADPH oxidase causes decreased enzymatic activity, resulting in the reduction of superoxide during immune responses (Yun et al., 2011Yun B.W. Feechan A. Yin M. Saidi N.B. Le Bihan T. Yu M. Moore J.W. Kang J.G. Kwon E. Spoel S.H. et al.S-Nitrosylation of NADPH oxidase regulates cell death in plant immunity.Nature. 2011; 478: 264-268Crossref PubMed Scopus (421) Google Scholar). In contrast, S-nitrosylation of ascorbate peroxidase 1 (APX1) enhances its enzymatic activity in scavenging H2O2, leading to increased resistance to oxidative stress (Yang et al., 2015Yang H. Mu J. Chen L. Feng J. Hu J. Li L. Zhou J.M. Zuo J. S-Nitrosylation positively regulates ascorbate peroxidase activity during plant stress responses.Plant Physiol. 2015; 167: 1604-1615Crossref PubMed Scopus (130) Google Scholar). S-nitrosylation Nitrosylation of peroxiredoxin (Prx) II E inhibits its peroxynitrite reductase activity, causing a dramatic increase in ONOO−-dependent nitrotyrosine residue formation (Romero-Puertas et al., 2007Romero-Puertas M.C. Laxa M. Matte A. Zaninotto F. Finkemeier I. Jones A.M. Perazzolli M. Vandelle E. Dietz K.J. Delledonne M. S-Nitrosylation of peroxiredoxin II E promotes peroxynitrite-mediated tyrosine nitration.Plant Cell. 2007; 19: 4120-4130Crossref PubMed Scopus (253) Google Scholar). Furthermore, mutation in OsCATC, encoding a rice ortholog of Arabidopsis catalase CAT2, causes overaccumulation of H2O2, which promotes NO production via the activation of nitrate reductase (NR) and further increases the SNO level, causing the S-nitrosylation of GADPH, GSNOR1, and others. (Lin et al., 2012Lin A. Wang Y. Tang J. Xue P. Li C. Liu L. Hu B. Yang F. Loake G.J. Chu C. Nitric oxide and protein S-nitrosylation are integral to hydrogen peroxide-induced leaf cell death in rice.Plant Physiol. 2012; 158: 451-464Crossref PubMed Scopus (224) Google Scholar). Moreover, as active redox radicals, both RNS and ROS molecules can react with each other to form additional molecules, such as peroxynitrite (ONOO−) by the interaction of NO and superoxide (O2−), which can trigger irreversible damage to different biomolecules. Collectively, these findings establish the molecular network between RNS and ROS signaling pathways and implicate that their interplay plays a critical role in regulating stress responses (Figure 1). S-nitrosylation has been considered for a long time as a non-enzymatic process. With the increase in S-nitrosylated proteins identified in both animals and plants, a question about the selectivity of S-nitrosylation arises. Accumulating evidence in animal models and bacterial systems indicates that a small group of S-nitrosylated proteins can transfer the NO moiety to another protein and nitrosylate the latter, termed transnitrosylase (Seth et al., 2018Seth D. Hess D.T. Hausladen A. Wang L. Wang Y.J. Stamler J.S. A multiplex enzymatic machinery for cellular protein S-nitrosylation.Mol. Cell. 2018; 69: 451-464 e456Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar). However, the mechanisms of selective S-nitrosylation and transnitrosylation remain largely unclear, and no transnitrosylase has so far been characterized in plants. Recently, Chen et al., 2020Chen L. Wu R. Feng J. Feng T. Wang C. Hu J. Zhan N. Li Y. Ma X. Ren B. et al.Transnitrosylation mediated by the non-canonical catalase ROG1 regulates nitric oxide signaling in plants.Dev. Cell. 2020; 53: 1-14Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar reported the characterization of the first plant transnitrosylase in Arabidopsis. They performed a genetic screen for the "repressors of" gsnor1 (rog) mutant in the gsnor1-3 background (Feechan et al., 2005Feechan A. Kwon E. Yun B. Wang Y. Pallas J. Loake G. A central role for S-nitrosothiols in plant disease resistance.Proc. Natl. Acad. Sci. U S A. 2005; 102: 8054-8059Crossref PubMed Scopus (390) Google Scholar) and identified nine allelic mutants. Importantly, the increased SNO level in gsnor1 was remarkably reduced by the rog1-1 mutation, indicating that ROG1 is required for S-nitrosylation. ROG1 was identified as the non-canonical catalase, CAT3 and demonstrated to have the transnitrosylase activity that specifically modifies GSNOR1 at Cys-10. Moreover, the NO-induced degradation of GSNOR1 was found to be abolished in rog1-10, suggesting that the stability of GSNOR1 is regulated by ROG1-mediated transnitrosylation. Interestingly, the transnitrosylase activity of ROG1 is regulated by a unique and highly conserved residue, Cys-343, which is Thr-343 in CAT2, the enzyme responsible for the majority of catalase activity. The dwarf-and-bushy and paraquat-resistant phenotypes of gsnor1 is partially rescued by rog1, but not by cat2. Moreover, it was found that native ROG1 functions as a transnitrosylase with only very weak catalase activity. However, ROG1 C343T mutant displays increased catalase activity but decreased transnitrosylase activity, confirming that Thr-/Cys-343 is critical in determining the main activities of ROG1 as a catalase or a transnitrosylase. Consistently, the insensitive phenotype of rog1 to the inhibitory effect of GSNO was fully restored by ROG1 but not by ROG1C343T (Chen et al., 2020Chen L. Wu R. Feng J. Feng T. Wang C. Hu J. Zhan N. Li Y. Ma X. Ren B. et al.Transnitrosylation mediated by the non-canonical catalase ROG1 regulates nitric oxide signaling in plants.Dev. Cell. 2020; 53: 1-14Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar). Therefore, ROG1 and GSNOR1 form a positive feedback loop to modulate the intracellular RNS level, eventually regulating various physiological processes. Collectively, this study proposed that ROG1 functions as a transnitrosylase to regulate NO-based redox signaling in plants. The switch function of ROG1/CAT3 reveals the underpinning mechanism of ROS and RNS interplay, which is of great significance (Figure 1). The physiological processes of cells need a stable and balanced status of different SNOs, which is governed by the total activity of nitrosylases and denitrosylases. Thus, to elucidate the operation and consequences of S-nitrosylation in a cellular context, the SNO proteins should be considered as both targets and transducers of S-nitrosylation, upon enzymatically governed equilibria. To date, only a limited number of transnitrosylases have been identified in animals and bacteria. As the first characterized plant transnitrosylase, ROG1 and ROG1-like proteins are structurally distinguished from all other transnitrosylases identified in animals and bacteria, thus discovering a unique class of transnitrosylases that specifically regulate NO signaling in plants. However, there are still many unanswered questions, for example, whether ROG1/CAT3 has only specific targets or also has transnitrosylase activity with other proteins, and whether there are some other proteins that have transnitrosylase activity in plants. Catalase, an important enzyme in ROS scavenging, has transnitrosylase function; is this just by accident? The story of the underpinning mechanism of ROS and RNS interplay may have just begun! The research is funded by a grant from the National Natural Science Foundation of China (grant no. 31871586 ).