A Coupled GSH/GSNOR System Denitrosylates TRXh5 to Allow Activation of SA Signalling by Oxidative Stress

Accumulating evidence shows that reversible protein S-nitrosylation is essential for H₂O₂ homoeostasis and signalling. However, roles for denitrosylation in such oxidative signalling remain poorly understood. Here, we examined this question using the Arabidopsis catalase-defective mutant, cat2, in which oxidative stress induces both glutathione accumulation and salicylic acid (SA) pathways. Induction of these pathways was accompanied by enhanced thioredoxin (TRXH5) expression, and oxidative stress-induced activation of the SA pathway was compromised when TRXH5 expression was genetically disabled, whereas TRXH5 overexpression stimulates H₂O₂-triggered SA responses. Intriguingly, TRXh5-reinforced SA responses were antagonised by glutathione (GSH) deficiency when introducing additional pad2 mutation, localised in the GLUTAMATE-CYSTEINE LIGASE gene encoding the first enzyme of glutathione biosynthesis. Further analysis revealed that the two active cysteine residues of recombinant TRXh5 can be denitrosylated by GSH. Blocking glutathione accumulation increased more TRXh5-SNO formation in TRXH5-YFP cat2 pad2 trxh5 than in TRXH5-YFP cat2 trxh5. Furthermore, S-nitrosoglutathione reductase (GSNOR) was capable of physically interacting with TRXh5, and was also required for GSH-dependent TRXh5 denitrosylation and TRXh5-enhanced SA responses during oxidative stress. Collectively, these data suggest that GSH/GSNOR constitutes an active denitrosylating module that works together with the canonical NADPH-dependent TRX-reducing pathway to sustain cytosolic TRXh5 operation within the oxidative signalling framework.