Redox-Dependent Structural Modification of Nucleoredoxin Triggers Defense Responses against Alternaria brassicicola in Arabidopsis

In plants, thioredoxin (TRX) family proteins participate in various biological processes by regulating the oxidative stress response. However, their role in phytohormone signaling remains largely unknown. In this study, we investigated the functions of TRX proteins in Arabidopsis thaliana. Quantitative polymerase chain reaction (qPCR) experiments revealed that the expression of ARABIDOPSIS NUCLEOREDOXIN 1 (AtNRX1) is specifically induced by the application of jasmonic acid (JA) and upon inoculation with a necrotrophic fungal pathogen, Alternaria brassicicola. The AtNRX1 protein usually exists as a low molecular weight (LMW) monomer and functions as a reductase, but under oxidative stress AtNRX1 transforms into polymeric forms. However, the AtNRX1M3 mutant protein, harboring four cysteine-to-serine substitutions in the TRX domain, did not show structural modification under oxidative stress. The Arabidopsis atnrx1 null mutant showed greater resistance to A. brassicicola than wild-type plants. In addition, plants overexpressing both AtNRX1 and AtNRX1M3 were susceptible to A. brassicicola infection. Together, these findings suggest that AtNRX1 normally suppresses the expression of defense-responsive genes, as if it were a safety pin, but functions as a molecular sensor through its redox-dependent structural modification to induce disease resistance in plants.