Redox regulation of the NOR transcription factor is involved in the regulation of fruit ripening in tomato.

Transcription factors (TFs) are important regulators of plant growth and development and responses to stresses. TFs themselves are also prone to multiple post-translational modifications (PTMs). However, redox-mediated PTM of TFs in plants remains poorly understood. Here, we established that NON-RIPENING (NOR), a master TF regulating tomato (Solanum lycopersicum) fruit ripening, is a target of the methionine sulfoxide reductases A and B, namely E4 and SlMsrB2, respectively, in tomato. Methionine oxidation in NOR, i.e., sulfoxidation, or mimicking sulfoxidation by mutating Met138 to glutamine, reduces its DNA-binding capacity and transcriptional regulatory activity in vitro. E4 and SlMsrB2 partially repair oxidized NOR and restore its DNA-binding capacity. Transgenic complementation of the nor mutant with NOR partially rescues the ripening defects. However, transformation of nor with NOR-M138Q, containing mimicked methionine sulfoxidation, inhibits restoration of the fruit ripening phenotype and this is associated with the decreased DNA-binding and transcriptional activation of a number of ripening-related genes. Taken together, these observations reveal a PTM mechanism by which Msr-mediated redox modification of NOR regulates the expression of ripening-related genes, thereby influencing tomato fruit ripening. Our report describes how sulfoxidation of TFs regulates developmental processes in plants.