The redox power of thioredoxins: evolution, mechanisms, and redox regulation of carbon–nitrogen metabolism in plants

Abstract Thioredoxins (TRXs) are a ubiquitous family of proteins that play a crucial role in the redox regulation of other proteins and enzymes, thereby modulating the (de)activation of various biochemical pathways in plants, and also found in all domains of life. An important aspect of the role of TRXs across these domains is their coevolution with the increase in atmospheric concentrations of oxygen and its derivatives. Since their discovery, significant efforts have been dedicated to identifying thioredoxin targets, which are potential candidates for redox regulation. Traditionally, most studies have focused on the reducing capabilities of thioredoxins. The discovery of Thioredoxin-like 2.2 (TRX-like 2.2) introduces a new perspective on plant metabolism. Traditionally, TRXs are considered capable of donating reducing power through their free thiol groups. However, the recent findings of TRX-like 2.2 capable to oxidize proteins in the Calvin-Benson cycle and the plastid glycolytic pathway reshapes our understanding of redox metabolism. Furthermore, recent findings demonstrate that TRX-f activates and deactivates the photosynthetic and glycolytic pathways in illuminated leaves, while TRX-like 2/2, through oxidation, inactivates and activates FBPase and PFK5. This scenario raises the question of whether different TRX isoforms assume similar or additional roles in other cellular compartments, such as the cytosol and mitochondria. Moreover, recent results suggest that the mitochondrial thioredoxin (mTRX) system can coordinate fluxes throughout the tricarboxylic acid (TCA) cycle and associated pathways. In this review, we will explore the mechanistic roles of TRX proteins across evolutionary species and examine the consequences of redox regulation mediated by TRX-like 2.2. We will also consider the implications for future research targeting redox regulation through TRX and discuss TRX's involvement in carbon and nitrogen metabolism.