Glutaredoxins in Development and Stress Responses of Plants

Abstract

Glutaredoxins (GRXs) are small ubiquitous glutathione (GSH)-dependent oxidoreductases and are known to exert a crucial function in the response to oxidative stresses in a range of pro- and eukaryotes, such as Escherichia coli, yeast, and human. Plant genomes encode three GRX classes, the CPYC, the CGFS, and the CC-type; however, up till now little has been known about their functions. Whereas the CPYC and CGFS classes occur ubiquitously in all thus far analyzed species, the CC-type GRXs exist only in land plants. Moreover, only the CC-type class expanded markedly during the evolution of land plants, providing clues for their involvement in the formation of more complex plants adapted to life on land. Accumulating evidence indicates that GRXs participate in various cellular processes in plants. In this review, focus is given to the recently emerging functions of land plant-specific GRXs in flower development and pathogen resistance. Strikingly, comparisons of the involved signaling pathways that have been thus far considered to be unrelated with one another reveal that similar protein molecules, CC-type GRXs as well as TGA transcription factors, seem to have been recruited to participate in these different pathways. The small CC-type oxidoreductases likely have the potential to alter TGA activities in a redox-sensitive manner, thereby causing differential gene expression and consequently affecting associated downstream biological processes. The activities of plant CPYC and CGFS GRXs in the assembly and delivery of Fe-S clusters and arsenic resistance indicate a broad functional spectrum for the other two classes of GRXs. Furthermore, S-nitrosylation of protein thiols adds a new layer of complexity to redox regulation in plant cells. As GRXs require GSH to reduce their target proteins, GSH-associated developmental processes, which affect flowering time, root and shoot development, are also discussed. The identification of more plant GRX targets, albeit challenging, will help to uncover further roles of plant GRXs and facilitate the investigation of functional redundancies and crosstalk between GRXs and TRXs.