A dicotyledon-specific glutaredoxin GRXC1 family with dimer-dependent redox regulation is functionally redundant with GRXC2

The major known function of glutaredoxins (Grxs) is to reduce disulphide bridges. Recently, some have also been shown to interact with ironsulphur clusters. These can be classified in two subgroups: class II Grx are found in all living organisms and are implicated in assembly of ironsulphur clusters, while class I Grx are represented by only two members known to form a holodimer structure containing a cluster in vitro, but with an unknown function different from class II. Here, we report that in eukaryotic plants, GRXC1 (class I) orthologs are exclusively present in dicotyledonous plants, suggesting a specific function. Indeed, in Arabidopsis thaliana, reducing activity of recombinant GRXC1 is regulated by redox-dependent stability of the cluster. In planta, GRXC1 has been found predominantly in a holodimeric form, indicating the presence of the cluster in vivo. This suggests that GRXC1 acts as a redox sensor, reducing downstream pathways under oxidative conditions. GRXC2, the closest homolog of GRXC1, is unable to form a cluster in vitro. Knock-out mutants in grxc1 or grxc2 are aphenotypic, but the double mutant produces a lethal phenotype at an early stage after pollinization, suggesting that GRXC1 and GRXC2 share redundant and vital functions.