Emerging roles of glutaredoxins in plant metabolism.

Glutaredoxins (GRXs) constitute a distinct family within the large thioredoxin superfamily. Based on the historic order of discoveries and structural similarities, GRXs were originally annotated as glutathione-dependent oxidoreductases with a CxxC active site motif. These prototypical GRXs are now clustered as class I. Later, it was discovered that some GRXs, particularly those with the highly conserved active site motif CGFS, coordinate iron-sulfur clusters and exhibit little or no oxidoreductase activity. Throughout the course of evolution across kingdoms, the CGFS-containing GRXs form the most highly conserved class II cluster. Compared with non-plant species, the GRX family in spermatophytes has undergone significant enlargement and diversification, resulting in typically more than 30 isoforms that are separated into four classes. This expansion is largely due to the evolution of new plant-specific class III GRXs or ROXYs, characterized by a central CCxx active site motif. The increasing number of GRX copies in the genome, coupled with the modification of domains and addition of domains with different functions, has resulted in the functional diversification of plant GRXs. In this review, we aim to provide an overview of the established and emerging roles of GRXs in plant metabolism.