Plant glutaredoxins: still mysterious reducing systems

Glutaredoxins are ubiquitous oxidoreductases which are similar to thioredoxins and possess a typical glutathione-reducible CxxC or CxxS active site. We present here the current knowledge about these proteins in plants. At least 31 glutaredoxin genes are present in Arabidopsis thaliana, a value close to the thioredoxin gene number. Based essentially on active site sequences, a classification of these multiple genes is proposed. The specificity of the various apparently redundant forms within the glutaredoxin group or between glutaredoxin and thioredoxin can be analysed in terms of differential spatiotemporal expression of the genes, specificity vs. target proteins and mode of catalysis (glutathiolation/ deglutathiolation processes appear to be a specific function of glutaredoxin). Additional putative functions are proposed for plant glutaredoxins based on their targets in other organisms and in the light of the existence of hybrid proteins containing glutaredoxin modules in their N- or C-terminal part.