The roxy1 mutant from Arabidopsis thaliana initiates a reduced number of petal primordia and exhibits abnormalities during further petal development. The defects are restricted to the second whorl of the flower and independent of the organ identity. ROXY1 is predominantly expressed in tissues that give rise to new flower organs, including petal precursor cells and petal primordia. Glutaredoxins (GRXs) are small, ubiquitous oxidoreductases that have been intensively studied in E. coli, yeast and humans and oxidize or reduce conserved, cysteine containing motifs. They are involved in a large variety of cellular processes and exert a crucial function in the response to oxidative stress. Our data demonstrate that, unexpectedly, a plant glutaredoxin is involved in flower development, likely by mediating posttranslational modifications and thus affecting the activity of target proteins required for normal petal organ initiation and morphogenesis. Surprisingly, ROXY1 belongs to a novel subgroup, the CC-type, being specific for land plants. The existence of large CC-type subfamilies in angiosperms supports the assumption that their capability to posttranslationally modify target protein activity has been integrated into crucial plant specific processes coinciding with the development of complex flowers. Further analysis of ROXY1 and other CC-type members aims for an understanding of their biochemical properties, the identification of target proteins and unraveling the intriguing connection between redox regulation and flower development.