Dear Editor,GUN4 is a porphyrin-binding protein involved in chlorophyll biosynthesis regulation and intracellular signaling (Larkin et al.,2003).It directly interacts with the catalytic subunit of magnesium chelatase,a branch-point enzyme for the chlorophyll biosynthetic pathway from heme synthesis (Tanaka and Tanaka,2007).GUN4 can bind to both the substrate and the product of the chelation reaction,protoporphyrin Ⅸ (PⅨ) and magnesium protoporphyrin Ⅸ (MgPⅨ);the latter participates in plastid-to-nucleus retrograde signaling (Nott et al.,2006).GUN4 can also bind to heme and cobalt protoporphyrin Ⅸ (CoPⅨ) (Adhikari et al.,2009).With such a low ligand selectivity for porphyrins,GUN4 is suggested to protect plants from reactive oxygen species (ROS)-mediated damage by shielding photosensitizing ligands (Adhikari et al.,2011).