A Thylakoid Membrane Protein Functions Synergistically with GUN5 in Chlorophyll Biosynthesis.

Chlorophyll (Chl) is essential for photosynthetic reactions and chloroplast development. While the enzymatic pathway for Chl biosynthesis is well established, the regulatory mechanism underlying the homeostasis of Chl levels remains largely unknown. In this study, we identified (), which functions in the regulation of chlorophyll biosynthesis. The gene was expressed specifically in green tissues and its protein product was embedded in the thylakoid membrane. Furthermore, was precisely co-expressed and functionally correlated with . Analysis of chlorophyll metabolic intermediates indicated that and mutants over-accumulated magnesium protoporphyrin IX (Mg-Proto IX). In addition, the mutant thylakoid contained less Mg than the wild type not only as a result of lower Chl content, but also implicating CBD1 in Mg transport. This was supported by the finding that CBD1 complemented a Mg uptake-deficient strain under low Mg conditions. Taken together, these results indicate that CBD1 functions synergistically with CHLH/GUN5 in Mg-Proto IX processing, and may serve as a Mg-transport protein to maintain Mg homeostasis in the chloroplast.