Photosynthesis is the largest synthetic process on earth. Light-harvesting antennae play a central role in the conversion of solar energy to chemical energy and determine the light reception efficiency. The light-harvesting pigments (mainly chlorophylls and carotenoids in plants) and their binding proteins Light Harvesting Complex Proteins (LHCPs) constitute variable forms of antennae. Optimizing light-harvesting pigment biosynthesis is the first step to maximizing photosynthetic efficiency. Previously, we reported that ORANGE (OR) protein is the major post-translational regulator of phytoene synthase (PSY), the rate-limiting enzyme of carotenoid biosynthesis in Arabidopsis. Here we show that OR is also the major post-translational regulator of CHLI, a key component of the first committed enzyme in chlorophyll biosynthesis and chloroplast biogenesis. Examination of the or or-like double mutant in Arabidopsis reveals that lack of the Or genes results in impaired light harvesting complex assembling, defected thylakoid membrane stacking in chloroplasts, and reduced photosystem II efficiency. The or or-like double mutant not only has dramatically reduced carotenoid biosynthesis, but also shows impaired chlorophyll biosynthesis with accumulation of chlorophyll precursor protoporphyrin IX. OR was found to physically interact with CHLI protein, prevent CHLI protein aggregation, and promote CHLI protein stability. Consequently, the levels of chlorophylls along with carotenoids are extremely low in or or-like double mutant and high in the OR overexpression lines under excessive light condition comparing to wild type. These findings reveal a conserved posttranslational regulatory mechanism for plants to optimize the biosynthesis of photosynthetic pigments simultaneously in response to fluctuating environmental conditions.