The Plastid Terminal Oxidase is a Key Factor Balancing the Redox State of Thylakoid Membrane

Mitochondria possess oxygen-consuming respiratory electron transfer chains (RETCs), and the oxygen-evolving photosynthetic electron transfer chain (PETC) resides in chloroplasts. Evolutionarily mitochondria and chloroplasts are derived from ancient α-proteobacteria and cyanobacteria, respectively. However, cyanobacteria harbor both RETC and PETC on their thylakoid membranes. It is proposed that chloroplasts could possess a RETC on the thylakoid membrane, in addition to PETC. Identification of a plastid terminal oxidase (PTOX) in the chloroplast from the Arabidopsis variegation mutant immutans (im) demonstrated the presence of a RETC in chloroplasts, and the PTOX is the committed oxidase. PTOX is distantly related to the mitochondrial alternative oxidase (AOX), which is responsible for the CN-insensitive alternative RETC. Similar to AOX, an ubiquinol (UQH2) oxidase, PTOX is a plastoquinol (PQH2) oxidase on the chloroplast thylakoid membrane. Lack of PTOX, Arabidopsis im showed a light-dependent variegation phenotype; and mutant plants will not survive the mediocre light intensity during its early development stage. PTOX is very important for carotenoid biosynthesis, since the phytoene desaturation, a key step in the carotenoid biosynthesis, is blocked in the white sectors of Arabidopsis im mutant. PTOX is found to be a stress-related protein in numerous research instances. It is generally believed that PTOX can protect plants from various environmental stresses, especially high light stress. PTOX also plays significant roles in chloroplast development and plant morphogenesis. Global physiological roles played by PTOX could be a direct or indirect consequence of its PQH2 oxidase activity to maintain the PQ pool redox state on the thylakoid membrane. The PTOX-dependent chloroplast RETC (so-called chlororespiration) does not contribute significantly when chloroplast PETC is normally developed and functions well. However, PTOX-mediated RETC could be the major force to regulate the PQ pool redox balance in the darkness, under conditions of stress, in nonphotosynthetic plastids, especially in the early development from proplastids to chloroplasts.