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Key Words chloroplasts, complex I, photosynthesis, alternative oxidase, respiration
* Abstract Chlororespiration has been defined as a respiratory electron transport chain (ETC) in interaction with the photosynthetic ETC in thylakoid membranes of chloroplasts. The existence of chlororespiration has been disputed during the last decade, with the initial evidence mainly obtained with intact algal cells being possibly explained by redox interactions between chloroplasts and mitochondria. The discovery in higher-plant chloroplasts of a plastid-encoded NAD(P)H-dehydrogenase (Ndh) complex, homologous to the bacterial complex I, and of a nuclear-encoded plastid terminal oxidase (PTOX), homologous to the plant mitochondrial alternative oxidase, brought molecular support to the concept of chlororespiration. The functionality of these proteins in nonphotochemical reduction and oxidation of plastoquinones (PQs), respectively, has recently been demonstrated. In thylakoids of mature chloroplasts, chlororespiration appears to be a relatively minor pathway compared to linear photosynthetic electron flow from H20 to NADP+. However, chlororespiration might play a role in the regulation of photosynthesis by modulating the activity of cyclic electron flow around photosystem I (PS I). In nonphotosynthetic plastids, chlororespiratory electron carriers are more abundant and may play a significant bioenergetic role.
INTRODUCTION
In photosynthetic cells, two main bioenergetic processes, photosynthesis and respiration, involve electron transport reactions coupled to ATP synthesis. Whereas photosynthesis converts light energy into chemical energy (NADPH and ATP) and allows the net fixation of carbon, respiration converts reducing power contained in carbohydrates into phosphorylating power. In eukaryotic cells, these processes take place in two separate organelles: Photosynthesis occurs in chloroplasts and respiration in mitochondria. In photosynthetic bacteria and cyanobacteria, photosynthetic and respiratory electron transport chains (ETCs) operate in close interaction, sharing identical electron carriers such as the plastoquinone (PQ) pool (127). In the early 1960s, Goedheer (53), by studying photosynthetic luminescence transients in unicellular green algae, postulated that a dark oxidation of intersystem electron carriers could occur through "some kind of chloroplast respiration." Bennoun (8, 9), based on the effects of respiratory inhibitors on chlorophyll fluorescence induction curves in unicellular green algae, proposed the existence in thylakoid membranes of a respiratory chain connected to the photosynthetic ETC. This respiratory activity, called chlororespiration, was suggested to originate from the bacterial ancestor of chloroplasts (127). According to the initial model of chlororespiration, reducing equivalents would be supplied in...