Any reactive forms of oxygen and nitrogen (ROS/RNS) resulting from over-reduced electron transport chains are scavenged by multiple antioxidant systems. But on the other hand, chloroplasts and mitochondria are also the source of radicals. Chloroplasts are the major sites of origin of reducing equivalents and ATP required for assimilatory processes. In each compartment, specific isoenzymes of basic metabolism provide reductants and ATP, while others consume these energy carriers for assimilation of C, N, and S to produce biomass (for review: Scheibe and Dietz 2012). Particularly the ATP/ADP and NAD(P)H/NAD(P) + ratios need to be balanced in each cellular compartment as well as the ATP/NADPH ratios therein. Therefore, their energy metabolism requires permanent adjustment to avoid imbalances and formation of harmful radicals. Plants as sessile organisms which depend on light as the primary energy source cannot easily escape stressful conditions. Knowledge of all the flexible regulatory mechanisms, their responsiveness, and their interdependencies is needed when plant growth is to be engineered to optimize biomass and production of any desired molecules. In contrast, sudden light stress, as employed when analyzing stress responses in lab experiments, frequently results in cell destruction. Continuous adjustment under natural conditions allows for adaptive responses. Examples for regulated steps in this sequence of events are given in this review. For acclimation, any imbalance is sensed and elicits signal transduction to induce the required genes. A plant-specific respiratory pathway, the alternative oxidase (AOX), functions as a site to convert excess electrons into heat. Induction of the genes required to achieve an optimal response suitable for the respective conditions allows for growth when plants are exposed to different light intensities and nutrient conditions with varying rates of energy input and different assimilatory pathways for its consumption are the required in the long term. Its redox- and metabolite-dependent interactions with the mitochondrial outer membrane, the cytoskeleton, and its occurrence in the nucleus are examples of these additional functions. Integration of metabolic and redox signals involves the cytosolic enzyme glyceraldehyde-3-P dehydrogenase (GapC) and some of its many moonlighting functions. Shuttle systems for indirect transfer of reducing equivalents and ATP specifically distribute the energy fluxes between compartments for optimal biomass production. CO 2 assimilation in chloroplasts is controlled by various redox-regulated enzymes their activation state is strictly linked to metabolism due to the effects of small molecules on their actual activation state. Plants can sense an upcoming imbalance and correspondingly adapt to changed conditions not only by an increase of ROS scavengers, but also by using excess incoming light energy productively for assimilatory processes in actively metabolizing cells of growing leaves. Multiple systems are operating to avoid imbalances and subsequent oxidative stress by efficiently scavenging any formed ROS. Any excess of reducing power or lack of electron acceptors can lead to the formation of reactive oxygen species (ROS). Plants depend on light energy for the generation of ATP and reductant as well as on supply of nutrients (inorganic C, N, and S compounds) to successfully produce biomass.
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