FOCUS PAPER Strategies to maintain redox homeostasis during photosynthesis under changing conditions

Plants perform photosynthesis and assimilatory pro-cesses in a continuously changing environment. En-ergy production in the various cell compartments and energy consumption in endergonic processes have to be well adjusted to the varying conditions. In addition, dissipatory pathways are required to avoid any detri-mental effects caused by over-reduction. A large num-ber of short-term and long-term mechanisms interact with each other in a flexible way, depending on intensity and the type of impact. Therefore, all levels of regula-tion are involved, starting from energy absorption and electron flow events through to post-transcriptional control. The simultaneous presence of strong oxidants and strong reductants during oxygenic photosynthesis is the basis for regulation. However, redox-dependent control also interacts with other signal transduction pathways in order to adapt metabolic processes and redox-control to the developmental state. Examples are given here for short-term and long-term control follow-ing changes of light intensity and photoperiod, focus-ing on the dynamic nature of the plant regulatory systems. An integrating network of all these mechan-isms exists at all levels of control. Cellular homeostasis will be maintained as long as the mechanisms for acclimation are present in sufficiently high capacities. If an impact is too rapid, and acclimation on the level of gene expression cannot occur, cellular damage and cell death are initiated. Key words: Light acclimation, malate valve, over-reduction, oxidative stress, photosynthesis, poising mechanisms, redox control, redox homeostasis, regulatory networks