Plants perform photosynthesis and assimilatory processes in a continuously changing environment. Energy 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 detrimental effects caused by over-reduction. A large number 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 regulation 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 following changes of light intensity and photoperiod, focusing on the dynamic nature of the plant regulatory systems. An integrating network of all these mechanisms 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.

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A complete ferredoxin (Fd) cDNA clone was isolated from potato (Solanum tuberosum L. cv Desiree) leaves. By molecular and immunoblot analysis, the gene was identified as the leaf-specific Fd isoform I. Transgenic potato plants were constructed by introducing the homologous potato fed 1 cDNA clone as an antisense construct under the control of the constitutive cauliflower mosaic virus 35S promoter. Stable antisense lines with Fd contents between 40% and 80% of the wild-type level were selected by northern-and western-blot analysis. In short-term experiments, the distribution of electrons toward their stromal acceptors was altered in the mutant plants. Cyclic electron transport, as determined by the quantum yields of photosystems I and II, was enhanced. The CO 2 assimilation rate was decreased, but depending on the remaining Fd content, some lines showed photoinhibition. The leaf protein content remained largely constant, but the antisense plants had a lower total chlorophyll content per unit leaf area and an increased chlorophyll a/b ratio. In the antisense plants, the redox state of the quinone acceptor A in photosystem II (Q A ) was more reduced than that of the wild-type plants under all experimental conditions. Because the plants with lower Fd amounts reacted as if they were grown under a higher light intensity, the possibility that the altered chloroplast redox state affects light acclimation is discussed.Ferredoxins (Fds) are small, iron-and sulfurcontaining proteins that act as low-potential oneelectron carriers. In chloroplasts, Fd distributes the electrons from PSI onto the various electronconsuming reactions in the chloroplast stroma (Arnon, 1988). During nitrogen (N) and sulfur (S) assimilation, reduced Fd is directly used by nitrite reductase, Gln synthase, sulfite reductase (Knaff and Hirasawa, 1991), and by enzymes of secondary metabolism, such as choline monooxygenase (Brouquisse et al., 1989). Moreover, Fd supplies electrons via Ferredoxin-NADP ϩ -Reductase for NADP reduction (Knaff and Hirasawa, 1991). The generated NADPH serves as a soluble reductant in the chloroplast stroma, mainly for the reduction of 1,3bisphos-phoglycerate by NAD(P)-dependent glyceraldehyde 3-phosphate dehydrogenase during CO 2 assimilation (Leegood, 1996). Fd is involved in the regulation of redox-modulated chloroplast enzymes via electron flow toward ferrodoxin-thioredoxin-reductase (FTR) and thioredoxins (Knaff and Hirasawa, 1991). Thioredoxins reduce their target enzymes and, thus, in interplay with specific metabolic effectors, adjust the enzyme activation states to the actual demand (Scheibe, 1991). Moreover, the redox state of thioredoxins also acts on chloroplast gene expression by regulating transcription and translation of several chloroplast proteins (Kim and Mayfield, 1997;Link, 2001).Fd is furthermore involved in nonassimilatory electron fluxes that act to adjust the stromal ATP/2e Ϫ ratio. Electrons are often generated in excess to the amount required for CO 2 fixation or photorespiration (Backhausen et al., 1...

Ferredoxins are the major distributors for electrons to the various acceptor systems in plastids. In green tissues, ferredoxins are reduced by photosynthetic electron flow in the light, while in heterotrophic tissues, nicotinamide adenine dinucleotide (reduced) (NADPH) generated in the oxidative pentose-phosphate pathway (OPP) is the reductant. We have used a Ds-T-DNA insertion line of Arabidopsis thaliana for the gene encoding the major leaf ferredoxin (Fd2, At1g60950) to create a situation of high electron pressure in the thylakoids. Although these plants (Fd2-KO) possess only the minor fraction of leaf Fd1 (At1g10960), they grow photoautotrophically on soil, but with a lower growth rate and less chlorophyll. The more oxidized conditions in the stroma due to the formation of reactive oxygen species are causing a re-adjustment of the redox state in these plants that helps them to survive even under high light. Redox homeostasis is achieved by regulation at both, the post-translational and the transcriptional level. Over-reduction of the electron transport chain leads to increased transcription of the malate-valve enzyme NADP-malate dehydrogenase (MDH), and the oxidized stroma leads to an increased transcription of the OPP enzyme glucose-6-P dehydrogenase. In isolated spinach chloroplasts, oxidized conditions give rise to a decreased activation state of NADP-MDH and an activation of glucose-6-P dehydrogenase even in the light. In Fd2-KO plants, NADPH-requiring antioxidant systems are upregulated. These adjustments must be caused by plastid signals, and they prevent oxidative damage under rather severe conditions.