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Chloroplasts and mitochondria are the powerhouses of photosynthetic cells. The oxidation-reduction (redox) cascades of the photosynthetic and respiratory electron transport chains not only provide the driving forces for metabolism but also generate redox signals, which participate in and regulate every aspect of plant biology from gene expression and translation to enzyme chemistry. Plastoquinone, thioredoxin and reactive oxygen have all been shown to have signalling functions. Moreover, the intrinsic involvement of molecular oxygen in electron transport processes with the inherent generation of superoxide, hydrogen peroxide and singlet oxygen provides a repertoire of additional extremely powerful signals. Accumulating evidence implicates the major redox buffers of plant cells, ascorbate and glutathione, in redox signal transduction. The network of redox signals from energy-generating organelles orchestrates metabolism to adjust energy production to utilization, interfacing with hormone signalling to respond to environmental change at every stage of plant development.