In plants, the photons of light are absorbed at the level of the photosystems in the chloroplasts. The functioning of the photosynthetic electron transfer chain linked to this process is required to generate NADPH and ATP. In addition, the light signal promotes a regulatory cascade, situated in the stroma, that involves ferredoxin, ferredoxin-thioredoxin reductase, and thioredoxins. This redox-based signal transduction chain allows fine regulation of stromal enzymes and tight control of the photosynthetic process. The molecular properties and the functioning of this redox regulatory chain will be described in this review.