From the pioneering work that the Buchanan group started in the late 1960s, we know that the division of photosynthesis into light and dark reactions is inade- quate because the activity of a number of chloroplast enzymes, many involved in the Calvin–Benson cycle, is strictly controlled by light; that is, they are activated in the light and deactivated in the dark. An exception is glucose-6-phosphate dehydrogenase, which is regulated in an opposite manner; that is, activation in the dark and deactivation in the light. It took a decade to biochemically dissect the newly identifed redox regula- tory pathway responsible for this regulation. Known as the ferredoxin-thioredoxin system, the pathway is com- posed of three components: ferredoxin, ferredoxin- thioredoxin reductase (FTR), and thioredoxin (Trx) that relay the reducing power generated at photosystem 1 (PS1) to target regulatory enzymes (1, 2).