Significance Plant chloroplasts have acquired an evolutionary “redox switch” that regulates the activities of photosynthesis-related enzymes in response to a dynamically changing environment. Because the metabolic state in chloroplasts drastically fluctuates in response to changes in the surrounding environment, it has been considered essential to adjust the activities of chloroplast enzymes. Many efforts have been made to elucidate this regulation in detail. Its physiological role is thought to be due to the fact that fine-tuning of the activity is essential for efficient photosynthesis, but it is still unclear how this is really the case. We have shown that the redox switch of NADP-malate dehydrogenase plays a crucial role in the optimal growth of plants under fluctuating light conditions and prolonged darkness.