Plants use solar energy to synthesize sugars and this is the basis for most of life on Earth. Plants are extremely sensitive to light and specialized photoreceptors direct plant development to optimize light capture. Red-, far-red- and blue-light sensitive photoreceptors are well-studied. Surprisingly, even though green light effectively drives plant development, a green light photoreceptor remains unknown. In this project, we aim to illuminate the mechanism by which plants sense and respond to green light and how these responses foster plant performance in green-rich (shaded) environments. We hypothesize that the relatively unknown, chloroplast localized cryptochrome3 (cry3) is a green light photoreceptor. Importantly, we recently discovered that green light inhibits seedling greening via cry3. We propose to investigate this effect of green light on greening and photosynthetic capacity during seedling establishment in light. Genome-wide expression, DNA-binding and protein-interaction analysis will enable us to interrogate the potential function of cry3 as a green light sensor. Finally, we will combine the gained knowledge in a computational model, to predict how green light sensing via cry3 affects adult plant performance in different light environments. With this project, we fill the knowledge gap of how plants detect and process green light. Importantly, we address a completely new role for chloroplasts as environmental (light) signaling hubs, which will foster other studies to organelle-specific sensors and signaling processes. Lastly, the ability to actively adapt photosynthetic capacity via light signaling pathways opens doors for crop improvement in sub-optimal and artificial light environments.