Turning over a green leaf: integrating light signals toward chloroplast establishment.

Light is indispensable for transitioning from a heterotrophic to a photoautotrophic state. Etiolated seedlings, developed in darkness, are primed for rapid chloroplast development upon light exposure. In this transition, dark-adapted etioplasts differentiate into functional chloroplasts through a tightly regulated process controlled at multiple levels. Light steers gene transcription and translation, as well as posttranslational modifications of both plastid- and nuclear-encoded chloroplast and photosynthetic proteins. Light is sensed not only by dedicated nuclear photoreceptors like phytochromes and cryptochromes, but also directly by plastids themselves. For example, incoming light drives chlorophyll metabolism, which initiates a cascade of structural and metabolic changes. The shaping of functional chloroplasts involves adjustments to the membrane lipid composition and the incorporation of photosynthetic proteins, contributing to the internal membrane structure for the buildup of the photosystems. In this update, we review our current understanding of integrated light signals in plastids and the nucleus that drive the multi-level transition of non-photosynthetic etioplasts toward fully functioning chloroplasts.