Nocturnal hydration prepares desert cyanobacteria for dawn-light harvesting by inducing phycoerythrin synthesis

, thereby initiating transcription of PE biosynthetic genes. Notably, we also find that SigB1 is regulated by the chromatic acclimation factor RcaF, which suppresses PE synthesis as weak dawn light emerges. Together, these findings reveal that nocturnal rehydration triggers an anticipatory transcriptional program that primes PE synthesis, enabling desert cyanobacteria to maximize light harvesting at dawn. More broadly, our results provide insight into how desiccation-tolerant photosynthetic organisms dynamically optimize their light-harvesting apparatus in response to transient environmental cues.