Evolutionary co-option of the ancient protein AMBRA1 in plant photomorphogenesis
Photomorphogenesis, the light-driven developmental program of plants, is repressed in darkness by the CUL4-DDB1-COP1 ubiquitin ligase complex. In animals, AMBRA1 is a substrate receptor for the same CUL4-DDB1 machinery, regulating proteostasis, cell proliferation and autophagy, but its role in plants has remained unknown. Here we show that the moss Physcomitrium patens carries two light-regulated AMBRA1 genes, AMBRA1a and AMBRA1b, whose loss impairs vegetative growth and cell-fate transitions under light and prevents the elongation of gravitropic filaments in darkness, indicating that AMBRA1 is required to repress photomorphogenesis. AMBRA1 localizes to the nucleus under both long-day and dark conditions, and its loss causes widespread transcriptional reprogramming of developmental and metabolic pathways, including repression of photosynthesis-related genes in darkness. These findings identify AMBRA1 as a previously unrecognized nuclear regulator that links light availability to plant growth and as a key repressor of photomorphogenesis.