Control of Retrograde Signaling by Rapid Turnover of GENOMES UNCOUPLED 1

44 45 The exchange of signals between cellular compartments coordinates development and 46 differentiation, modulates metabolic pathways, and triggers responses to environmental 47 conditions. The proposed central regulator of plastid-to-nucleus retrograde signaling, 48 GENOMES UNCOUPLED 1 (GUN1), is present at very low levels, which has hampered the 49 discovery of its precise molecular function. Here, we show that GUN1 protein accumulates to 50 detectable levels only at very early stages of leaf development, where it functions in the 51 regulation of chloroplast biogenesis. GUN1 mRNA is present at high levels in all tissues, but 52 GUN1 protein undergoes rapid degradation (with an estimated half-life of ~4 h) in all tissues 53 where chloroplast biogenesis has been completed. The rapid turnover of GUN1 is mainly 54 controlled by the chaperone ClpC1, suggesting degradation of GUN1 by the Clp protease. 55 Degradation of GUN1 slows under stress conditions that alter retrograde signaling, thus ensuring 56 that the plant has sufficient GUN1 protein. We also find that the pentatricopeptide repeat (PPR) 57 motifs of GUN1 are important determinants of GUN1 stability. Moreover, overexpression of 58 GUN1 causes an early flowering phenotype, suggesting a function of GUN1 in developmental 59 phase transitions beyond chloroplast biogenesis. Taken together, our results provide new insight 60 into the regulation of GUN1 by proteolytic degradation, uncover its function in early chloroplast 61 biogenesis, and suggest a role in developmental phase transitions. 62 63