GENOMES UNCOUPLED PROTEIN1 binds to plastid RNAs and promotes their maturation.

Plastid biogenesis and the coordination of plastid and nuclear genome expression through anterograde and retrograde signaling are essential for plant development. GENOMES UNCOUPLED1 (GUN1) plays a central role in retrograde signaling during early plant development. The putative function of GUN1 has been extensively studied, but its molecular function remains controversial. Here, we evaluate published transcriptome data and generate our own data from gun1 mutants grown under signaling relevant conditions to show that editing and splicing are not relevant for GUN1-dependent retrograde signaling. Our study of the plastid (post)-transcriptome of gun1 seedlings with white and pale cotyledons demonstrates that GUN1 deficiency significantly alters the entire plastid transcriptome. By combining this result with a PPR code-based prediction and experimental validation by RNA immunoprecipitation experiments, several putative targets of GUN1 were identified, including tRNAs and RNAs derived from ycf1.2, rpoC1 and rpoC2, and the ndhH-ndhA-ndhI-ndhG-ndhE-psaC-ndhD gene cluster. The absence of plastid rRNAs and the significant reduction of almost all plastid transcripts in white gun1 mutants account for the cotyledon phenotype. Our study provides evidence for RNA binding and maturation as the long-sought molecular function of GUN1 and resolves long-standing controversies. We anticipate that our findings will serve as a basis for subsequent studies investigating the mechanism of plastid gene expression and will facilitate the elucidation of GUN1's function in retrograde signaling.