GUN1 interacts with MORF2 to regulate plastid RNA editing during retrograde signaling.

Abstract

During development or under stress, chloroplasts generate signals that regulate the expression of a large number of nuclear genes, a process called retrograde signaling. GENOMES UNCOUPLED 1 (GUN1) is an important regulator of this pathway. In this study, we have discovered an unexpected role for GUN1 in plastid RNA editing, as <i>gun1</i> mutations affect RNA-editing efficiency at multiple sites in plastids during retrograde signaling. GUN1 plays a direct role in RNA editing by physically interacting with MULTIPLE ORGANELLAR RNA EDITING FACTOR 2 (MORF2). <i>MORF2</i> overexpression causes widespread RNA-editing changes and a strong <i>genomes uncoupled</i> (<i>gun</i>) molecular phenotype similar to <i>gun1</i> MORF2 further interacts with RNA-editing site-specificity factors: ORGANELLE TRANSCRIPT PROCESSING 81 (OTP81), ORGANELLE TRANSCRIPT PROCESSING 84 (OTP84), and YELLOW SEEDLINGS 1 (YS1). We further show that <i>otp81</i>, <i>otp84</i>, and <i>ys1</i> single mutants each exhibit a very weak <i>gun</i> phenotype, but combining the three mutations enhances the phenotype. Our study uncovers a role for GUN1 in the regulation of RNA-editing efficiency in damaged chloroplasts and suggests that MORF2 is involved in retrograde signaling.