Loss of Chloroplast GNAT Acetyltransferases Results in Distinct Metabolic Phenotypes in Arabidopsis

Acetylation is one of the most common chemical modifications found on a variety of molecules ranging from metabolites to proteins. Although numerous chloroplast proteins have been shown to be acetylated, the role of acetylation in the regulation of chloroplast functions has remained mainly enigmatic. The chloroplast acetylation machinery in Arabidopsis thaliana consists of eight General control non-repressible 5 (GCN5)-related N-acetyltransferase (GNAT)-family enzymes that catalyze both N-terminal and lysine acetylation of proteins. Additionally, two plastid GNATs have also been reported to be involved in the biosynthesis of melatonin. Here, we have characterized six plastid GNATs (GNAT1, GNAT2, GNAT4, GNAT6, GNAT7 and GNAT10) using a reverse genetics approach with an emphasis on the metabolomes and photosynthesis of the knock-out plants. Our results reveal the impact of GNAT enzymes on the accumulation of chloroplast-related compounds, such as oxylipins and ascorbate, and the GNAT enzymes also affect the accumulation of amino acids and their derivatives. Specifically, the amount of acetylated arginine and proline was significantly decreased in the gnat2 and gnat7 mutants, respectively, as compared to the wild-type Col-0 plants. Additionally, our results show that the loss of the GNAT enzymes results in increased accumulation of Rubisco and Rubisco activase (RCA) at the thylakoids. Nevertheless, the reallocation of Rubisco and RCA did not have consequent effects on carbon assimilation under the studied conditions. Taken together, our results show that chloroplast GNATs affect diverse aspects of plant metabolism and pave way for future research into the role of protein acetylation.