The PPR protein RARE1-mediated editing of chloroplast accD transcripts is required for fatty acid biosynthesis and heat tolerance in Arabidopsis

It has been reported that Arabidopsis chloroplast accD transcripts undergo RNA editing and that loss of accD-C794 RNA editing does not affect plant growth under normal conditions.To date,the exact biological role of accD-C794 editing has remained elusive.Here,we reveal an unexpected role for accD-C794 editing in response to heat stress.Loss of accD-C794 editing results in a yellow and dwarf phenotype with decreased chloroplast gene expression under heat stress,and artificial improvement of C794-edited accD gene expression enhances heat tolerance in Arabidopsis.These data suggest that accD-C794 editing confers heat tolerance in planta.We also found that treatment with the product of acetyl coenzyme A carboxylase(ACCase)could allay mutant phenotypic characteristics and showed that a mutation in the CAC3 gene for the a-subunit of ACCase was associated with dwarfism under heat stress.These observations indicate that defective accD-C794 editing may be intrinsic to reduced ACCase activity,thereby contributing to heat sensitivity.ACCase catalyzes the committed step of de novo fatty acid(FA)biosynthesis.FA content analysis revealed that unsaturated oleic(C18:1)and linoleic acids(C18:2)were low in the accD-C794 editing-defective mutant but high in the C794-edited accD-overexpressing plants compared with the wild type.Supplying exogenous C18:1 and C18:2 could rescue the mutant phenotype,suggesting that these FAs play an essential role in tolerance to heat stress.Transmission electron microscopy observations showed that heat stress seriously affected the membrane architecture in accD editing-defective mutants but not in accD-overexpressing plants.These results provide the first evidence that accD-C794 editing regulates FA biosynthesis for maintenance of membrane structural homeostasis under heat stress.