Mitochondrial fatty acid synthesis in plants: enzymes and biological roles

Mitochondria are intracellular organelles located in the cytoplasm, that are essential for life and health. It is also known as the powerhouse of the cells and use a process called oxidative phosphorylation to generate energy. In side mitochondria, the lipid monomer fatty acids act as energy sources, membrane constituents, molecules for post translational modifications of proteins and cellular signaling factors. Plant cells possess couple of distinct type 2 Fatty Acid Synthase (FAS) systems. The two systems take place in plastids (ptFAS) and mitochondria (mtFAS). Lots of mtFAS components have been identified but, the physiological functions of mtFAS are still poorly understood. In this review, we have characterized five Arabidopsis mtFAS components, namely Mitochondrial beta-Ketoacyl-ACP Synthase (mtKAS), Mitochondrial 3-Hydroxyacyl-ACP Dehydratase (mtHD), Mitochondrial Enoyl-ACP reductase (mtER), Mitochondrial Phosphopantetheinyl Transferase (mtPPT), and Mitochondrial Malonyl-CoA Synthetase (mtMCS). MtKAS, mtHD, and mtER catalyze 3 reactions of the 4 reactions of the acyl chain elongation cycle; mtPPT is responsible for activating Mitochondrial Acyl Carrier Protein (mtACP) by the addition of a phosphopantetheine cofactor; and mtMCS generates malonyl-CoA, the donor of 2-carbon elongation unit for the mtFAS system. Mitochondrial octanoyl-ACP is the precursor for the synthesis of lipoic acid, which is the coenzyme of the H-subunit of Glycine Decarboxylase Complex (GDC), a key enzyme of photorespiration. Accordingly, mutations in the mtFAS system can lead to a typical deficiency in photorespiration due to the depletion of the lipoylation of the H-subunit of GDC. These mutants are altered as an inferior effect of the deficiency in photorespiration. In addition, mutant analyses demonstrate that mitochondrial 3-hydroxymyristyl-ACP contributes to the biosynthesis of lipid alike molecules, suggesting a novel synthetic destination of the mtFAS intermediates. RNA-seq transcriptomic analyses of the mutants establish a regulatory network that is associated with the mtFAS functions is crucial for plant metabolic homeostasis.