Mitochondrial ascorbate synthesis acts as a pro-oxidant pathway and down-regulate energy supply in plants

Abstract

31Attempts to improve the ascorbate (AsA) content of plants are still dealing with the limited 32 understanding of why exists a wide variability of this powerful anti-oxidant molecule in different 33 plant sources, species and environmental situations. In plant mitochondria, the last step of AsA 34 synthesis is catalyzed by the enzyme L-galactone-1,4-lactone dehydrogenase (L-GalLDH). By using 35GalLDH-RNAi silencing plant lines, biochemical and proteomic approaches, we here discovered 36 that, in addition to accumulate this antioxidant, mitochondria synthesize AsA to down-regulate the 37 respiratory activity and the cellular energy provision. The work reveals that the AsA synthesis 38 pathway within mitochondria is a branched electron transfer process that channels electrons 39 towards the alternative oxidase, interfering with conventional electron transport. It was 40 unexpectedly found that significant hydrogen peroxide is generated during AsA synthesis, which 41 affects the AsA level. The induced AsA synthesis shows proteomic alterations of mitochondrial 42 and extra-mitochondrial proteins related to oxidative and energetic metabolism. The most 43 identified proteins were known components of plant responses to high light acclimation, 44 programmed cell death, oxidative stress, senescence, cell expansion, iron and phosphorus 45 starvation, different abiotic stress/pathogen attack responses and others. We propose that 46 changing the electron flux associated with AsA synthesis might be part of a new mechanism by 47 which the L-GalLDH enzyme would adapt plant mitochondria to fluctuating energy demands and 48 redox status occurring under different physiological contexts. 49 50 to mETC but via flavin dinucleotide (FAD) (Sweetlove et al., 2010). These oxidation reactions are all 60 coupled to reduction of the ubiquinone (UQ) to ubiquinol (UQH 2 ) (Schertl and Braun, 2014). 61