Analysis of a Range of Catabolic Mutants Provides Evidence That Phytanoyl-Coenzyme A Does Not Act as a Substrate of the Electron-Transfer Flavoprotein/Electron-Transfer Flavoprotein:Ubiquinone Oxidoreductase Complex in Arabidopsis during Dark-Induced Senescence    

Abstract

The process of dark-induced senescence in plants is not fully understood, however, the functional involvement of an electrontransfer flavoprotein/electron-transfer flavoprotein:ubiquinone oxidoreductase (ETF/ETFQO), has been demonstrated. Recent studies have revealed that the enzymes isovaleryl-coenzyme A (CoA) dehydrogenase and 2-hydroxyglutarate dehydrogenase act as important electron donors to this complex. In addition both enzymes play a role in the breakdown of cellular carbon storage reserves with isovaleryl-CoA dehydrogenase being involved in degradation of the branched-chain amino acids, phytol, and lysine while 2-hydroxyglutarate dehydrogenase is exclusively involved in lysine degradation. Given that the chlorophyll breakdown intermediate phytanoyl-CoA accumulates dramatically both in knockout mutants of the ETF/ETFQO complex and of isovaleryl-CoA dehydrogenase following growth in extended dark periods we have investigated the direct importance of chlorophyll breakdown for the supply of carbon and electrons during this process. For this purpose we isolated three independent Arabidopsis (Arabidopsis thaliana) knockout mutants of phytanoyl-CoA 2-hydroxylase and grew them under the same extended darkness regime as previously used. Despite the fact that these mutants accumulated phytanoyl-CoA and also 2-hydroxyglutarate they exhibited no morphological changes in comparison to the other mutants previously characterized. These results are consistent with a single entry point of phytol breakdown into the ETF/ETFQO system and furthermore suggest that phytol is not primarily metabolized by this pathway. Furthermore analysis of isovaleryl-CoA dehydrogenase/ 2-hydroxyglutarate dehydrogenase double mutants generated here suggest that these two enzymes essentially account for the entire electron input via the ETF complex.

Model-based Confirmation of Alternative Substrates of Mitochondrial Electron Transport Chain

“…We show that the presented model predictions and the experimentally observed behavior in A. thaliana are in excellent agreement, supporting the claim that both lysine and 2-hydroxyglutarate are the major substrates supplying electrons to the mitochondrial ETC under carbon limitation conditions. Moreover, our modeling approach provides compelling evidence that the phytanoyl-CoA breakdown does not contribute to the electron donation via the ETF-ETFQO complex, as suggested very recently on the basis of experimental data (46).…”

“…As a result, we are able to not only ascertain the experimental data but also provide modeling support in confirming the hypotheses concerning the feeding of electrons into the ETC from IVDH and D2HGDH during dark-induced senescence (1). However, it should be noted that, although these pathways are particularly prominent during dark-induced senescence (1,46), recent evidence suggests that they also operate in the dark period of a normal light-dark cycle in the photosynthetic tissue of Arabidopsis (50,51).…”

“…As a simplification of this leaf-based model, other substrates, such as NAD, are excluded. However, we point out that during dark-induced senescence, the main respiratory substrates are completely consumed within few days of stress (1,29,30,46), making the presence of alternative pathways an essential condition to maintain respiratory rates, which further justifies the exclusion of other substrates of the ETC in the modeling.…”

Omics Study for Abiotic Stress Responses in Plants

“…72 In other metabolite profiling studies, an increase in BCAAs, i.e.valine, leucine, isoleucine, and other amino acids sharing synthetic pathways with BCAAs i.e. lysine, threonine and methionine were reported under abiotic stress conditions.…”

Transcription Factor Arabidopsis Activating Factor1 Integrates Carbon Starvation Responses with Trehalose Metabolism

“…Likewise, genes encoding enzymes involved in amino acid breakdown, such as isovaleryl-CoA dehydrogenase, D-2-hydroxyglutarate dehydrogenase, and Glu dehydrogenase, and genes encoding mitochondrial electron transfer flavoprotein/electron transfer flavoprotein:ubiquinone oxidoreductase subunits are up-regulated upon carbon limitation (Ishizaki et al, 2006;Miyashita and Good, 2008;Araújo et al, 2011;Izumi et al, 2013). Isovaleryl-CoA dehydrogenase and D-2-hydroxyglutarate dehydrogenase act as electron donors to the electron transfer flavoprotein/electron transfer flavoprotein:ubiquinone oxidoreductase-mediated alternative pathway of respiration (Araújo et al, 2010).…”