https://orcid.org/0000-0001-5268-4094
Plant mitochondria facilitate complex metabolic networks differently from animals, integrating various substrates and external signals into vital energy and metabolite products for successful plant development and adaptation. Despite the conserved pathway architecture of mitochondrial respiration and primary metabolisms among the kingdoms, plant mitochondrial respiration is more tightly linked to their dynamic metabolic status in response to fluctuating environmental conditions and/or in their developmental-/tissue-specific manner (Millar et al. 2011). In many plant species, for example, the respiration rate decreases throughout the night, accompanied by a similar decrease in the levels of several primary metabolites, including sugars and amino acids. This positive correlation between respiration activity and metabolite accumulation is similarly observed when leaf discs are incubated with exogenously supplied carbohydrates or amino acids (O’Leary et al. 2017; Bruhn et al. 2022). However, the molecular mechanism behind this phenomenon has been elusive for a long time, which has hindered our understanding of the crosstalk between mitochondrial respiration and central carbon metabolism in plants.
One of the most notable unique features in plant mitochondria is alternative oxidase (AOX), a membrane-bound component in the mitochondrial electron transport chain that is found only in plants and a few microorganisms such as Trypanosoma brucei. AOX bypasses electron transport to mitigate reactive oxygen species production via the dissipation of excess reducing power, which has been well characterized as one of the protection mechanisms under abiotic stresses such as high light (Millar et al. 2011; Del-Saz et al. 2018). Recently, AOX has been proposed to play a crucial role in central carbon metabolism in mitochondria (Del-Saz et al. 2018). For example, external proline treatment stimulates nocturnal respiration and upregulates the expression of 2 AOX isoforms, AOX1a and AOX1d, likely to help mitigate proline-induced oxidative stress (Oh et al. 2022). However, the mechanism underlying AOX-mediated metabolic contributions has not been fully elucidated.
In this issue of Plant Physiology, Oh et al. (2023) tackled this question by focusing on alanine (Ala), which is not only one of the 20 proteogenic amino acids but also functions as a key hub metabolite in central carbon metabolism. In plant mitochondria, Ala is reversibly converted into pyruvate, which is further catabolized in the mitochondrial tricarboxylic acid (TCA) cycle (Hildebrandt et al. 2015; Zhang and Fernie 2018). The authors utilized an artificial chemical treatment system where exogenous Ala was applied to Arabidopsis thaliana leaf discs to investigate its physiological, transcriptomic/proteomic, and metabolic impacts.
Ala feeding induced nighttime respiration, accompanied by an elevated AOX1d amount at both transcriptional and protein levels. Similarly, the AOX expression was upregulated by proline, serine, and glycine but not by sucrose or some TCA cycle intermediates. These results highlight that AOX upregulation is induced by products of amino acid catabolism. Meanwhile, the Ala-induced respiration was not altered in the aox knock-out mutant, indicating that AOX capacity per se was not required for respiratory upregulation by Ala. Instead, AOX capacity elevated the levels of the mitochondrial TCA cycle intermediates, particularly citrate, under the high Ala condition. Also, Ala treatment triggered antioxidant accumulation (e.g. ascorbate), which protects from oxidative stress, but this was not observed in the aox mutant. Overall, Ala treatment upregulates AOX expression to lower reactive oxygen species production. This AOX-mediated redox change affects various metabolic processes, including high citrate accumulation (Figure), because carbon flux in the TCA cycle was likely mitigated due to the redox hypersensitivity of some enzymes involved in citrate metabolism (Møller et al. 2020).
The role of AOX in Ala-induced metabolism in plant mitochondria. In the wild-type mitochondria, upregulated AOX capacity induced by exogenous Ala treatment likely suppresses reactive oxygen species (ROS) production, ensuring efficient TCA cycle operation including citrate production.
What is the role of Ala-induced AOX expression in plant-specific mitochondria metabolism? Plant central metabolism is flexibly changed in response to cellar redox status during the day/night cycle. In this context, citrate might be one of the key metabolites to focus on. In the night, mitochondria produce and export citrate for storage in the vacuole, which has been proposed as a redox valve to decrease an excessively elevated NADH:NAD redox ratio (Igamberdiev and Bykova 2018). Also, citrate stored at night is proposed to support nitrogen assimilation during the following day (Gauthier et al. 2010). AOX might play a crucial role in such citrate-related metabolic adjustment. However, a lot of questions still remain unsolved, including when the Ala level is elevated enough to induce AOX expression under physiological conditions. Generally, the amino acid concentration is drastically increased during senescence because of protein degradation, where typical respiratory carbohydrates are depleted (Araújo et al. 2011). Coincidently, AOX1d expression is reportedly upregulated in senescent leaves (Clifton et al. 2006). Under this situation, amino acid–induced AOX expression may help catabolize amino acids as alternative respiratory substrates. More detailed physiological comparison using the aox mutants will be needed to test this hypothesis. Also, it is still a mystery what downstream metabolite(s) upregulate AOX expression. Possible answers to these questions may provide us some clues in understanding why AOX is essential in plants but does not exist in animals.
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Author notes
Conflict of interest statement. None declared.
