Alternative oxidase 1a and 1d enable metabolic flexibility during Ala catabolism in Arabidopsis

Ala is a central metabolite in leaf cells whose abundance is related to pyruvate metabolism and nocturnal respiration rate. Exposure of Arabidopsis (Arabidopsis thaliana) leaf discs to certain exogenous amino acids including Ala led to substantial increases in nighttime respiration rates as well as increases in alternative oxidase (AOX) 1d transcript and protein levels. During Ala-treatment, AOX1d accumulation, but not AOX1a accumulation, was dependent upon the catabolism of Ala. Complete loss of AOX expression in aox1a aox1d leaf discs did not significantly affect oxygen consumption rates (OCR) under Ala treatment, indicating that AOX capacity per se was not essential for respiratory stimulation by Ala. Rather, Ala treatments caused induction of select antioxidant mechanisms in leaf discs, including a large increase of the ascorbate pool, which was substantially more oxidized in aox1a aox1d leaf discs. Furthermore, we observed differences in the accumulation of a sequence of TCA cycle intermediates from pyruvate to 2-oxoglutarate in WT upon Ala treatment that did not occur in aox1a aox1d leaf discs. The results indicate that AOX induction during enhanced Ala catabolism in leaves mediates mitochondrial redox status, allowing greater metabolic flexibility in mitochondrial organic acid metabolism.