Plants have evolved the ability to derive the benzenoid moiety of the respiratory cofactor and antioxidant, ubiquinone (coenzyme Q), either from the b-oxidative metabolism of p-coumarate or from the peroxidative cleavage of kaempferol. Here, isotopic feeding assays, gene coexpression analysis and reverse genetics identified Arabidopsis 4-COUMARATE-COA LIGASE 8(4-CL8; At5g38120) as a contributor to the b-oxidation of p-coumarate for ubiquinone biosynthesis. The enzyme is part of the same clade (V) of acyl-activating enzymes than At4g19010, a  p-coumarate CoA ligase known to play a central role in the conversion of p-coumarate into 4-hydroxybenzoate. A 4-cl8T-DNA knockout displayed a 20% decrease in ubiquinone content compared with wild-type plants, while 4-CL8 overexpression boosted ubiquinone content up to 150% of the control level.Similarly, the isotopic enrichment of ubiquinone's ring was decreased by 28% in the 4-cl8knockout as compared to wild-type controls when Phe-[Ring-13C6] was fed to the plants. This metabolic blockage could be bypassed via the exogenous supply of 4-hydroxybenzoate, the product of p-coumarateb-oxidation. Arabidopsis4-CL8 displays a canonical peroxisomal targeting sequence type 1, and confocal microscopy experiments using fused fluorescent reporters demonstrated that this enzyme is imported into peroxisomes. Time course feeding assays using Phe-[Ring-13C6] in a series of Arabidopsis single and double knockouts blocked in the b-oxidative metabolism of p-coumarate (4-cl8; at4g19010; at4g19010x 4-cl8), flavonol biosynthesis (flavanone-3-hydroxylase), or both (at4g19010x flavanone-3-hydroxylase) indicated that continuous high light treatments (500 µE m-2s-1; 24h) markedly stimulated the de novobiosynthesis of ubiquinone independently of kaempferol catabolism.

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