Involved in the biosynthesis of ascorbate (PubMed:18190525). Catalyzes the final step of ascorbate biosynthesis (PubMed:18190525). Uses L-galactono-1,4-lactone and L-gulono-1,4-lactone as substrates, but not D-galactono-1,4-lactone, D-gulono-1,4-lactone, L-mannono-1,4-lactone or D-galactonic acid (PubMed:18190525). Also active with phenazine methosulfate and 1,4-benzoquinone as electron acceptors (PubMed:18190525). Involved in the regulation of the accumulation of the mitochondrial respiratory complex I (PubMed:18799460, PubMed:33060577). Structural part of one of the plant-specific mitochondrial complex I assembly intermediates, lacking the whole distal (PD) module (PubMed:33060577). Prevents the binding of the plant specific P1 protein (CPN60/HSP60), responsible for the linkage of the proximal (PP) to the distal (PD) module (PubMed:33060577). 4 Fe(III)-[cytochrome c] + L-galactono-1,4-lactone = 4 Fe(II)-[cytochrome c] + 5 H(+) + L-dehydroascorbate Binds FAD non-covalently. Optimum pH is 8.8. Cofactor biosynthesis; L-ascorbate biosynthesis. Delayed germination, chlorotic cotyledons and death at the cotyledon stage.