Catalyzes the conversion of monodehydroascorbate to ascorbate, oxidizing NADH in the process (PubMed:16146528). Required for producing sufficient ascorbate to maintain the interaction between Piriformospora indica and Arabidopsis in a mutualistic state (PubMed:19386380). H(+) + 2 monodehydro-L-ascorbate radical + NADH = 2 L-ascorbate + NAD(+) A number of isoforms are produced. According to EST sequences. Up-regulated by drought stress and in roots colonized by the beneficial endophytic fungus Piriformospora indica. Loss of growth and seed production promotion by Piriformospora indica. Belongs to the FAD-dependent oxidoreductase family.