Flavonoids and some other phenolics as substrates of peroxidase: Physiological significance of the redox reactions

Flavonoids and other phenolics are used for various reactions in plants. When used for lignification and browning on aging, the oxidation products are accumulated. When used as substrates of peroxidase (POX) in the presence of ascorbic acid (AA), phenolics are initially oxidized by POX and the phenoxyl radicals formed are reduced by AA producing monodehydroascorbic acid and dehydroascorbic acid. Since the oxidation forms of AA are reduced to AA in plant cells, the phenolics/AA/POX systems can scavenge H2O2 without accumulating oxidation products of phenolics. Scavenging of H2O2 by the systems can proceed in vacuoles and the apoplast, because phenolics, AA and POX are normal components of the compartments. AA seems to control lignification because it reduces radicals of lignin monomers which are formed by POX-dependent reactions. On lignification, oxidation of sinapyl alcohol is enhanced by radicals of coniferyl alcohol and hydroxycinnamic acid esters when apoplastic POX rapidly oxidizes coniferyl alcohol and the esters but slowly oxidizes sinapyl alcohol. POX seems to participate in the browning of tobacco leaves and onion scales on aging. H2O2, which is required for the POX-dependent reactions, can be formed by autooxidation of the phenolics that are transformed to brown components. It is discussed that browning involves the formation of antimicrobial substances.