Altered Activity of Peroxidase and Oxalate Oxidase Influences Lignification in Transgenic Tobacco

Peroxidase and hydrogen peroxide both play important roles in the final stages of the lignification pathway. Peroxidase, in the presence of H2O2 catalyses the oxidation of monolignols to give lignin. In order to examine this process we looked at lignification in transgenic tobacco plants expressing a barley peroxidase gene, HvPrx8, either alone or in combination with a wheat germin gene, g.f 2.8, which encodes oxalate oxidase, thereby providing a source of H2O2. Elevated activity of the antioxidant ascorbate peroxidase was found in plants expressing oxalate peroxidase and was greatly increased by co-expression with the barley peroxidase, although the latter had no effect when expressed alone. An increase was observed in the oxidation of the lignin monomer, syringaldazine in cell lines over-expressing barley peroxidase, while a decrease was observed in double transformants. Plants over-expressing barley peroxidase have elevated levels of lignin deposition compared to that of wild type tobacco plants. Over-expression of the individual enzymes was also shown to enhance heat-induced programmed cell death (PCD) in cell suspension cultures, an effect which was greatly reduced in the double-expressing lines.