Dehydrogenation polymerization products of p-hydroxycinnamyl alcohols by isoperoxidases obtained from downy mildew-infected roots of Japanese radish ( Raphanus sativus)

Abstract

Lignification of cell walls in roots of Japanese radish ( Raphanus sativus) infected with downy mildew is followed by an increase in peroxidase activity. In an attempt to determine the rôle of enhanced peroxidase activity the formation of dehydrogenation polymerization products (DHPs) from p-hydroxycinnamyl alcohols was investigated by using isoperoxidases isolated from the tissues. DHPs were formed from p-coumaryl and coniferyl alcohols with the crude peroxidase preparation and from sinapyl alcohol when this was mixed with p-coumaryl and coniferyl alcohols. Characteristics properties of DHPs were similar to those of lignins isolated from healthy and diseased roots. DHP formation was examined with peroxidase fractions obtained from the crude peroxidase preparation by cellulose column chromatography. Of 11 fractions containing peroxidase activity (A 1-A 5, B-G), Fractions D and F did not form DHP from any of the alcohols, while Fraction G formed DHP only from p-coumaryl alcohol. When the mixture of sinapyl and coniferyl alcohols was used as substrate, DHP was formed by Fractions A 2, A 3, B and E. The significance of these peroxidase fractions in cell wall lignification is discussed.