Light-enhanced induction of ascorbate peroxidase in Japanese radish roots during postgerminative growth

During postgerminative growth of Japanese radish (Raphanus sativus L. var. hortensis) in continuous light, the specific activity of APX (EC 1.11.1.11) in roots markedly increased 2 days after imbibition. This increase in APX activity in roots was accompanied by increases in activities of the enzymes in the ascorbate-dependent H2O2-scavenging system, superoxide dismutase (E C 1.15.1.1), monodehydroascorbate reductase (EC 1.6.5.4), dehydroascorbate reductase (EC 1.8.5.1) and glutathione reductase (EC 1.6.4.2) which may par ticipate in the detoxification of H2O2 generated via the beta-oxidation of fatty acid in seeds and consequent active cell division in the meristem/det ermination zone of the root. Catalase (EC 1.11.1.6) activity was not detect ed and guaiacol peroxidase (EC 1.11.1.7) activity was extremely low in root s at this stage of postgermination. A prominent increase in APX activity in roots under light conditions reflected the increases in protein level and mRNA accumulation of cytosolic APX. These results indicate that at the earl y stages of postgermination of Japanese radish, cytosolic APX is rapidly in duced in growing roots and its induction may be regulated by light. The rol e of cytosolic APX in roots is discussed in relation to detoxification of H 2O2 generated in the process of postgerminative growth. (C) 1999 Elsevier S cience Ireland Ltd. All rights reserved.