Enzymatic activities for the synthesis of chlorophyll in pigment-deficient variegated leaves of Euonymus japonicus [Celastraceae]

The enzymes involved in the biosynthesis of chlorophyll (Chl) in pigment-deficient variegated leaves of Euonymus japonicus were investigated. Each variegated leaf was composed of clearly delineated green and white sectors. The white sectors contained almost no Chls. The rate of synthesis of 5-aminolevulinic acid (ALA) in the white sectors in vivo was twice that in the green sectors. The level of glutamate 1-semialdehyde aminotransferase in the white sectors was much higher than that in the green sectors. Plastidic tRNAGlu was also present at substantial levels in the white sectors, indicating that the system for synthesis of ALA was very active in the white sectors. The activity of porphobilinogen (PBG) synthase in the white sectors in vitro was twice that in the green sectors. In the white sectors the rate of porphyrin synthesis from PBG was 4- to 6-fold higher than in the green sectors. We measured Mg-chelatase activity indirectly in both sectors by monitoring the accumulation of Mg-protoporphyrin IX in the presence of 2,2′-dipyridyl, which inhibits isocyclic ring formation with the resultant accumulation of Mg-protoporphyrin IX. When sectors were incubated in darkness with 2,2′-dipyridyl, large amounts of protoporphyrin IX accumulated in the white sectors, whereas Mg-protoporphyrin IX mainly accumulated in the green sectors. These results suggest that the enzymes for the synthesis of porphyrin that catalyze conversion of ALA to protoporphyrin IX were very active and that the Mg-insertion step might be blocked in the white sectors, with the resultant failure to synthesize Chl. The deficiency is discussed in a comparison with that in other Chl-deficient plants.