CHLOROPHYLL SYNTHESIS IN A DEETIOLATED-(DET340) MUTANT OF ARABIDOPSISWITHOUT NADPH-PROTOCHLOROPHYLLIDE (PCHLIDE) OXIDOREDUCTASE (POR)-A AND PHOTOACTIVE PCHLIDE-F655

Chlorophyll (Chl) synthesis in Arabidopsis is controlled by two light- dependent NADPH-protochlorophyllide (PChlide) oxidoreductases (PORs), one (FOR A) that is active transiently in etiolated seedlings at the b eginning of illumination and another (FOR B) that also operates in gre en plants, The function of these two enzymes during the light-induced greening of dark-grown seedlings has been studied in the wild type and a deetiolated (det340) mutant of Arabidopsis. One of the consequences of the def mutation is that FOR A is constitutively down-regulated, a nd therefore, synthesis of the FOR A enzyme is shut off. When grown in the dark, the det340 mutant lacks FOR A and the photoactive PChlide-F 655 species but maintains the second PChlide reductase, FOR B, Previou sly, photoactive PChlide-F655 has often been considered to be the only PChlide form that leads to Chi formation. Despite its deficiency in F OR A and photoactive PChlide-F655, the det340 mutant is able to green when placed in the light. Chi accumulation, however, proceeds abnormal ly, At the beginning of illumination, seedlings of det340 mutants are extremely susceptible to photooxidative damage and accumulate Chi only at extremely low light intensities, They form core complexes of photo systems I and II but are almost completely devoid of light-harvesting structures. The results of this study demonstrate that in addition to the route of Chi synthesis that has been studied extensively in illumi nated dark-grown wild-type plants, a second branch of Chi synthesis ex ists that is driven by POR B and does not require POR A.