CORRELATED CHANGES IN THE ACTIVITY, AMOUNT OF PROTEIN, AND ABUNDANCE OF TRANSCRIPT OF NADPH - PROTOCHLOROPHYLLIDE OXIDOREDUCTASE AND CHLOROPHYLL ACCUMULATION DURING GREENING OF CUCUMBER COTYLEDONS

Changes in the activity and abundance of NADPH:protochlorophyllide oxi doreductase (NPR) and the abundance of mRNA encoding it were examined during the greening of 5-d-old etiolated cucumber cotyledons under con tinuous illumination. To measure NPR activity in the extracts from ful ly greened tissues, we have developed an improved method of assay. Upo n exposure of etiolated cotyledons to light, NPR activity decreased ra pidly within the first 2 h of exposure. Thereafter, enzymatic activity increased transiently, reaching a submaximum level at 12 h, and decre ased slowly. The level of immunodetectable NPR protein followed the sa me pattern of changes during 96 h of greening as observed for NPR acti vity. The NPR mRNA in etiolated cotyledons disappeared quickly in the ist h of irradiation. However, the level of mRNA increased thereafter to reach 3-fold or more of the dark level at 12 h and then decreased. The changes in the activity, protein level, and mRNA level after the f irst rapid decreases corresponded chronologically and nearly parallele d the increase in the rate of chlorophyll accumulation. These findings suggest that the greening of cucumber cotyledons is regulated basical ly by the level of NPR protein without activation or repression of enz ymatic activity and that NPR mRNA increased by light maintains the lev el of enzyme protein necessary for greening.