Density-labeling evidence against a de novo formation of peroxisomes during greening of fat-storing cotyledons.

During greening of fat-storing cotyledons, the functional properties of the microbody population changes from that of glyoxysomes to that of leaf peroxisomes. The amount of microbody protein per cell is assumed to be substantially reduced [ 1,2,3] . Little is known about the mechansim of this change of microbody function. Recently, data obtained from labeling of microbody membranes with Cl4 and H3 choline were interpreted as evidence that light triggers both a selective destruction of glyoxysomes and a concomitant de novo formation of leaf peroxisomes [4]. In contrast, electron microscopic examinations [.5,6] as well as studies on the change of microbody isoenzyme spectra [2,7] had previously led to the conclusion that in greening cotyledons leaf peroxisomes do arise from glyoxysomes. Direct evidence for a de novo synthesis of proteins is provided by density labeling with stable heavy isotopes [8]. In this paper the lack of light-dependent density labeling of leaf peroxisomes is contrasted with light-dependent density labeling of plastids, and with density labeling of glyoxysomes in early stages of germination of sunflower seeds.