Phytochrome control of degradation of storage protein and formation of plastids in the cotyledons of the mustard seedling (Sinapis alba L.)

: Histological and histochemical methods have been used to investigate the behaviour of storage protein and structural protein (=plastids) in the cytoledons of the mustard seedling during phytochrome-mediated photomorphogenesis.-It has been shown that under continuous standard far-red light, which maintains a low but virtually stationary concentration of the active phytochrome (=P730) in the tissue, the degradation of storage protein (dark, dense bodies in Fig. 2) is accelerated (Fig. 1,2). Degradation of storage protein is not homogeneous throughout the cotyledonary tissue; we rather observe a complicated pattern. The epidermal layer, e.g., is characterized by the most rapid degradation. Within the mesophyll the palisade parenchyma degrades the storage protein faster than the spongy parenchyma. In the lamina there exists a gradient insofar as the degradation is always faster in the basal part than in the anterior part. - Whereas the rate of degradation of storage protein is controlled by P730, the pattern of degradation is exactly the same in light and dark. - Under continuous far-red light, large plastids are formed in the mesophyll cells of the cotyledons. They are virtually identical with chloroplasts formed under white light as far as size and shape are concerned (Fig. 2). These plastids generally contain some but only traces of chlorophyll.Data from electron microscopic studies (Fig. 3, 4, 5) support the conclusion that the plastids formed under continuous far-red are homologous to chloroplasts.It is obvious that P730 accelerates the degration of storage protein in the cotyledons; at the same time, however, P730 reduces the rate of translocation of N-containing material from the cotyledons to the axis of the seedling and increases the rate of protein synthesis in the cotyledons (JACOBS and MOHR, 1966). Most of the protein which is synthesized under the control of P730 is structural protein of the newly formed plastids.In conclusion it can be stated that under the influence of P730 the cotyledons are transformed from storage organs to photosynthetic organs. This transformation can be followed at the level of the plastids as well as at the level of the tissues (Fig. 6). Photosynthesis is not involved in this transformation.