[The action of phytochrome and actinomycin D on chlorophyll a formation in mustard seedlings (Sinapis alba L.)].

In the mustard seedling chlorophyll a synthesis under white light is enhanced by a pretreatment with far-red which maintains a low but virtually stationary concentration of active phytochrome (=P730) (Fig. 1) during the period of irradiation (4 hours). - On the other hand chlorophyll a synthesis is inhibited or delayed by relatively low concentrations of Actinomycin D(=Act) (Fig. 2)The inhibitory action of Act (on a percent basis) is exactly the same with and without a far-red pre-irradiation (Fig. 3). Act in relatively low doses (5 or 10 μg/ml) greatly extends the lag-phase of chlorophyll synthesis; however, these doses do not influence the effect of the far-red pretreatment on the rate of chlorophyll synthesis when it finally takes place (Fig.4,5,6). The data presented in this paper indicate that Act does not inhibit protochlorophyll synthesis as such; we have rather to conclude that Act inhibits the de novo synthesis of some specific structural proteins which are prerequisites of chlorophyll accumulation and maintenance in the plastids (Table 1). Synthesis of these structural proteins seems to be under the control of phytochrome too.It is concluded that those genes which are already in function are relatively resistant to Act (e. g. those genes which are needed for protochlorophyll synthesis) whereas potentially active genes (e. g. those which code some specific structural proteins of the plastids) are very sensitive to Act. -A similar conclusion has been reached in an earlier paper in connection with phytochrome-induced antocyanin synthesis (LANGE and MOHR, 1965). Our argumentation is further supported by SCHOPFER's data on control of ascorbate synthesis in the mustard seedling by phytochrome and Act (SCHOPFER, 1966).