["Primary" and "secondary" differentiation in connection with photomorphogenesis of seedlings (Sinapis alba L.)]

Anthocyanin synthesis of the mustard seedling (Sinapis alba L.), a typical phytochrome-dependent photoresponse has been further investigated. - It has been found that only two types of tissue can synthesize anthocyanin under the influence of active phytochrome (=P730), namely, the epidermis of the votyledons and the subepidermal layer in the hypocotyl (Fig. 2, 3). - Under our standard conditions (25° C; cf. methods) phytochrome-potentiated anthocyanin synthesis is only possible 24 hours after sowing and it ceases about 60 hours after sowing, independent of the amount of anthocyanin which has been accumulated (Fig. 5, 6). On the basis of the whole seedling the highest sensitivity of the anthocyanin producing system to light is around 36 hours after sowing (Fig. 8). Within the tissues which are capable of forming anthocyanin there is a characteristic shift of the ability to respond to P730 as the seedling ages. If we devide the seedling into 4 segments (Fig. 9) it turns out that in the basal and middle part of the hypocotyl the ability to form anthocyanin is rapidly lost whereas in the upper part of the hypocotyl and in the cotyledons this ability even increases at first. The following decrease is slower than in the basal parts (Fig. 10, 11).It is argued that this specific and dynamic cellular pattern of responsiveness to P730 can be regarded as a manifestation of a "primary differentiation" in the course of which the genotype of each individual cell in the dark-grownt seedling is devided into 3 functional types of genes: active, inactive, and potentially active genes (P730) (Fig. 4). - In connection with anthocyanin synthesis P730 is thought to act exclusively at the level of "secondary differentiation", i.e., it is thought to initiate the action of potentially active genes via a signal-chain. The action of P730 is non-specific. The specificity of the photoresponse of an individual cell is determined by the status of its "primary" differentiation (Fig. 4).If the process of differentiation is slowed down (e.g. by the application of low doses of Actiomycin D) anthocaynin synthesis can continue much longer than under our standard conditions where it ceases around 60 hours after sowing (Fig. 12). This fact seems to indicate that the loss of the ability to form anthocyanin is due to an inactivation of pertinent genes by the process of "primary differentiation", which is itself, as one would expect, under the control of genes.