[Kinetical studies on phytochrome-induced protein synthesis]

Control by phytochrome of the appearance of ribulose-1,5-bisphosphate carboxylase and the mRNA for its small subunit

We have measured levels of ribulose-1,5-bisphosphate carboxylase (RuBPCase) and levels of in-vitro-translatable mRNA for the small subunit (SSU) of RuBPCase up to 96 h after sowing in mustard (Sinapis alba L.) cotyledons, in order to investigate to what extent the rate of enzyme synthesis is related to the level of SSU-mRNA. Both enzyme and mRNA level are controlled strongly by phytochrome, but the rate of RuBPCase accumulation was found to be unrelated to the level of translatable SSU-mRNA. As an example, it was found that the amount of SSU-mRNA in far-red light (FR)-grown mustard seedlings doubles between 54 and 84 h after sowing while the rate of RuBPCase accumulation remains constant over this period. Since the holoenzyme shows zero turnover during this period it is concluded that the rate of enzyme synthesis remains constant although the level of SSU-mRNA increases strongly. Following an FR→dark transition, with different levels of physiologically active phytochrome (Pfr) established at the end of the light period, no correlation was found between the time course of mRNA levels in darkness and the rate of enzyme synthesis. Rather, the data indicate that there is at least one translational or post-translational regulatory step which is also phytochrome-dependent. It is concluded that coarse control of the appearance of translatable SSU-mRNA is essential for RuBPCase to appear at a high rate but that fine tuning by phytochrome of the actual appearance of RuBPCase is not transcriptional.

Rapid activation by phytochrome of nitrate reductase in the cotyledons of Sinapis alba

Nitrate reductase in the cotyledons of etiolated seedlings of Sinapis alba L. responds rapidly to the addition of nitrate. The response is inhibited by cycloheximide at low concentrations. The enzyme is also under phytochrome control. Five minutes of red light irradiation leads instantaneously to a 45% increase in enzyme activity. Increases in activity, linear with respect to time and with no lag phases are promoted by continuous far-red or blue irradiation. These increases are insensitive to cycloheximide. Thus, light and nitrate act through different mechanisms in controlling nitrate reductase activity and phytochrome does not act via controlling the rate of synthesis of the enzyme.

Phytochrome effect on free amino acid metabolism of higher plants

Activated phytochrome lowers the free amino acid pools of all plants investigated by about 15%. The action of red light is preferentially directed to Asp, Glu and Phe. Exogenously supplied Leu-U-(14)C is incorporated more quickly into protein of red-light-treated samples compared with dark controls. In contrast, red light decreases the amount of Asp-U-(14)C incorporated into protein, but increases the amount of (14)CO2 respired after feeding with Asp-U-(14)C. Red light has no effect on the amount of (14)CO2 respired after feeding with Leu-U-(14)C. Red-light-mediated stimulation of incorporation of Leu-U-(14)C into protein occurred within 15 min, well before the red-light-mediated increase in (14)CO2 production following feeding with Asp-U-(14)C could be detected.

Differential effects of the embryonic axis on chlorophyll production and photosynthesis of mustard cotyledons

Mustard cotyledons detached from the dry seed or at intervals during five days dark germination produced consistently more chlorophyll but a lower photosynthetic rate when maintained in the dark on moist filter paper for 24 hr before exposure to light than cotyledons which were attached to the seedling axis during this time.The results suggest that the embryonic axis exerts a differential effect on the developing photosynthetic system, competing for material involved in chlorophyll formation and at the same time stimulating the development of other photosynthetic components.

[The control by actinomycin D and puromycin of the phytochrome-mediated inhibition of hypocotyl lengthening in the mustard seedling (Sinapis alba L.)]

Inhibition of hypocotyl lengthening by phytochrome can be regarded as a prototype of a "negative" photoresponse (MOHR, 1966b). A characteristic feature of a negative photoresponse is the short lag-phase (Fig. 3). - The hypothesis has been advanced that "negative" photoresponses are the consequence of a differential gene repression which is exerted by P730, the active phytochrome (Fig. 1). In the present paper a number of experiments with the intact seedling are described which support this hypothesis. Continuous irradiation with far-red has been used to establish a stationary concentration of P730 over the whole period of experimentation. 1. Under steady state conditions of hypocotyl lengthening P730 and actinomycin D (=Act) lower the growth velocity of the organ (Fig. 3, 4). The inhibitory action of Act (on a percentage basis) is the same with and without far-red (Fig. 5, 6). This fact means that Act and P730 act as two independent factors in a multiplicative system. 2. Under steady state conditions of hypocotyl lengthening puromycin (=Pu) lowers the growth velocity of the organ (Fig. 7). The inhibitory action of Pu (on a percentage basis) is the same with and without far-red (Fig. 8, 9). This fact means that Pu and P730 act as two independent factors in a multiplicative system. 3. The model in Fig. 10 agrees with all the facts which are available at the moment. The model is useful only if one can assume that the mRNA and enzymes which are coded by the repressible genes have a very short "life-time". This assumption has not yet been proved. - The model is supported by the fact that RNA synthesis in the hypocotyl is strongly inhibited by P730 and is very sensitive toward Act (WEIDNER, unpublished data). - Furthermore LINK (unpublished data) has shown that both P730 and Act suppress the incorporation of (14)C-leucine into the protein fraction of the hypocotyl. It seems that in this case also, Act and P730 behave as two independent factors in a multiplicative system.

[Protein and RNA contents of the hypocotyl during steady state growth lengthening in the dark and under the influence of phytochrome (seedlings of sinapis alba L.)]

Inhibition of hypocotyl lengthening by phytochrome can be regarded as a prototype of a "negative" photoresponse. The hypothesis has been advanced (SCHOPFER, 1967) that negative photoresponses are the consequence of a differential gene repression which is exerted by P730, the active phytochrome. This hypothesis is mainly based on experiments with specific inhibitors of RNA- and protein synthesis. -The present paper is part of an experimental program which has been designed to check this hypothesis.-Continuous irradiation with standard far-red has been used to establish a virtually stationary concentration of P730 over the whole period of experimentation (36-60 hours after sowing). To correlate more strictly the growth response of the hypocotyl with "molecular" changes in this organ the axis system without cotyledons has been used (Fig. 1). Even under these conditions the growth rate of the hypocotyl is nearly constant in light (continuous far-red) and dark during the whole period of experimentation (36-60 hours after sowing) (Fig. 2, 3). It is known from earlier experiments that cell division in the hypocotyl are very rare during this period and that there is virtually no increase in the DNA contents of the organ during the period of our experimentation (WEIDNER, 1967). Obviously the number of cells per hypocotyl is virtually constant between 36 and 60 hours after sowing. Organ (i.e. hypocotyl) lengthening is nearly exclusively due to cellular lengthening.-If we follow the protein contents of the hypocotyl we find (Fig. 4) that the total protein of the organ decreases steadily in spite of the fact that the organ grows at a constant rate. There is no significant difference in protein contents between dark-grown and far-red grown systems although the growth rates differ by a factor of 4 (Fig. 2, 3).-The situation is some-what different with respect to total RNA (Fig. 5). The RNA contents eventually decrease in far-red as well as in dark-grown systems but the decrease is significantly faster in the far-red treated systems than in the dark controls.-It is concluded that only a very small part of the total RNA and total protein of a cell can be related to the control of cellular growth. Changes in bulk RNA and bulk protein obviously do not necessarily reflect changes in the growth rate or growth capacity of an organ or a cell.

[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.

[Studies on regulation of RNA-synthesis by phytochrome in the mustard seedling (Sinapis alba L.)]

P730, the active phytochrome, increases the rate of RNA synthesis (Table) and the RNA contents in the cotyledons of the mustard seedling (Sinapis alba L.) (Fig. 1) whereas the RNA contents in the hypocotyl is decreased under the influence of P730 (Fig. 2).-It takes about 6 hours until changes in the RNA contents-which must be attributed to the formation of P730-can be measured after the onset of light (continuous far-red). Since the lag-phases of "positive" photoresponses in the cotyledons and "negative" photoresponses in the hypocotyl (MOHR, 1966) are in general much shorter than 6 hours, the changes of the RNA contents of the organs cannot be regarded as being directly connected with the formation of characteristic "positive" photoresponses such as anthocyanin synthesis, induced enzyme synthesis, ascorbic acid synthesis, etc., or "negative" photoresponses such as inhibition of hypocotyl lengthening.We have rather to conclude that the changes of RNA contents are secondary adaptations of the organs which lead to an increase (cotyledons) or decrease (hypocotyl) of protein synthesizing capacity of the cells and tissues. The P730-dependent increase of bulk RNA in the cotyledons is probably due to a differential gene activation and the P730-dependent decrease of bulk RNA in the hypocotyl is due to a differential gene repression. The causalities of these processes are possibly complex.The hypothesis of differential gene activation or repression by P730 (MOHR, 1966; SCHOPFER, 1967a, b) is not disproved by these results. We have rather to reach a conclusion which has already been suggested by other data (e.g. KAROW and MOHR, 1966), namely, that positive as well as negative photoresponses are due to changes in the activity of a limited (possibly small) number of enzymes. Correspondingly changes in only a minute amount of the total RNA are directly involved in the formation of photoresponses. These changes cannot be detected by following RNA contents.-It seems to be of great interest, however, that P730 eventually brings about strong tissue specific changes in the RNA contents per cell as described in the present paper.

GROWTH RETARDANTS IN RELATION TO THE GERMINATION OF SEEDS: I. A PARADOXICAL CONCENTRATION EFFECT OF N,N-DIMETHYLAMINO-SUCCINAMIC ACID ON THE GERMINATION OF KALE SEEDS AFFECTED WITH COUMARIN

As the concentration of N,N-dimethylaminosuccinamic acid (DMASA) increases, the germination of kale seeds (Brassica oleracea L. var. acephala) treated with 100 mg of coumarin per liter is initially reduced and then increased. A curve of germination versus concentration of DMASA is parabola-like with a minimum at 10−3 − 5 × 10−3 M. Kinetin at 10−4 M and gibberellic acid at 10−4 M reduce and reverse the inhibition of germination caused by coumarin and the growth retardant. On the other hand, DMASA at 5 × 10−3 M, in spite of its slight inhibitory activity, reduces the toxic effect of kinetin applied in the supra-optimal concentration of 5 × 10−4 M. Indole-3-acetic acid increases the inhibitory effects of coumarin and DMASA.In the germinating seed of kale, DMASA possibly acts as the antagonist of gibberellin biosynthesis; coumarin possibly blocks an unknown metabolic pathway, controlled by kinetin.The paradoxical concentration effect of DMASA remains unexplained.

[The DNA contents of cotyledons and hypocotyl of the mustard seedling (Sinapis alba L.) during phytochrome-mediated photomorphogenesis]

DNA contents and accordingly cell numbers of cotyledons and hypocotyl of the mustard seedling were virtually constant during the experimental period (between 36-60 hours after sowing) in the dark as well as under the influence of P730, the active phytochrome (Table).-Therefore the phytochromemediated increase of the RNA and protein contents (WEIDNER et al., 1965) must be understood as an increase of RNA and protein per cell. This fact is in agreement with the conception of differential gene activation mediated by P730 (MOHR, 1966). The virtually constant DNA contents during the period of time which is regularly used for experimentation on photomorphogenesis in our laboratory (36-60 hours after sowing; MOHR, 1966) is an important prerequisite for comparing biochemical data under the point of view of differential gene activation, e.g. in kinetical studies in the dark and under continuous far-red light.

[Changes of activity of isocitritase (E C 4. 1. 3. 1.) during photomorphogenesis in mustard seedlings (Sinapis alba L.)]

In a previous paper (HOCK, KüHNERT and MOHR, 1965) we have shown that fat degradation in the cotyledons of the mustard seedling (Sinapis alba L). is under the control of active phytochrome (=P730), at least to a certain extent. One could imagine that P730 might exert this control over fat consumption through isocitritase (E C 4. 1. 3. 1). In the present paper we demonstrate that P730 does not have any influence on the activity of isocitritase in the mustard seedling, although the enzyme activity does show a strong increase and a following decline during the time of our experimentation (Fig. 8, 9). - Isocitritase could be assayed in a satisfactory manner in the raw extract only after the adsorption of phenolic compounds on "Divergan" (Fig. 1, 3). Furthermore the biological unit of the "seedling" turned out to be the only reliable system of reference for the enzyme activity determined in the extract. Puromycin strongly inhibits isocitritase synthesis whereas Actinomycin D is hardly effective (Fig. 10). It has been concluded that there might exist a relatively stable m-RNA which has been formed during the early stage of germination. Regulation of enzyme synthesis might occur on the level of translation. The evidence for this conclusion is not convincing, however; other interpretations are not excluded.