Phytochrome system

Alkaline pyrophosphatase dependent on Mg2+ was found in every sample examined from a broad spectrum of the plant kingdom (SI). Plants which fix C02 by the dicarboxylic acid pathway have characteristic high levels of alkaline pyrophosphatase in their chloroplasts presumably this performs the rather specific function of driving the synthesis of phosphoenolpyruvate, the immediate precursor of C02 fixation (32). Biosynthesis of the maize chloroplast enzyme is controlled by light acting through the phytochrome system (S3). Pyrophosphatase from spinach chloroplasts has been partially purified (34, 35). 

Phases of susceptibility in which secondary metabolism is influenced by appropriate signals have also been detected in higher plants. The induction of antho-cyanin biosynthesis (D 22.3.3) by phytochrome in the epidermal cells of the cotyledons of Sinapis alba for instance, first begins 27 h after sowing, despite the fact that the phytochrome system per se is fully functional immediately after sowing. Competence with respect to anthocyanin formation is determined endogenously, i.e., by the state of differentiation of the epidermis cells in which the anthocyanins are formed and accumulated. 

Fig. 178. Proof that the phytochrome system is a factor in the germination of the achenes of the lettuce (Lactuca sativa). RL = red light, FRL = far red light (from Goodwin 1965).

The phytochrome system is widely distributed in plants, even if in small concentrations. It can be extracted without too much difficulty, particularly from seedlings. Phytochrome suspended in a test-tube can also be converted from one form into the other by irradiation with RL or FRL. It has been shown that phytochrome is a chromoproteid. The pigment bound to protein in this chromoproteid is related to certain pigments of blue algae and algae. It is a phycobilin, thus, in principle, a chain of four pyrrole rings which are linked with each other by C atoms (Fig. 180). 

An unspecific secondary effect of actinomycin can be excluded. If this were the case then actinomycin given later, e.g. 6 hours after the beginning of irradiation, should also have had an inhibitory effect. The result of the experiment, which has been supplemented by additional data, provided the first experimental evidence that the phytochrome system is capable of activating genetic material, in this case the genes for anthocyanin synthesis. 

Anthocyanin synthesis is a very complicated process in which many genes are involved (page 132). Various authors Such as Zucker, Mohr, and Zenk discovered in the 1960s that light is capable of stimulating the synthesis of the phenylalanine-ammonium-lyase via the phytochrome system. Thus, light activates the genetic material for the synthesis of a key... 

Assembly of the photosynthetic apparatus completes the seedling phase of development. The development of this typical plastid structure, which we discussed earlier (page 54), can only take place in light. In darkness, it development goes awry. The phytochrome system is also engaged in the differentiation of the plastids. 

Fig. 206. Hypothesis concerning mode of action of the phytochrome system in seeds which are stimulated and inhibited by light.

Light. Illumination usually leads to opening of the stomata. In many cases blue light, which is absorbed by a special pigment system, is particularly effective. However, chlorophyll and the phytochrome system can also be engaged by appropriate illumination. 

Judging from action spectra, several photochemical processes appear to be involved in the maturation of etiolated proplastids. First, the photoreduction of the protochlorophyllide to chlorophyllide is driven by light absorbed by the protochlorophyllide-protein complex itself (Frank, 1949 Koski et al., 1951 Smith, 1952). Second, the undefined chemistry of the lag phase appears to be under the control of the red, far-red phytochrome system. The latter is based on the observations (Price and Klein, 1961 Mitrakos, 1961 Virgin, 1955) that (1) if etiolated leaves are briefly illuminated with red light and then returned to darkness for a few hours, chlorophyll synthesis begins rapidly without the intervention of an additional lag period upon reillumination (this effect of red light is reported to be reversed by... 

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