J.-P. Verbelen

PHYTOCHROME-MEDIATED INTER-ORGAN DEPENDENCE AND RAPID TRANSMISSION OF THE LIGHT STIMULUS

The chapter discusses phytochrome-mediated interorgan dependence and rapid transmission of the light stimulus. A study of the transition from heterotrophic to autotrophic growth in the Phaseolus system shows that the embryonic axis is functioning as a coordinating center for phytochrome-mediated morphogenic reactions taking place in the primary leaves and in the hook region. In this study, particular attention was paid to short-term effects induced by light. Rapid changes in P i levels and adenosine triphosphate (ATP) content of hook and leaf tissue were observed within the first few minutes of irradiation. The activity of ATPase(s) was very high in both dark controls and light-stimulated plants. By means of a histochemical method and electron microscopy, the enzyme complex was localized at the level of the cytoplasmic membrane, predominantly in the outer-stelar cell layers of the embryonic axis. Within the same time range of illumination, fast changes in the surface potential of the embryonic axis were recorded. This could be the result of a changed ionic permeability of the cells belonging to the surface tissues. If the active form of phytochrome is part of the cell membrane or at least associated with it, Pfr may modulate the activity of ATP-dependent ion pump systems and phosphorylating activities at the membrane level in many rapid energy-dependent photomorphogenic processes.

PHOTOMORPHOGENETIC REACTIONS and THEIR RELATION TO TIME

Abstract— Hook‐opening and leaf expansion were selected as a measure of the photoactivity of etiolated Phaseolus vulgaris L. seedlings. The experimental procedure consisted of a standard red illumination, followed by a 24‐h reaction period. The magnitude of both morphogenic responses was dependent upon the time of day when the light pulse was given. The photoactivity of the etiolated seedlings changed rhythmically with a periodicity of about 6 h.

CIRCANNUAL COURSE OF PHOTOMORPHOGENETIC REACTIVITY IN ETIOLATED BEAN SEEDLINGS

Hook opening and leaf expansion, measured 24 h after standard red light illumination, were considered as parameters determining the photoactivity of etiolated Phaseolus vulgaris L. seedlings. The experiment was repeated every h during one 17‐h period each month for 1 year. The mean values for each experimental period indicated that the photoactivity of the etiolated seedlings changed markedly throughout the year according to a seasonal pattern.

Chloroplast Development Controlled by Phytochrome and Interorgan Correlation

The first physiological evidence for the existence of a photoreversible pigment controlling plant development was brought by Parker et al. (1945) demonstrating through action spectroscopy the photoperiodic control of flowering induction with soybean, Glycine max. var. Biloxi. In 1959 the pigment phytochrome was detected spectrophotometrically by Butler and coworkers. In the meanwhile and after the discovery of phytochrome it became clear that this pigment is controlling many other steps in the life cycle of a plant.

Phytochrome controls ATP-breakdown and -synthesis in bean hooks

Abstract Illumination of etiolated bean seedlings causes a fast change in the ATP-content of the stem hook. This change is mediated through the phytochrome system. We present evidence that breakdown as well as synthesis pathways of the ATP-turnover cycle can be affected. Both in vivo and in vitro illuminations are effective. This proves that even in a cell-free system a functional relationship exists between the photoreceptor and enzymes involved in ATP-metabolism.