Arabidopsis rbcS genes are differentially regulated by light.

Individual members of the Arabidopsis thaliana ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit (rbcS) gene family are differentially regulated by light of different qualities. In 10-d-old etiolated seedlings, the expression of only three of the four genes is under inductive phytochrome control. rbcS mRNA levels reach a maximum (3- to 5-fold higher than the dark level) about 6 h after a red light pulse, but the rate of decay differs among the genes. Moreover, rbcS 2B requires a higher fluence for induction. At early stages of development, rbcS 1A, 2B, and 3B are highly expressed in the dark and cannot be further induced by red light, indicating a developmental component in the overall regulatory mechanism. Continuous light experiments indicate that high-irradiance responses may play a role in the induction of at least three of the four rbcS genes. Under conditions of phytochrome saturation, rbcS 1A is insensitive to blue light pulses, whereas among the three B locus genes, at least rbcS 3B appears to respond to a blue-light photoreceptor. These results add to the data suggesting that individual members of rbcS gene families in higher plants may be subject to a variety of differing regulatory mechanisms.

Long-Lasting Light Effects in Imbibed Kalanchoë blossfeldiana Seeds in the Presence of Gibberellic Acid.

Two brief red (R) irradiations, separated by 24 hours, given to Kalanchoë blossfeldiana Poelln. cv Feuerblüte seeds, made secondarily dormant by a prolonged dark incubation period on water and transferred to GA(3), induce very low germination. Some effect of these irradiations is preserved, however, during a long dark interval in fully imbibed seeds and greatly increases the germination induced by another brief R exposure. This long-lasting light effect is, at 20 degrees C, only lost after a dark interval of about 1 month. It can also be induced by two brief far-red (FR) exposures. Its preservation is temperature-dependent, low temperatures being favorable. Light-induced changes in the ATP-content were demonstrated during preservation and expression of the long-lasting light effect, indicating a long-lasting metabolic change. In seeds with primary dormancy sown on GA(3), an analogous long-lasting light effect is induced by one or two brief R or FR irradiations, even when they are given before germination can take place. The presence of GA(3), which was shown to induce a very low fluence germination response in Kalanchoë seeds, is required for the occurrence of the long-lasting light effect. The data suggest long-term preservation of some effect(s) of Pfr rather than persistent presence of Pfr itself.

Fluence-response curves and action spectra for the very low fluence and the low fluence response for the induction of kalanchoë seed germination.

Germination of Kalanchoë blossfeldiana Poelln. seeds is absolutely light-requiring. Germination of one seed is the result of one out of three reactions, viz. the very low fluence response (VLFR), the low fluence response (LFR) and the high fluence response/high irradiance response. In order to demonstrate the involvement of phytochrome for both photoresponses, i.e. VLFR and LFR, action spectra for induction were determined. Fluence-response data are analyzed by means of probit analysis in order to calculate the seed population parameters, with special attention to mu, or the fluence for half-maximal induction, and B, the slope in the probit diagram. Laser light was used between 620 and 800 nanometers to analyze the VLFR. Phytochrome is responsible for both photoresponses: the VLFR action spectrum demonstrates an exponential decrease in apparent photoconversion cross-section (P(r) --> P(fr)) up to about 800 nanometers. Assuming that P(r):P(fr)-X and P(fr):P(fr)-X are the effectors for the VLFR and the LFR, respectively, we estimate an average induction threshold of about 0.003% P(r):P(fr) for the VLFR and about 1% P(fr):P(fr) for the LFR among individuals of the seed population.

Biphasic fluence-response curves for phytochrome-mediated kalanchoë seed germination : sensitization by gibberellic Acid.

The fluence-response curves for the effect of two red pulses separated by 24 hours on the germination of Kalanchoe blossfeldiana Poelln. cv Vesuv seeds, incubated on gibberellic acid (GA(3)) are biphasic for suboptimal concentrations. The response in the low fluence range corresponds with a classical red/far-red reversible phytochrome mediated reaction. GA(3) induces an additional response in the very low fluence range, which is also phytochrome mediated. The sensitivity to phytochrome-far-red absorbing form (Pfr), however, is increased about 20,000-fold, so that even far-red fluences become saturating. Both in the very low and low fluence response range, the maximal responses induced by saturating fluences are modulated by the GA(3) concentration. GA(3) having no direct influence on the phytochrome phototransformations, alters the Pfr requirement and determines the responding seed population fraction in the very low and low fluence range. The effet of GA(3) appears to be on the transduction chain of the phytochrome signal.