Circadian changes in activity of the far-red form of phytochrome: Physiological and in vivo spectrophotometric studies

Circadian dysfunction causes aberrant hypocotyl elongation patterns in Arabidopsis

Many endogenous and environmental signals control seedling growth, including several phototransduction pathways. We demonstrate that the circadian clock controls the elongation of the Arabidopsis hypocotyl immediately upon germination. The pattern of hypocotyl elongation in constant light includes a daily growth arrest spanning subjective dawn and an interval of rapid growth at subjective dusk. Maximal hypocotyl growth coincides with the phase during which the cotyledons are raised, in the previously described rhythm of cotyledon movement. The rhythm of hypocotyl elongation was entrained by light-dark cycles applied to the imbibed seed and its period was shortened in the toc1-1 mutant, indicating that it is controlled by a similar circadian system to other rhythmic markers. The daily groth arrest is abolished by the early flowering 3 (elf3) mutation, suggesting that this defect may cause its long-hypocotyl phenotype. Mutations that affect the circadian system can therefore cause gross morphological phenotypes, not because the wild-type gene functions pleiotropically in several signalling pathways, but rather because the circadian clock exerts wide-spread control over plant physiology.

Circadian oscillations of cytosolic and chloroplastic free calcium in plants

Tobacco and Arabidopsis plants, expressing a transgene for the calcium-sensitive luminescent protein apoaequorin, revealed circadian oscillations in free cytosolic calcium that can be phase-shifted by light-dark signals. When apoaequorin was targeted to the chloroplast, circadian chloroplast calcium rhythms were likewise observed after transfer of the seedlings to constant darkness. Circadian oscillations in free calcium concentrations can be expected to control many calcium-dependent enzymes and processes accounting for circadian outputs. Regulation of calcium flux is therefore fundamental to the organization of circadian systems.

The regulation of circadian period by phototransduction pathways in Arabidopsis

Transgenic Arabidopsis plants expressing a luciferase gene fused to a circadian-regulated promoter exhibited robust rhythms in bioluminescence. The cyclic luminescence has a 24.7-hour period in white light but 30- to 36-hour periods under constant darkness. Either red or blue light shortened the period of the wild type to 25 hours. A phytochrome-deficient mutation lengthened the period in continuous red light but had little effect in continuous blue light, whereas seedlings carrying mutations that activate light-dependent pathways in darkness maintained shorter periods in constant darkness. These results suggest that both phytochrome- and blue light-responsive photoreceptor pathways control the period of the circadian clock.

A partial characterization of the long-wavelength "Activated" far-red absorbing form of phytochrome

The spectral properties of Pfr in extracts prepared from red-irradiated oat tissue are similar to those for Pfr as measured in vivo, with a spectral peak at about 732 nm. The presence or absence of Mg(2+) or EDTA in the extraction medium is without effect on the spectral properties of the Pfr extracted from red-irradiated tissue. In contrast, the spectral properties of Pfr formed in vitro in extracts of non-irradiated oat tissue cleared of particulate matter are markedly different, the absorption peak being at about 725 nm. However, the red irradiation of the crude extract prior to its being cleared of particulate matter, results in the formation of a Pfr-species with spectral properties similar to Pfr as seen in vivo, suggesting the requirement for some particulate fraction for the formation of the long-wavelength Pfr form. The long-wavelength "activated" form of Pfr once extracted and partially purified is highly stable both at 0° and 25°C. Photoconversion of this long-wavelength form of Pfr to Pr initially results in the formation of a Pr species with a peak at 662 nm. In contrast, the peak location of Pr as isolated from non-irradiated tissue is at 667 nm.The 662 nm form of Pr is unstable and with time is transformed in a temperature-dependent reaction to a form with a peak at 667 nm. At 25°C this reaction is rapid, occurring in minutes, while at 0°-4°C it requires a few hours. At 25°C, immediate photoconversion of the 662 nm form of Pr back to Pfr results in the reformation of the long-wavelength form of Pfr. However, after being incubated for 15 min in the Pr form the peak shifts to 667 nm and the Pfr subsequently formed exhibits spectral characteristics identical to those of Pfr as formed in vitro, i.e., with a peak at 725 nm.