植物中的光敏色素

光敏色素是植物体内的光受体。本介绍了光敏色素的结构、特征及由光敏色素引发的昼夜节律生物钟,着重介绍了在信号转导和昼夜节律系统中的光敏色素作用因子。     

Impaired phytochrome-mediated shade-avoidance responses in the aurea mutant of tomato

Internode extension-growth responses to neighbouring plants and to red to far-red ratios (R:FR) were investigated in wild-type (WT) and aurea (au)-mutant seedlings of tomato grown under natural radiation. The genomic location of the au mutant is not known, but one of its consequences is the reduced phytochrome level. In WT seedlings, internode growth was promoted by the presence of non-shading neighbours reflecting far-red light (FR), the shade of a tall canopy, FR provided as a supplement during the photoperiod, and FR pulses either provided at the end of the day or delayed into the dark period. Supplementary FR during the photoperiod also promoted growth in herbicide-treated partially bleached WT seedlings. The au mutant showed higher background extension-growth rates, but only responded to the most severe treatments: deep shade light and very low R:FR at the end of the day, i.e. au-mutant seedlings were less sensitive than WT seedlings to R:FR signals. Wild-type seedlings were transferred from the glasshouse to a growth room and exposed to white light with two levels of phytochrome-absorbable radiation but similar phytochrome photoequilibria and radiation for photosynthesis. The plants exposed to the lowest level showed a transient increase of internode extension growth rate and a simultaneous reduction of response to FR pulses, i.e. reproduced some of the features of au-mutant seedlings. Phytochrome itself could set the degree of response to Pfr during neighbour detection.     

The photoreceptors in the high irradiance response of plants

Several studies show that the high irradiance response (HIR) of plants is probably due to two photoreceptors. One of the photoreceptors is phytochrome, and the other is an unidentified pigment provisionally named heliochrome. One of the functions of heliochrome is the synthesis of phytochrome, using far-red and blue radiations of high intensities, to replace the phytochrome destroyed by light. Another possible function could be an interaction of heliochrome with a substance produced by phytochrome. The data presented show that heliochrome is a pigment with different properties from phytochrome. It shows a far-red/green reversibility. Heliochrome has been shown to participate with phytochrome in such HIRs as leaf movement in Albizzia and flowering in a long-day plant. The first event initiated by phytochrome and by heliochrome could be the generation of a strong positive, electrostatic charge in the cell membrane.     

Temperature perception and signal transduction in plants

Plants can show remarkable responses to small changes in temperature, yet one of the great unknowns in plant science is how that temperature signal is perceived. The identity of the early components of the temperature signal transduction pathway also remains a mystery. To understand the consequences of anthropogenic environmental change we will have to learn much more about the basic biology of how plants sense temperature. Recent advances show that many known plant-temperature responses share common signalling components, and suggest ways in which these might be linked to form a plant temperature signalling network.     

Cotton plant size and fiber developmental responses to FR/R ratio reflected from the soil surface

Cotton ( Gossypium hirsutum L.) plants were grown in irrigated field plots over red, green or white soil covers (mulches). The far-red (FR) to red (R) light ratios were higher in upwardly reflected light over the red and green surfaces than in incoming sunlight. Plants that grew over the white mulches received higher photosynhetic photon flux (PPF), but the reflected FR/R ratio did not differ significantly from that in incoming sunlight. At five weeks after emergence, seedling stern lengths were significantly longer over red and green than over white surfaces. At maturity, plants that had grown over the red and green surfaces had longer stems, larger shoots, more bolls (fruit), more seed cotton, and longer fibers than plants grown over the white surfaces even though those grown over the white surfaces had received more reflected photosynthetic light during growth and development.     

Timing of Photoperiodic Flowering:Light Perception and Circadian Clock

Flowering symbolizes the transition of a plant from vegetative phase to reproductive phase and is controlled by fairly complex and highly coordinated regulatory pathways. Over the last decade, genetic studies in Arabidopsis have aided the discovery of many signaling components involved in these pathways. In this review, we discuss how the timing of flowering is regulated by photoperiod and the involvement of light perception and the circadian clock in this process. The specific regulatory mechanisms on CONSTANS expression and CONSTANS stability by the circadian clock and photoreceptors are described in detail. In addition, the roles of CONSTANS, FLOWERING LOCUS T, and several other light signaling and circadiandependent components in photoperiodic flowering are also highlighted.     

The function, action and adaptive significance of phytochrome in light-grown plants

Abstract. It has previously been proposed that the fundamental function of phytochrome in the natural environment is the perception of the relative proportions of red and far-red light, i.e. the red: far-red ratio. This paper re-evaluates this hypothesis, for vegetative green plants, in the light of recent findings. Essentially, three issues are considered: (a) the modulation of the response to red: far-red by fluence rate: (b) the anticipation of competition for light by perception of changes in red: far-red that precede actual shading: and (c) characteristics of phytochrome that may be important in the mechanism of photoperception (i.e. the accumulation of photoconversion intermediates, and the stability of Pfr). We conclude: (a) the red: far-red ratio provides a reliable signal of plant density, even before shading by neighbours occurs: (b) plants are able to perceive and respond to these signals, and that possible ambiguities due to low red: far-red at low solar angles may be avoided by modulation of the perception process by fluence-rate dependent mechanisms; (c) although direct experimental evidence does not yet exist, circumstantial evidence suggests that the perception of red: far-red may confer positive adaptive advantage; and (d) plants of certain species perceive and respond to fluence rate changes, mediated perhaps by a blue-light absorbing photoreceptor or by phytochrome, but that these responses do not necessarily lead to shade avoidance reactions and their ecological relevance is not fully understood.     

Flowering responses to light-breaks in photomorphogenic mutants of Arabidopsis thaliana, a long-day plant

Flowering response and plant form of photomorphogenic mutants (hy1, hy2, hy3, hy4 and hy5) of Arabidopsis thaliana (L.), a long-day plant, were examined in long and short days. There were only slight differences among genotypes including Landsberg wild type with respect to the flowering time under long days. The effect of 1 h light-(night)-breaks of far-red, red, blue and white light given in the middle of the dark period of plants grown under short days, was studied. Effects of far-red light applied at the end or the beginning of the main photoperiod on flowering and plant form were also examined. The light-breaks with all the above mentioned light qualities promoted floral initiation of all the genotypes including the wild type in terms of both the flowering time and the number of rosette leaves. In general, far-red light was most effective. It is possible to classify the hy-mutants into 3 groups by their responses to light-breaks under short day conditions: (a) Mutants hy2 and hy3, which have a reduced number of rosette leaves, and flower early. Red light is as effective as far-red light. The wavelength of light-breaks is relatively unimportant for flowering response. (b) Mutants hy4, hy5 and Landsberg wild type, which have a greater number of rosette leaves, and flower relatively late. The effectiveness of light-breaks is in the following order, far-red, blue, and red light, which is in reverse order to the transformation of phytochrome to the P_fr form. (c) Mutant hy1, which behaves anomalously with respect to relations between flowering time and number of rosette leaves; late flowering with reduced number of rosette leaves. Red, blue and far-red light are effective, but white light is ineffective for reducing the number of rosette leaves. When far-red light was given in the middle of the night or at the end of the main photoperiod, it markedly reduced the number of rosette leaves compared to those grown under short days for all the genotypes, while when applied at the beginning of the main photoperiod far-red light did not affect the number of rosette leaves. Different effects on the plant form dependent on the time of treatment with far-red light-breaks are also discussed.     

Phytochrome is required for the occurrence of time-dependent phototropism in maize coleoptiles

Time-dependent phototropism (TDP), sometimes called second positive curvature, occurs when the duration of phototropic stimulation with blue light (B) exceeds a few minutes. TDP was characterized in maize (Zea mays L.) coleoptiles raised under continuous red light (R). Subsequently, coleoptiles adapted to darkness were used to investigate the effect of R on TDP. It was found that TDP, which is induced in R-grown coleoptiles, does not occur in dark-adapted coleoptiles and that dark-adapted coleoptiles begin to show TDP after treatment with R. The TDP responsiveness became maximal 1-2 h after treatment with a R pulse and decreased during the next few hours. At least 10 min was required after a short pulse of R before the coleoptile began to respond to B for the induction of TDP. The effect of R in establishing the TDP responsiveness was totally suppressed by a pulse of far-red light given immediately after an inductive pulse of R. It is concluded that the mechanism of TDP requires for its establishment a R signal perceived by phytochrome. The TDP of R-grown and R-pretreated coleoptiles showed relationships to stimulation times and fluence rates that are similar to those reported for oat coleoptiles, except that TDP of maize showed a sharp increase in its magnitude within a narrow range of stimulation times as short as 5-10 min.     

Role of phytochrome in photoperiodic regulation of bulbing and growth in the long day plant Allium cepa

Plant elongation in Allium cepa L. cv. Dorata di Parma was stimulated by end-of-day far-red radiation, while the same treatment was ineffective with respect to bulbing response. It was concluded that the P_fr-dependent reactions which control bulbing are completed during the long daily light period (18 h). Day breaks of 3 h fluorescent white light, in the middle of the inductive photoperiod were inhibitory to bulbing. Repeated brief far-red irradiations could substitute for continuous far-red irradiations lasting 3 h in the middle of the photoperiod. Red light alone or applied immediately after each far-red irradiation inhibits bulb formation.     

Mapping-by-Sequencing Identifies HvPHYTOCHROME C as a Candidate Gene for the early maturity 5 Locus Modulating the Circadian Clock and Photoperiodic Flowering in Barley

Phytochromes play an important role in light signaling and photoperiodic control of flowering time in plants. Here we propose that the red/far-red light photoreceptor HvPHYTOCHROME C (HvPHYC), carrying a mutation in a conserved region of the GAF domain, is a candidate underlying the early maturity 5 locus in barley (Hordeum vulgare L.). We fine mapped the gene using a mapping-by-sequencing approach applied on the whole-exome capture data from bulked early flowering segregants derived from a backcross of the Bowman(eam5) introgression line. We demonstrate that eam5 disrupts circadian expression of clock genes. Moreover, it interacts with the major photoperiod response gene Ppd-H1 to accelerate flowering under noninductive short days. Our results suggest that HvPHYC participates in transmission of light signals to the circadian clock and thus modulates light-dependent processes such as photoperiodic regulation of flowering.     

Morphological variation in Plantago lanceolata L.: effects of light quality and growth regulators on sun and shade populations

The effects of different ratios of red to far-red light (R/FR-ratio) and of exogenously applied growth regulators on the morphology of plants from sun and shade populations were studied. Large differences in growth form were found between populations adapted to either sun or shaded habitats. Low R/FR-ratios, simulating vegetation shade, induced a growth form similar to that of plants from the shade population. High R/FR-ratios, simulating sunlight, had the opposite effect. Most morphological differences between shade and sun populations and effects of low R/FR-ratios on growth form could be mimicked by exogenously applied gibberellin (GA₃). In contrast, application of a gibberellin inhibitor (CCC) induced a growth form similar to that of the sun population and of plants grown under a high R/FR-ratio. Interactions between genetic background, the R/FR-ratio, and hormone treatment, were small and the factors exerted their action independently. The results are discussed in relation to the influence of developmental constraints on the evolvability of optimal phenotypes and the plastic responses therein.     

A eukaryotic LOV-histidine kinase with circadian clock function in the picoalga Ostreococcus

The marine environment has unique properties of light transmission, with an attenuation of long wavelengths within the first meters of the water column. Marine organisms have therefore evolved specific blue‐light receptors such as aureochromes to absorb shorter‐wavelength light. Here, we identify and characterize a light, oxygen, or voltage sensing (LOV) containing histidine kinase (LOV‐HK) that functions as a new class of eukaryotic blue‐light receptor in the pico‐phytoplanktonic cell Ostreococcus tauri. This LOV‐HK is related to the large family of LOV‐HKs found in prokaryotes. Phylogenetic analysis indicates that the LOV domains from LOV‐HKs, including O. tauri LOV‐HK, and phototropins (phot; plant and green algal LOV serine/threonine kinases) have different evolutionary histories. Photochemical analysis shows that the LOV domain of LOV‐HK binds a flavin cofactor and absorbs blue light with a fast photocycle compared with its prokaryotic counterparts. Ostreococcus tauri LOV‐HK expression is induced by blue light and is under circadian control. Further, both overexpression and downregulation of LOV‐HK result in arrhythmia of the circadian reporter CCA1:Luc under constant blue light. In contrast, photochemical inactivation of O. tauri LOV‐HK is without effect, demonstrating its importance for function of the circadian clock under blue light. Overexpression/downregulation of O. tauriLOV‐HK alters CCA1 rhythmicity under constant red light, irrespective of LOV‐HK’s photochemical reactivity, suggesting that O. tauri LOV‐HK also participates in regulation of the circadian clock independent of its blue‐light‐sensing property. Molecular characterization of O. tauri LOV‐HK demonstrates that this type of photoreceptor family is not limited to prokaryotes.