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.     

拟南芥PKS2与CAM4蛋白互作研究

植物蓝光受体向光素(phototropin,PHOT)介导许多生理反应,现已从拟南芥中分离了其下游的一些信号转导组分。前期研究表明,拟南芥光敏色素底物PKS家族成员PKS1与部分Ca2+结合蛋白钙调素(calmodulin,CAM)成员互作,参与PHOT2介导的强蓝光诱导下胚轴向光反应。旨在探讨PKS2和CAM4之间的互作关系,首先用RT-PCR技术得到PKS2和CAM4的c DNA全长序列。通过酵母双杂交和双分子荧光互补技术,从体外与体内证实PKS2和CAM4能相互作用。此结果进一步丰富了PKS家族与CAM之间的联系,为深入解析PHOT功能研究奠定基础。     

Light receptor action is critical for maintaining plant biomass at warm ambient temperatures

The ability to withstand environmental temperature variation is essential for plant survival. Former studies in Arabidopsis revealed that light signalling pathways had a potentially unique role in shielding plant growth and development from seasonal and daily fluctuations in temperature. In this paper we describe the molecular circuitry through which the light receptors cry1 and phyB buffer the impact of warm ambient temperatures. We show that the light signalling component HFR1 acts to minimise the potentially devastating effects of elevated temperature on plant physiology. Light is known to stabilise levels of HFR1 protein by suppressing proteasome-mediated destruction of HFR1. We demonstrate that light-dependent accumulation and activity of HFR1 are highly temperature dependent. The increased potency of HFR1 at warmer temperatures provides an important restraint on PIF4 that drives elongation growth. We show that warm ambient temperatures promote the accumulation of phosphorylated PIF4. However, repression of PIF4 activity by phyB and cry1 (via HFR1) is critical for controlling growth and maintaining physiology as temperatures rise. Loss of this light-mediated restraint has severe consequences for adult plants which have greatly reduced biomass.     

Functional ecology of a blue light photoreceptor: effects of phototropin-1 on root growth enhance drought tolerance in Arabidopsis thaliana

* The blue light photoreceptor phototropin-1 has been shown to enhance fitness in Arabidosis thaliana under field conditions. Here, we ask whether performance consequences of phototropin-1 reflect its impact on root growth and drought tolerance. * We used a PHOT1-GFP gene construct to test whether phototropin-1 abundance in roots is highest at shallow soil depths where light penetration is greatest. We then compared root growth efficiency and size at maturity between individuals with and without functional phototropin-1. Comparisons were made under wet and dry conditions to assess the impact of phototropin-1 on drought tolerance. * Phototropin-1 was most abundant in upper root regions and its impact on root growth efficiency decreased with soil depth. Roots of plants with functional phototropin-1 made fewer random turns and traveled further for a given length (higher efficiency) than roots of phot1 mutants. In dry (but not wet) soil, enhancement of root growth efficiency by phototropin-1 increased plant size at maturity. * Results indicate that phototropin-1 enhances performance under drought by mediating plastic increases in root growth efficiency near the soil surface.     

Mutational analysis of blue-light sensing in Arabidopsis

Blue light regulates many physiological and developmental processes in higher plants through the action of multiple photosensory systems. The analysis of photomorphogenic mutants is leading to a better understanding of how the different photosensory systems mediate the wide range of responses observed in blue light. A review of the current literature on photomorphogenic mutants makes it apparent that redundancies exist in photoreceptor function. For example, many blue-light responses that have been shown to be regulated by a blue-light photosensory system are also under phytochrome control. The study of various light-response mutants suggests that a complex sensory network regulates light-mediated responses. This article attempts to piece together information regarding the sensory systems responsible for blue-light-regulated responses.     

新疆北部拟南芥自然居群表型变异与协变

拟南芥(Arabidopsis thaliana)自然居群的表型特征代表其在自然环境下的适应状况, 不同居群间特征的对比可以为了解拟南芥表型变化规律, 进而分析其形成过程和机制提供重要线索。本研究以分布于新疆北部天山、塔尔巴哈台山和阿尔泰山的10个种群的9个表型性状为基础, 对比分析了小尺度、局域尺度和区域尺度环境下原生境拟南芥种群表型性状的变化。结果发现, 不同性状对环境变化的反应不同, 其中株高、株重、根重、根长、单个果实重、果实开裂力度在3种环境尺度下种群间的差异均达到极显著水平, 而分枝数、果实长度的种群间变化不显著, 种群间的表型分化系数较低。不同环境尺度下株重、根重、单株果数均表现出一致的协变格局, 反映了生理功能性状之间整合对拟南芥适应环境的重要性。同时, 各种群间整体的性状协变差异性明显, 根长、单个果实重、分枝数、果实长度、果实开裂力度等特征与其他特征协变具有明显的局部性, 局域尺度和区域尺度环境之间的变化较大。聚类分析发现区域尺度上的不同种群聚合在一起的现象非常突出, 进一步表明拟南芥的表型特征受微环境的强烈影响。Mantel检验表明, 小尺度上10个种群株高、株重、根重、单个果实重、果实长度、果实开裂力度6个性状变化存在显著的空间相关性, 而分枝数、根长的相关性却不显著。因此, 我们认为拟南芥表型变化受小尺度环境的影响强烈, 但在表型层面并非所有性状都与原生境气候存在遗传关联。     

What makes the Arabidopsis clock tick on time? A review on entrainment

Entrainment, the synchronization of a circadian clock with the external environment, is a crucial step in daily life. Although many signals contribute to entrainment, light and temperature are typically the strongest resetting cues. Much progress has been made concerning light resetting in the model plant Arabidopsis thaliana. Multiple photoreceptors (phytochromes, cryptochromes, LOV-domain proteins) are involved in light perception. The clock genes CCA1, LHY and TOC1 are all probable targets of light signalling, although the details of these pathways are not completely established. Temperature can entrain the clock, but little is known about the mechanism underlying this resetting; no obvious clock gene candidate for temperature resetting has been identified. Although circadian research has emphasized oscillations in free-running conditions, in the real world the circadian clock is entrained. During entrainment, short or long period mutants exhibit a 24-h period, but a mutant phenotype is often manifested as an altered phase relationship with the entraining cycle; short and long period mutants show leading and lagging phases, respectively, and this may be detrimental under some conditions. Arrhythmic CCA1-overexpressing plants display increased lethality under very short photoperiods, consistent with the circadian clock being of adaptive significance to life on a rotating world.     

Interactions within a network of phytochrome, cryptochrome and UV-B phototransduction pathways regulate chalcone synthase gene expression in Arabidopsis leaf tissue

The Arabidopsis gene encoding the key flavonoid biosynthesis enzyme chalcone synthase (CHS) is regulated by several environmental and endogenous stimuli. Here we dissect the network of light signalling pathways that control CHS expression in mature leaves using cryptochrome (cry) and phytochrome (phy) deficient mutants. The UV-A/blue light induction of CHS is mediated principally by cry1, but neither cry1 nor cry2 is involved in UV-B induction or in the UV-A and blue light signalling pathways that interact synergistically with the UV-B pathway to enhance CHS expression. Moreover, these synergistic responses do not require phyA or phyB. Phytochrome is a positive regulator of the cry1 inductive pathway, mediating distinct potentiation and coaction effects. A red light pretreatment enhances subsequent cry1-mediated CHS induction. This potentiation is unaltered in phyA and phyB mutants but much reduced in a phyA phyB double mutant, indicating that it requires principally phyA or phyB. In contrast, the cry1-mediated induction of CHS, without pretreatment, is much reduced in phyB but not phyA mutants, indicating coaction between cry1 and phyB. Further experiments with phy-deficient mutants demonstrate that phyB is a negative regulator of the UV-B inductive pathway. We further show that phyB acts upstream of the points of interaction of the UV-A and blue synergism pathways with the UV-B pathway. We propose that phyB functions to balance flux through the cry1 and UV-B signalling pathways.     

Phototropin LOV domains exhibit distinct roles in regulating photoreceptor function

Phototropins (phot1 and phot2) are autophosphorylating serine/threonine kinases that function as photoreceptors for phototropism, light-induced chloroplast movement, and stomatal opening in Arabidopsis. The N-terminal region of phot1 and phot2 contains two specialized PAS domains, designated LOV1 and LOV2, which function as binding sites for the chromophore flavin mononucleotide (FMN). Both LOV1 and LOV2 undergo a self-contained photocycle, which involves the formation of a covalent adduct between the FMN chromophore and a conserved active-site cysteine residue (Cys39). Replacement of Cys39 with alanine abolishes the light-induced photochemical reaction of LOV1 and LOV2. Here we have used the Cys39Ala mutation to investigate the role of LOV1 and LOV2 in regulating phototropin function. Photochemical analysis of a bacterially expressed LOV1 + LOV2 fusion protein indicates that LOV2 functions as the predominant light-sensing domain for phot1. LOV2 also plays a major role in mediating light-dependent autophosphorylation of full-length phot1 expressed in insect cells and transgenic Arabidopsis. Moreover, photochemically active LOV2 alone in full-length phot1 is sufficient to elicit hypocotyl phototropism in transgenic Arabidopsis, whereas photochemically active LOV1 alone is not. Further photochemical and biochemical analyses also indicate that the LOV1 and LOV2 domains of phot2 exhibit distinct roles. The significance for the different roles of the phototropin LOV domains is discussed.     

苔藓植物蓝光/紫外光-A 信号传导的研究

种子植物含有5个已分离的光受体和至少1个未鉴定的蓝光/紫外光-A受体。隐花色素(CRY1、CRY2和CRY3) 调节植物的生长发育,而向光蛋白(PHOT1和PHOT2) 调节植物对光的营养反应。黄素可以吸收蓝光和紫外光-A,是生色团。对这些光受体的结构和作用模式已了解很多。苔藓植物小立碗藓中含有2个已分离的隐花色素(CRY1a和CRY1b),负责调节侧枝形成和生长素代谢;有4个向光蛋白(PHOTA1,PHOTA2,PHOTB1,PHOTB2) 调节叶绿体的运动。苔藓细胞内蓝光/紫外光-A刺激引发的信号转导有Ca2+参与。     

The Effect of Epibrassinolide on Arabidopsis Seedling Morphogenesis and Hormonal Balance under Green Light

We studied the effect of 24-epibrassinolide (EB) on the levels of endogenous hormones and photomorphogenesis of Arabidopsis thaliana (L.) Heynh wild-type (Ler) and mutant (hy4) seedlings. This mutant is deficient in the cryptochrome 1 (CRY1) synthesis. CRY1, which is a product of the HY4 gene, is a blue light photoreceptor in wild-type plants, but is sensitive to green light as well. In dark-grown seven-day-old mutant seedlings, the ABA/zeatin ratio differed from this ratio in wild-type seedlings. Thehy4 mutant exhibited a lower zeatin and higher free-ABA contents, which could retard its hypocotyl growth in darkness. EB retarded the growth of hypocotyls in etiolated hy4 seedlings and enlarged their cotyledons more efficiently than in wild-type seedlings. Green light (GL) did not affect the growth of hypocotyls but enlarged cotyledons of hy4 seedlings, which might be associated with some increase in the level of free IAA and a considerable decrease in free ABA and also with a decrease in the cytokinin level in seedlings. The hy4 cotyledon response to GL depended evidently on photoreceptors other than CRY1. GL enhanced the effects of EB on the morphogenesis of both Ler and hy4 seedlings, which was coupled with changes in the balance of endogenous IAA, ABA, and cytokinins. We may suppose that EB is involved in the control of photomorphogenesis by interaction with endogenous hormones, which are involved in the transduction of a light signal absorbed by the GL photoreceptors.     

Both phytochrome A and phytochrome B are required for the normal expression of phototropism in Arabidopsis thaliana seedlings

The role of phytochrome A (phyA) and phytochrome B (phyB) in phototropism was investigated by using the phytochrome-deficient mutants phyA-101 , phyB-1 and a phyA/phyB double mutant. The red-light-induced enhancement of phototropism, which is normally observed in wild-type seedlings, could not be detected in the phyA/phyB mutant at fluences of red light between 0.1 and 19 000 μmol m⁻². The loss of phyB has been shown to have no apparent effect on enhancement, while the loss of phyA resulted in a loss of enhancement only in the low fluence range (Janoudi et al. 1997). The conclusions of the aforementioned study can now be modified based on the current results which indicate that phototropic enhancement in the high fluence range is mediated by either phyA or phyB, and that other phytochromes have no role in enhancement. First positive phototropism was unaffected in phyA-101 and phyB-1 However, the magnitude of first positive phototropism in the phyA/phyB mutant was significantly lower than that of the wild-type Landsberg parent. Thus, the presence of either phyA or phyB is required for normal expression of first positive phototropism. The time threshold for second positive phototropism is unaltered in the phyA-101 and phyB mutants. However, the time threshold in the phyA/phyB mutant is about 2 h, approximately six times that of the wild type. Finally, the magnitude of second positive phototropism in both phyA-101 and phyB-1 is diminished in comparison with the wild-type response. Thus, phyA and phyB, acting independently or in combination, regulate the magnitude of phototropic curvature and the time threshold for second positive phototropism. We conclude that the presence of phyA and phyB is required, but not sufficient, for the expression of normal phototropism.     

Alterations in light-induced stomatal opening in a starch-deficient mutant of Arabidopsis thaliana L. deficient in chloroplast phosphoglucomutase activity

Stomatal responses to light of Arabidopsis thaliana wild-type plants and mutant plants deficient in starch (phosphoglucomutase deficient) were compared in gas exchange experiments. Stomatal density, size and ultrastructure were identical for the two phenotypes, but no starch was observed in guard cells of the mutant plants whatever the time of day. The overall extent of changes in stomatal conductance during 14 h light–10 h dark cycles was similar for the two phenotypes. However, the slow endogenous stomatal opening occurring in darkness in the wild type was not observed in the mutant plants. Stomata in the mutant plants responded much more slowly to blue light (70 μmol m^?2 s^?1) though the response to red light (250 μmol m^?2 s^?1) was similar to that of wild-type plants. In paradermal sections, stomatal responses to red light (300 μmol m^?2 s^?1) were weak for wild-type plants as well as for mutant plants. Stomatal opening was greater under low blue light (75 μmol m^?2 s^?1) than under red light for the two genotypes. However, in mutant plants, a high chloride concentration (50 mol m^?3) was necessary to achieve the same stomatal aperture as observed for the wild-type plants. These results suggest that starch metabolism, via the synthesis of a counter-ion to potassium (probably malate), is required for full stomatal response to blue light but is not involved in the stomatal response to red light.     

Phytochrome regulation of pea phototropin

Type 1 phototropin, one of the blue light receptors responsible for phototropism, is encoded in peas by at least two genes, PsPHOT1A and PsPHOT1B (formerly PsPK4 and PsPK5), both of which are more similar to Arabidopsis PHOT1 than to Arabidopsis PHOT2. We show here that PsPHOT1B encodes a full-length phototropin, whose expression pattern suggests that Psphot1b is the predominant phot1-type phototropin in etiolated seedlings. The gene encoding the other type 1 phototropin, PsPHOT1A, is expressed at low levels, with its highest levels in the leaves and stems of more mature, light-grown plants. Studies with phyA, phyB and the phyAphyB double mutants show that phyA and phyB have partially redundant roles in the reduction of PsPHOT1B expression under red light.     

Unexpected roles for cryptochrome 2 and phototropin revealed by high-resolution analysis of blue light-mediated hypocotyl growth inhibition

Blue light (BL) rapidly and strongly inhibits hypocotyl elongation during the photomorphogenic response known as de-etiolation, the transformation of a dark-grown seedling into a pigmented, photoautotrophic organism. In Arabidopsis thaliana, high-resolution studies of hypocotyl growth accomplished by computer-assisted electronic image capture and analysis revealed that inhibition occurs in two genetically independent phases, the first beginning within 30 sec of illumination. The present work demonstrates that phototropin (nph1), the photoreceptor responsible for phototropism, is largely responsible for the initial, rapid inhibition. Signaling from phototropin during the curvature response is dependent upon interaction with NPH3, but the results presented here demonstrate that NPH3 is not necessary for phototropin-dependent growth inhibition. Activation of anion channels, which transiently depolarizes the plasma membrane within seconds of BL, is an early event in the cryptochrome signaling pathway leading to a phase of growth inhibition that replaces the transient phototropin-dependent phase after approximately 30 min of BL. Surprisingly, cry1 and cry2 were found to contribute equally and non-redundantly to anion-channel activation and to growth inhibition between 30 and 120 min of BL. Inspection of the inhibition kinetics displayed by nph1 and nph1cry1 mutants revealed that the cryptochrome phase of inhibition is delayed in seedlings lacking phototropin. This result indicates that BL-activation of phototropin influences cryptochrome signaling leading to growth inhibition. Mutations in the NPQ1 gene, which inhibit BL-induced stomatal opening, do not affect any aspect of the growth inhibition within the first 120 min examined here, and NPQ1 does not affect the activation of anion channels.