Phytochrome modulation of blue-light-induced phototropism

Red light enhances hypocotyl phototropism toward unilateral blue light through a phytochrome‐mediated response. This study demonstrates how the phytochromes modulate blue‐light‐induced phototropism in the absence of a red light pre‐treatment. It was found that phytochromes A, B, and D have conditionally overlapping functions in the promotion of blue‐light‐induced phototropism. Under very low blue light intensities (0.01 µmol m^?2 s^?1) phyA activity is necessary for the progression of a normal phototropic response, whereas above 1.0 µmol m^?1 s^?2 phyB and phyD have functional redundancy with phyA to promote phototropism. PhyA also contributes to attenuation of phototropism under high fluence rates of unilateral blue light, which was previously shown to be dependent on the phototropins and cryptochromes. From these results, it appears that phytochromes are required to develop a robust phototropic response under low fluence rates, whereas under high irradiances where phototropism may be less important, phyA suppresses phototropism.     

Hypocotyl growth orientation in blue light is determined by phytochrome A inhibition of gravitropism and phototropin promotion of phototropism

How developing seedlings integrate gravitropic and phototropic stimuli to determine their direction of growth is poorly understood. In this study we tested whether blue light influences hypocotyl gravitropism in Arabidopsis. Phototropin1 (phot1) triggers phototropism under low fluence rates of blue light but, at least in the dark, has no effect on gravitropism. By analyzing the growth orientation of phototropism-deficient seedlings in response to gravitropic and phototropic stimulations we show that blue light not only triggers phototropism but also represses hypocotyl gravitropism. At low fluence rates of blue light phot1 mutants were agravitropic. In contrast, phyAphot1 double mutants grew exclusively according to gravity demonstrating that phytochrome A (phyA) is necessary to inhibit gravitropism. Analyses of phot1cry1cry2 triple mutants indicate that cryptochromes play a minor role in this response. Thus the optimal growth orientation of hypocotyls is determined by the action of phyA-suppressing gravitropism and the phototropin-triggering phototropism. It has long been known that phytochromes promote phototropism but the mechanism involved is still unknown. Our data show that by inhibiting gravitropism phyA acts as a positive regulator of phototropism.     

Cryptochrome‐mediated hypocotyl phototropism was regulated antagonistically by gibberellic acid and sucrose in Arabidopsis

Both phototropins(phot1 and phot2) and cryptochromes(cry1 and cry2) were proven as the Arabidopsis thaliana blue light receptors. Phototropins predominately function in photomovement, and cryptochromes play a role in photomorphogenesis. Although cryptochromes have been proposed to serve as positive modulators of phototropic responses, the underlying mechanism remains unknown. Here, we report that depleting sucrose from the medium or adding gibberellic acids(GAs) can partially restore the defects in phototropic curvature of the phot1 phot2 double mutants under high-intensity blue light; this restoration does not occur in phot1 phot2 cry1 cry2 quadruple mutants and nph3(nonphototropic hypocotyl 3) mutants which were impaired phototropic response in sucrose-containing medium. These results indicate that GAs and sucrose antagonistically regulate hypocotyl phototropism in a cryptochromes dependent manner, but it showed a crosstalk with phototropin signaling on NPH3.Furthermore, cryptochromes activation by blue light inhibit GAs synthesis, thus stabilizing DELLAs to block hypocotyl growth, which result in the higher GAs content in the shade side than the lit side of hypocotyl to support the asymmetric growth of hypocotyl. Through modulation of the abundance of DELLAs by sucrose depletion or added GAs, it revealed that cryptochromes have a function in mediating phototropic curvature.     

Plant blue-light receptors

Plants have several blue-light receptors, which regulate different aspects of growth and development. Recent studies have identified three such receptors: cryptochrome 1, cryptochrome 2 and phototropin. Cryptochromes 1 and 2 are photolyase-like receptors that regulate hypocotyl growth and flowering time; phototropin mediates phototropism in response to blue light. In addition, phytochrome A has also been found to mediate various blue-light responses. Although the signal-transduction mechanisms of blue-light receptors remain largely unclear, phototropin is probably a protein kinase that regulates cytoplasmic calcium concentrations, whereas the cryptochromes might regulate anion-channel activity and changes in gene expression.     

Reduced phototropism in pks mutants may be due to altered auxin‐regulated gene expression or reduced lateral auxin transport

Phototropism allows plants to orient their photosynthetic organs towards the light. In Arabidopsis, phototropins 1 and 2 sense directional blue light such that phot1 triggers phototropism in response to low fluence rates, while both phot1 and phot2 mediate this response under higher light conditions. Phototropism results from asymmetric growth in the hypocotyl elongation zone that depends on an auxin gradient across the embryonic stem. How phototropin activation leads to this growth response is still poorly understood. Members of the phytochrome kinase substrate (PKS) family may act early in this pathway, because PKS1, PKS2 and PKS4 are needed for a normal phototropic response and they associate with phot1 in vivo. Here we show that PKS proteins are needed both for phot1‐ and phot2‐mediated phototropism. The phototropic response is conditioned by the developmental asymmetry of dicotyledonous seedlings, such that there is a faster growth reorientation when cotyledons face away from the light compared with seedlings whose cotyledons face the light. The molecular basis for this developmental effect on phototropism is unknown; here we show that PKS proteins play a role at the interface between development and phototropism. Moreover, we present evidence for a role of PKS genes in hypocotyl gravi‐reorientation that is independent of photoreceptors. pks mutants have normal levels of auxin and normal polar auxin transport, however they show altered expression patterns of auxin marker genes. This situation suggests that PKS proteins are involved in auxin signaling and/or lateral auxin redistribution.     

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。     

Genetic separation of phototropism and blue light inhibition of stem elongation

Blue light-induced regulation of cell elongation is a component of the signal response pathway for both phototropic curvature and inhibition of stem elongation in higher plants. To determine if blue light regulates cell elongation in these responses through shared or discrete pathways, phototropism and hypocotyl elongation were investigated in several blue light response mutants in Arabidopsis thaliana. Specifically, the blu mutants that lack blue light-dependent inhibition of hypocotyl elongation were found to exhibit a normal phototropic response. In contrast, a phototropic null mutant (JK218) and a mutant that has a 20- to 30-fold shift in the fluence dependence for first positive phototropism (JK224) showed normal inhibition of hypocotyl elongation in blue light. F1 progeny of crosses between the blu mutants and JK218 showed normal phototropism and inhibition of hypocotyl elongation, and approximately 1 in 16 F2 progeny were double mutants lacking both responses. Thus, blue light-dependent inhibition of hypocotyl elongation and phototropism operate through at least some genetically distinct components.     

Photobiology: Light signal transduction and photomorphogenesis

正Light is crucial for plants, not only because of photosynthesis, but also because of photomorphogenesis. As one of the most important environmental cues, light influences multiple responses in plants,including seed germination, seedling de-etiolation,shade avoidance, phototropism, stomata and chloroplast movement, circadian rhythms, and flowering     

Seeing the world in red and blue: insight into plant vision and photoreceptors.

Plants see light through multiple photoreceptors, including phytochromes and cryptochromes. Cryptochromes are flavoproteins that participate in many blue-light responses, including phototropism in plants and entrainment of circadian rhythms in plants and animals. A novel flavoprotein, NPH1, is also implicated in plant phototropism. Phytochromes function as serine/threonine kinases whose potential interacting partners include cryptochrome (CRY1 and CRY2).     

Arabidopsis ROOT PHOTOTROPISM2 Contributes to the Adaptation to High-Intensity Light in Phototropic Responses

Living organisms adapt to changing light environments via mechanisms that enhance photosensitivity under darkness and attenuate photosensitivity under bright light conditions. In hypocotyl phototropism, phototropin1 (phot1) blue light photoreceptors mediate both the pulse light-induced, first positive phototropism and the continuous light-induced, second positive phototropism, suggesting the existence of a mechanism that alters their photosensitivity. Here, we show that light induction of ROOT PHOTOTROPISM2 (RPT2) underlies photosensory adaptation in hypocotyl phototropism of Arabidopsis thaliana. rpt2 loss-of-function mutants exhibited increased photosensitivity to very low fluence blue light but were insensitive to low fluence blue light. Expression of RPT2 prior to phototropic stimulation in etiolated seedlings reduced photosensitivity during first positive phototropism and accelerated second positive phototropism. Our microscopy and biochemical analyses indicated that blue light irradiation causes dephosphorylation of NONPHOTOTROPIC HYPOCOTYL3 (NPH3) proteins and mediates their release from the plasma membrane. These phenomena correlate closely with the desensitization of phot1 signaling during the transition period from first positive phototropism to second positive phototropism. RPT2 modulated the phosphorylation of NPH3 and promoted reconstruction of the phot1-NPH3 complex on the plasma membrane. We conclude that photosensitivity is increased in the absence of RPT2 and that this results in the desensitization of phot1. Light-mediated induction of RPT2 then reduces the photosensitivity of phot1, which is required for second positive phototropism under bright light conditions.     

Phototropin 1 and cryptochrome action in response to green light in combination with other wavelengths

Genetic studies have shown the effects of various photoreceptors on early photomorphogenic processes, defining the precise time course of red (RL), far-red (FrL) and blue light (BL) action. In this study, the effect of green wavebands in conjunction with these responses is examined. Longer-term (end point; 24–96 h) analysis of hypocotyl elongation in enriched green environments shows an increase in growth compared to seedlings under blue, red or both together. The effect was only observed at lower fluence rates (<10 μmol/m² s). Genetic analyses demonstrate that cryptochromes are required for this GL effect, consistent with earlier findings, and that the phy receptors have no influence. However, analysis of early (minutes to hours) stem growth kinetics indicates that GL cannot reverse the cryptochrome-mediated BL effect during early stem growth inhibition, and instead acts additively with BL to drive cryptochrome-mediated inhibition. Green light (GL) treatments antagonize RL and FrL-mediated hypocotyl inhibition. The GL opposition of RL responses persists in phyA, phyB, cry1cry2 and phot2 mutants. The response requires phot1 and NPH3, suggesting that this is not a GL response, but instead a response to extremely low-fluence rate BL. Tests with dim BL (<0.1 μmol/m² s) confirm a previously uncharacterized phot1-dependent promotion of stem growth, opposing the effects of RL. These findings demonstrate how enriched green environments may adjust RL and BL photomorphogenic responses through both the crys and phot1 receptors, and define a new role for phot1 in stem growth promotion.     

Evidence for two photoreceptors controlling growth in de-etiolated seedlings

The effect of blue light on hypocotyl extension in de-etiolated seedlings of lettuce, cucumber and tomato was investigated under conditions which precluded the involvement of phytochrome. Small but highly inhibitory amounts of blue light were added to a high intensity background illumination from low pressure sodium lamps. A log-linear response for inhibition of hypocotyl extension against the blue light fluence rate was obtained for lettuce and cucumber, and inhibition in tomato was also related to the blue light fluence rate. The added blue light did not alter phytochrome photostationary state and its effect was independent of the total fluence rate. Growth inhibition by Pfr could be demonstrated in tomato and cucumber but not in lettuce. The results indicate that two photoreceptors may normally be involved in the control of seedling growth but their relative importance varies greatly between species.Abbreviations HIR high irradiance reaction - Pfr far red absorbing form of phytochrome - Pr red absorbing form of phytochrome     

Magnetic intensity affects cryptochrome-dependent responses in Arabidopsis thaliana

Cryptochromes are blue-light absorbing photoreceptors found in many organisms where they have been involved in numerous growth, developmental, and circadian responses. In Arabidopsis thaliana, two cryptochromes, CRY1 and CRY2, mediate several blue-light-dependent responses including hypocotyl growth inhibition. Our study shows that an increase in the intensity of the ambient magnetic field from 33–44 to 500 μT enhanced growth inhibition in A. thaliana under blue light, when cryptochromes are the mediating photoreceptor, but not under red light when the mediating receptors are phytochromes, or in total darkness. Hypocotyl growth of Arabidopsis mutants lacking cryptochromes was unaffected by the increase in magnetic intensity. Additional cryptochrome-dependent responses, such as blue-light-dependent anthocyanin accumulation and blue-light-dependent degradation of CRY2 protein, were also enhanced at the higher magnetic intensity. These findings show that higher plants are sensitive to the magnetic field in responses that are linked to cryptochrome-dependent signaling pathways. Because cryptochromes form radical pairs after photoexcitation, our results can best be explained by the radical-pair model. Recent evidence indicates that the magnetic compass of birds involves a radical pair mechanism, and cryptochrome is a likely candidate for the avian magnetoreception molecule. Our findings thus suggest intriguing parallels in magnetoreception of animals and plants that appear to be based on common physical properties of photoexcited cryptochromes.     

Reception of far-ultraviolet light in Phycomyces: antagonistic interaction with blue and red light

Phototropism experiments were done with sporangiophores of the fungus Phycomyces blakesleeanus to characterize the interaction between far-UV, blue and red light. Far-UV light elicits negative phototropism (bending away from the light source) while blue light elicits positive phototropism (bending toward the light source). In contrast, red light above 600 nm is phototropically inert. Phototropism was analyzed with light regimens of bilateral or unilateral irradiation with far-UV and blue light. Under bilateral irradiation, in which the two light sources were facing each other, blue light partially inhibited the far-UV-elicited phototropism. A fluence-response curve for this inhibition showed that blue light was maximally effective at fluence rates which exceeded 3 to 57 times that of the far-UV. Tonic red light, which was given from above, abolished to a large extent the antagonistic action of blue light. With a regimen of unilateral irradiation, i.e. when far-UV and blue light were given from the same side, a phototropic balance could be achieved with approximately equal fluence rates of blue and UV light. Above or below this critical balance point the bending was either negative or positive. In this setup the effect of tonic red light was complex. First, it caused an enhancement of the positive or negative bending, and second, it caused at some fluence rates a sign reversal from positive to negative phototropism. The balance point itself was only marginally affected. The data cannot be explained on the basis of a single photoreceptor and support the previous notion of separate far-UV and blue-light receptors. The antagonism between these two receptors probably occurs on the level of a red-light-absorbing receptor intermediate. Received: 16 November 1997 / Accepted: 18 December 1997     

Mapping of low- and high-fluence autophosphorylation sites in phototropin 1

Phototropins, originally detected by their blue light-dependent autophosphorylation, are plant photoreceptors involved in several blue light responses such as phototropism, chloroplast relocation, leaf expansion, rapid inhibition of hypocotyl growth, and stomatal opening. Three domains have been identified in phototropin sequences, two chromophore binding domains (LOV1 and LOV2) and a kinase domain. We describe here two additional domains, the N-terminus upstream of LOV1 and the hinge region between LOV1 and LOV2, as the regions for autophosphorylation; the phosphorylation sites were identified by site-directed mutagenesis as S27, S30, S274, S300, S317, S325, S332, and S349 of the PHOT1a sequence of Avena sativa. Investigation of the autophosphorylation in vivo revealed that serines close to the LOV1 domain are phosphorylated at lower fluence of blue light than the serines close to the LOV2 domain. Recovery of phosphorylation in vivo during a dark period after saturating irradiation is caused by dephosphorylation rather than by degradation of the phosphorylated form and new synthesis of nonphosphorylated phototropin. The results were obtained by a combination of autophosphorylation of phototropin with phosphorylation of recombinant domains by protein kinase A, which turned out to have the same site specificity as the phototropin kinase, followed by proteolysis and separation of phosphopeptides. With the knowledge of the phosphorylation sites, the physiological and biochemical consequences of autophosphorylation can now be approached by site-directed mutagenesis of phototropins.