Low-fluence Blue Light Research Articles

Biphasic Effects of Blue Light Irradiation on Human Umbilical Vein Endothelial Cells.

Blue light regulates biological function in various cells, such as proliferation, oxidative stress, and cell death. We employed blue light illumination on human umbilical vein endothelial cells utilizing a LED device at 453 nm wavelength and revealed a novel biphasic response on human umbilical vein endothelial cells (HUVECs). The results showed that low fluence blue light irradiation promoted the fundamental cell activities, including cell viability, migration and angiogenesis by activating the angiogenic pathways such as the VEGF signaling pathway. In contrast, high fluence illumination caused the opposite effect on those activities by upregulating pro-apoptotic signaling cascades like ferroptosis, necroptosis and the p53 signaling pathways. Our results provide an underlying insight into photobiomodulation by blue light and may help to implement potential treatment strategies for treating angiogenesis-dependent diseases.

Low-fluence blue light-induced phosphorylation of Zmphot1 mediates the first positive phototropism.

Phototropin1 (phot1) perceives low- to high-fluence blue light stimuli and mediates both the first and second positive phototropisms. High-fluence blue light is known to induce autophosphorylation of phot1, leading to the second positive phototropism. However, the phosphorylation status of phot1 by low-fluence blue light that induces the first positive phototropism had not been observed. Here, we conducted a phosphoproteomic analysis of maize coleoptiles to investigate the fluence-dependent phosphorylation status of Zmphot1. High-fluence blue light induced phosphorylation of Zmphot1 at several sites. Notably, low-fluence blue light significantly increased the phosphorylation level of Ser291 in Zmphot1. Furthermore, Ser291-phosphorylated and Ser369Ser376-diphosphorylated peptides were found to be more abundant in the low-fluence blue light-irradiated sides than in the shaded sides of coleoptiles. The roles of these phosphorylation events in phototropism were explored by heterologous expression of ZmPHOT1 in the Arabidopsis thaliana phot1phot2 mutant. The first positive phototropism was restored in wild-type ZmPHOT1-expressing plants; however, plants expressing S291A-ZmPHOT1 or S369AS376A-ZmPHOT1 showed significantly reduced complementation rates. All transgenic plants tested in this study exhibited a normal second positive phototropism. These findings provide the first indication that low-fluence blue light induces phosphorylation of Zmphot1 and that this induced phosphorylation is crucial for the first positive phototropism.

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.

Blue-light regulation of ZmPHOT1 and ZmPHOT2 gene expression and the possible involvement of Zmphot1 in phototropism in maize coleoptiles.

ZmPHOT1 and ZmPHOT2 are expressed differentially in maize coleoptiles and leaves, with Zmphot1 possibly involved in first-positive phototropic curvature of red-light-adapted maize coleoptiles exposed to pulsed low-fluence blue light. Unilateral blue-light perception by phototropin(s) is the first event of phototropism, with the subsequent signal causing lateral transport of auxin at the coleoptile tip region of monocots. In this study, we analyzed the behavior of two maize phototropin genes: ZmPHOT1 and ZmPHOT2, the latter identified from the maize genome database and newly characterized. Quantitative real-time PCR analysis demonstrated that ZmPHOT1 was abundantly expressed in etiolated coleoptiles, while lower expressions of both ZmPHOT1 and ZmPHOT2 were observed in young leaves. Interestingly, these genes were not specifically expressed in the coleoptile tip region, a key position for photoperception in phototropism. Exposure to pulsed low-fluence blue light (LBL) (0.33 µmol m(-2) s(-1) × 8 s) and continuous high-fluence blue light (HBL) (10 µmol m(-2) s(-1)) rapidly decreased ZmPHOT1 gene expression in coleoptiles, with levels of ZmPHOT2 not significantly altered in that tissue. In young leaves, no drastic expression changes were induced in either ZmPHOT1 or ZmPHOT2 by LBL or HBL irradiation. The Zmphot1 protein was investigated by Western blot analysis with anti-Osphot1 antibodies. Zmphot1 was detected in microsomal fractions, with higher levels in coleoptiles than in leaves. HBL caused rapid phosphorylation of the protein, whereas no phot1 phosphorylation was induced by LBL. The involvement of Zmphot1 in LBL-induced phototropic curvature of maize coleoptiles is discussed.

Light quality modulates metabolic synchronization over the diel phases of crassulacean acid metabolism.

Temporal compartmentation of carboxylation processes is a defining feature of crassulacean acid metabolism and involves circadian control of key metabolic and transport steps that regulate the supply and demand for carbon over a 24h cycle. Recent insights on the molecular workings of the circadian clock and its connection with environmental inputs raise new questions on the importance of light quality and, by analogy, certain photoreceptors for synchronizing the metabolic components of CAM. The present work tested the hypothesis that optimal coupling of stomatal conductance, net CO2 uptake, and the reciprocal turnover of carbohydrates and organic acids over the diel CAM cycle requires both blue and red light input signals. Contrasting monochromatic wavelengths of blue, green, and red light (i.e. 475, 530, 630nm) with low fluence rates (10 μmol m(-2) s(-1)) were administered for 16 hours each diel cycle for a total treatment time of 48 hours to the obligate CAM bromeliad, Aechmea 'Maya'. Of the light treatments imposed, low-fluence blue light was a key determinant in regulating stomatal responses, organic acid mobilization from the vacuole, and daytime decarboxylation. However, the reciprocal relationship between starch and organic acid turnover that is typical for CAM was uncoupled under low-fluence blue light. Under low-fluence red or green light, the diel turnover of storage carbohydrates was orchestrated in line with the requirements of CAM, but a consistent delay in acid consumption at dawn compared with plants under white or low-fluence blue light was noted. Consistent with the acknowledged influences of both red and blue light as input signals for the circadian clock, the data stress the importance of both red and blue-light signalling pathways for synchronizing the metabolic and physiological components of CAM over the day/night cycle.

Blue Light-Induced Phototropism of Inflorescence Stems and Petioles is Mediated by Phototropin Family Members phot1 and phot2

Phototropin family photoreceptors, phot1 and phot2, in Arabidopsis thaliana control the blue light (BL)-mediated phototropic responses of the hypocotyl, chloroplast relocation movement and stomatal opening. Phototropic responses in dark-grown tissues have been well studied but those in de-etiolated green plants are not well understood. Here, we analyzed phototropic responses of inflorescence stems and petioles of wild-type and phototropin mutant plants of A. thaliana. Similar to the results obtained from dark-grown seedlings, inflorescence stems and petioles in wild-type and phot2 mutant plants showed phototropic bending towards low fluence BL, while in phot1 mutant plants, a high fluence rate of BL was required. phot1 phot2 double mutant plants did not show any phototropic responses even under very high fluence rates of BL. We further studied the photoreceptive sites for phototropic responses of stems and petioles by partial tissue irradiation. The whole part of the inflorescence stem is sensitive to BL and shows phototropism, but in the petiole only the irradiated abaxial side is sensitive. Similar to dark-grown etiolated seedlings, phot1 plays a major role in phototropic responses under weak light, but phot2 functions under high fluence rate conditions in green plants.

High pigment1 mutation negatively regulates phototropic signal transduction in tomato seedlings.

Phototropins and phytochromes are the major photosensory receptors in plants and they regulate distinct photomorphogenic responses. The molecular mechanisms underlying functional interactions of phototropins and phytochromes remain largely unclear. We show that the tomato (Lycopersicon esculentum) phytochrome A deficient mutant fri lacks phototropic curvature to low fluence blue light, indicating requirement for phytochrome A for expression of phototropic response. The hp1 mutant that exhibits hypersensitive responses to blue light and red light reverses the impairment of second-positive phototropic response in tomato in phytochrome A-deficient background. Physiological analyses indicate that HP1 functions as a negative regulator of phototropic signal transduction pathway, which is removed via action of phytochrome A. The loss of HP1 gene product in frihp1 double mutant allows the unhindered operation of phototropic signal transduction chain, obviating the need for the phytochrome action. Our results also indicate that the role of phytochrome in regulating phototropism is restricted to low fluence blue light only, and at high fluence blue light, the phytochrome A-deficient fri mutant shows the normal phototropic response.

Purification and characterization of a probable light receptor with kinase activity from beet root plasma membranes.

A 120-kDa glycoprotein was found in beet root (Beta vulgaris L.) plasma membranes. This protein could be phosphorylated in a Ca2+-independent manner. Its carbohydrate moiety was composed of both O-linked galactose-beta(1-3)-N-acetylgalactosamine disaccharides (which bind peanut agglutinin) and N-linked concanavalin A (ConA)-binding oligosaccharides. The phosphorylation of this protein was rapid, half-saturated with 6 microM ATP and higher at alkaline pH values. This protein was phosphorylated more efficiently with Mn-ATP as substrate than with Mg-ATP. This phosphorylation increased when plasma membranes were illuminated with low-fluence blue light, a fact suggesting that the 120-kDa glycoprotein could be similar to phototropin: a blue-light photoreceptor involved in phototropism. This protein was purified using a ConA-Sepharose column. The phosphorylation of the purified protein could be observed, but it was much lower than that of the 120-kDa protein in plasma membranes. In addition, it was not enhanced by light. Some possible explanations for this photosensitivity loss are discussed.

Phytochromes and cryptochromes in the entrainment of the Arabidopsis circadian clock.

Circadian clocks are synchronized by environmental cues such as light. Photoreceptor-deficient Arabidopsis thaliana mutants were used to measure the effect of light fluence rate on circadian period in plants. Phytochrome B is the primary high-intensity red light photoreceptor for circadian control, and phytochrome A acts under low-intensity red light. Cryptochrome 1 and phytochrome A both act to transmit low-fluence blue light to the clock. Cryptochrome 1 mediates high-intensity blue light signals for period length control. The presence of cryptochromes in both plants and animals suggests that circadian input pathways have been conserved throughout evolution.

Light-responsive gene expression in cyanobacteria

Light-responsive gene expression in cyanobacteria

Blue and red light reversibly control psbA expression in the cyanobacterium Synechococcus sp. strain PCC 7942.

The three psbA genes encoding the photosystem II D1 protein in Synechococcus sp. strain PCC 7942 respond differentially to an increase in intensity of white light through transcriptional induction of psbAII and psbAIII and accelerated degradation of psbAI and psbAIII messages. We report that the genes exhibit a novel photoreversible response involving blue and red light that is almost indistinguishable from the high-white light response. Transfer of cells from white to low-fluence blue light caused a decrease in the level of the psbAI message and increased levels of psbAII and psbAIII messages, whereas transfer to red or far-red light had little effect. Five min of blue light was sufficient to trigger psbAII and psbAIII induction; five min of subsequent red irradiation attenuated this response, whereas subsequent green or far-red light (or darkness) had no effect. Response to both high and blue light was insensitive to inhibitors of photosynthetic electron transport. We propose that Synechococcus modulates photosystem II biosynthesis in a variable light environment through a photoreception signal pathway, which is independent of photosystem II activity and which is distinct from red/green-reversible control of chromatically adapting cyanobacteria and the red/far-red-reversible phytochrome of plants.

Photomorphogenic mutants of Arabidopsis thaliana reveal activities of multiple photosensory systems during light-stimulated apical-hook opening

Fluence rate-response curves were generated for red-, far-red-, and blue-light-stimulated apical-hook opening in seedlings of several photomorphogenic mutants of Arabidopsis thaliana (L.) Heynh. Compared to wild-type plants, hook opening was reduced in the phytochrome-deficient hy1, hy2, and hy6 mutants in red and far-red light at all fluence rates tested, and in low-fluence blue light, but was normal under high-irradiance blue light. In contrast, the blue-light-response mutants (blu1, blu2, and blu3) lacked the high-irradiance-dependent hook-opening response in blue light while hook opening was normal in low-fluence blue light and in red and farred light at all fluence rates tested. Hook opening in the phytochrome-B-deficient hy3 mutant was similar to wild type in all light conditions tested. The effects of the different mutations on light-induced hook opening indicate that a phytochrome(s) other than phytochrome B mediates hook opening stimulated by red, far-red and lowfluence blue light, while a blue-light-absorbing photoreceptor mediates the blue-light-sensitive high-irradiance response. Although the phytochrome and blue-light photosensory systems appear to work independently for the most part, some of their signal-transduction components may interact since the hy4, and hy5 mutants showed reduced hook-opening responses under conditions dependent on the phytochrome and blue-light-photosensory systems.

Light-Stimulated Apical Hook Opening in Wild-Type Arabidopsis thaliana Seedlings.

Apical hook opening and cotyledon unfolding are characteristic responses that occur during deetiolation of dicotyledonous seedlings. Light-stimulated apical hook opening and cotyledon unfolding in etiolated Arabidopsis thaliana seedlings appears to involve the activities of multiple photosensory systems. Red, far-red, and blue light are all effective in stimulating these responses in Arabidopsis. Stimulation of hook opening by red light and low fluence blue light is inductive, far-red reversible, and exhibits reciprocity, as is characteristic of many low fluence-dependent phytochrome-mediated responses. Far-red and high-fluence blue light appear to stimulate hook opening and cotyledon unfolding through high-irradiance-response systems during long-term light treatments. Although a phytochrome high-irradiance-response system presumably mediates the responses in far-red light, the responses to high-fluence blue light may be mediated by a blue light-specific photosensory system.

Changes in synthesis and abundance of specific polypeptides at early and late stages of blue-light-induced sporulation of Trichoderma

The temporal relationship between photoinduced morphogenesis and the translation and abundance of specific polypeptides was studied in Trichoderma harzianum. Sporulation of this common soil fungus is induced by a brief pulse of low fluence blue light. The minute light signal is amplified to cause a local switch from vegetative mycelia to the production of aerial, branched conidiophores. Patterns of polypeptide synthesis and abundance were analysed to detect changes both before and after visible structural changes occur. The early phases of morphogenesis (up to 4–8 h after light) occur without masive changes in morphology, or in the pattern of polypeptide abundance. Two-dimensional electrophoresis of polypeptides pulse labelled in vivo demonstrated that a major in vivo translation product at 65 kD and near neutral pH disappeared within 2 h after light and another labelled spot at slightly more acidic pH and 90 kD increased during the first 4 h after light. Major differences in polypeptide abundance were detected only when mature conidiophores were compared with vegetative mycelia. Four major soluble polypeptides were specific to vegetative mycelia, while two major polypeptides were specific to membrane fractions from conidiophores. A polyclonal antibody was obtained against the most abundant of these membrane proteins, a 48 kD polypeptide. The antibody recognized a 48 kD polypeptide on immunoblots of membrane fractions from sporulating colonies at 24 h after photoinduction, but not from dark control colonies of the same age. On immunoblots of total protein, the 48 kD polypeptide was detectable at 12–16 h after photoinduction, together with the completion of conidiophore branching, and increased further by 24 h, when pigmented spores are present.