Light is made up of electromagnetic particles that travel in waves. These waves emit energy, and range in length and strength. The length of the waves is measured in nanometers (nm). Every wavelength is represented by a different color, and is grouped into the following categories: gamma rays, x-rays, ultraviolet (UV) rays, visible light, infrared light, and radio waves. Together these wavelengths make up the electromagnetic spectrum.
However, the human eye is sensitive to visible light only. Visible light is that part of the electromagnetic spectrum that is seen as colors: violet, indigo, blue, green, yellow, orange and red. Blue light has a very short wavelength, and so produces a higher amount of energy.
Blue light affects many aspects of plant growth and development. Plant blue-light responses include inhibition of hypocotyl elongation, stimulation of cotyledon expansion, regulation of flowering time, phototropic curvature, stomatal opening, entrainment of the circadian clock and regulation of gene expression. During the past decade, molecular genetic studies using Arabidopsis as a model system have identified three blue light receptors: cryptochrome 1, cryptochrome 2 and phototropin, which regulate primarily hypocotyl inhibition, flowering time and phototropism, respectively.
Cryptochromes and blue-light regulation of hypocotyl growth
Cryptochromes (CRY) are photosensory receptors that regulate growth and development in plants. Plant cryptochromes are best studied in Arabidopsis thaliana. The Arabidopsis genome encodes three cryptochrome genes, CRY1, CRY2, and CRY3. The major photoreceptor mediating blue-light inhibition of hypocotyl elongation is cry1. In nature, seeds are often buried under soil and germinate in the dark. Young seedlings of dicot plants germinated in the dark develop rapidly elongating hypocotyls to push unopened cotyledons above the soil surface. Upon exposure to light, hypocotyl elongation is inhibited and the cotyledons start to expand and to become photosynthetically competent. These developmental changes are collectively referred to as de-etiolation. Transgenic Arabidopsis plants over expressing CRY2 also showed a short hypocotyl phenotype in blue light, which led to the hypothesis that cry2 is also involved in hypocotyl inhibition. Cryptochrome 2 is a flavin-type blue light receptor mediating floral induction in response to photoperiod and a blue light-induced hypocotyl growth inhibition. cry2 is also required for the elevated expression of the flowering-time gene CO in response to long-day photoperiods. The carboxyl domain of cry2 bears a basic bipartite nuclear localization signal, and the cry2 protein was co-fractionated with the nucleus. Analysis of transgenic plants expressing a fusion protein of CRY2 and the reporter enzyme GUS (GUS±CRY2) indicated that the GUS±CRY2 fusion protein accumulated in the nucleus of transgenic plants grown in dark or light. The C-terminal domain of cry2 that contains the basic bipartite nuclear localization signal was sufficient to confer nuclear localization of the fusion protein. Phenotypic analysis of transgenic plants expressing the fusion protein GUS±CRY2 demonstrated that GUS±CRY2 acts as a functional photoreceptor in vivo, mediating the blue light-induced inhibition of hypocotyl elongation. Research on Arabidopsis has also shown that phytochrome A (phyA) and phytochrome B (phyB) function in far-red and red light, respectively. It has been found that, Arabidopsis hy4–cry1 mutants are insensitive to blue light, especially high intensity blue light; by contrast, transgenic Arabidopsis plants over expressing CRY1 have enhanced blue-light sensitivity. Hence, it was concluded that cry1 is the major blue-light receptor regulating de-etiolation. Blue-light receptors function not only in blue light but also, to varying degrees, in long-wavelength UV light. Therefore, cryptochrome was historically defined as a photoreceptor with a two-peak action spectrum, one in the blue-light region and the other in the UV-A region. Cryptochromes have also been shown to mediate blue light regulation of other aspects of de-etiolation. The Arabidopsis cry1 (hy4) mutant is defective in light-dependent anthocyanin accumulation, indicating its function in this blue light response. The cry2 mutant showed reduced cotyledon opening in low-irradiance blue light, suggesting a role of cry2 in this response. Blue-light-induced hypocotyl inhibition has two kinetic phases: a rapid phase and a slow phase. The rapid response occurs transiently within a few minutes or even seconds of a blue-light pulse, and the slow response occurs hours later and lasts much longer. The rapid growth inhibition induced by blue light is preceded by a transient plasma-membrane depolarization in hypocotyl cells of various plant species, including Arabidopsis. This membrane depolarization might result from the opening of ion channels, because blue light has also been shown to trigger a rapid activation of anion-channel opening.
Role of Cryptochromes in the Control of Flowering Time
Plant flowering time is controlled by a network of signal transduction cascades that connects various environmental signals to developmental programs. One of the most important environmental signals affecting flowering time is day length, or the photoperiod. The function of cryptochrome in the control of flowering time has been investigated by studies of Arabidopsis photoreceptor mutants under photoperiodic conditions with either white light illumination or light of specific wavelengths. The function of Arabidopsis CRY2 in flowering-time control has been studied using the CRY2 mutant. CRY2/fha mutant flowers later than the wild type in long day but not in short day, whereas transgenic plants over expressing CRY2 flowered slightly early in short day but not in long day. The flowering-promotion function of CRY2 is dependent on both blue light and red light. According to a recent study, the early day length insensitive locus, which is largely responsible for the dominant photoperiod insensitive and early-flowering traits of the Cvi accession, was identified to be the CRY2 gene. CRY2-Cvi encodes CRY2 protein with a methionine substitution for the valine at position 376 (V367M). Val376 was completely conserved among 8 different cryptochrome genes compared, except for CRY2-Cvi. Transgenic plants expressing the mutated CRY2-Ler (CRY2 gene of Ler accession) with the V367M substitution flowered similar to Cvi, whereas plants expressing the mutated CRY2-Cvi with a M367V substitution flowered just like the Ler wild type. It was confirmed that the single V367M substitution in the CRY2-Cvi protein is indeed the cause of the photoperiodic-insensitive early flowering of the Cvi ecotype.
Role of Cryptochromes in Regulating the Circadian Clock and Light-Dependent Gene Expression
The role of cryptochromes in regulating the period lengths of the circadian clock was systematically studied recently using Arabidopsis mutants impaired in either the CRY1 or the CRY2 gene. The period length of the circadian rhythm of the CAB promoter activity was analyzed under various light intensities in the photoreceptor mutant lines. The cry1 mutant had period length longer than those of the wild type in both high and low intensities of blue light, indicating the function of cry1 in the regulation of the circadian clock over a wide range of light intensities. The cry2 mutant showed a slight change in period length only in relatively low intensities of blue light, suggesting that cry2 was not the major blue light receptor setting the clock. However, the role of cry2 in the regulation of the circadian clock was clearly demonstrated when the cry1 cry2 double mutant was found to have much longer period lengths than either the cry1 or cry2 monogenic mutants in both low and high intensities of blue light. Clearly, cry1 and cry2 act redundantly in the regulation of the circadian clock. Cryptochromes also mediate light regulation of gene expression. Cry1 is well known to be the major blue light receptor regulating light induction of expression of flavonoid biosynthesis genes such as CHS (chalcone synthase) in Arabidopsis. cry1 and cry2 act redundantly in mediating blue light induction of CHS expression, which showed a more pronounced defect in the cry1 cry2 double mutant than in the cry1 or cry2 monogenic mutants. Cryptochrome regulation of CHS gene expression occurs at the transcription level: transgenic plants expressing PCHS::GUS transgene exhibited lower PCHS::GUS transgene expression in response to blue/UV-A light in the cry1 mutant than in the wild-type background.
Phototropins and Phototropism
Plants are immobile organisms. However, plant organs and organelles do move in response to various environmental stimuli, especially light. For example, hypocotyls bend toward light to maximize photosynthesis in cotyledons, whereas roots curve away from blue light to ensure that they stay in soil for water and nutrient absorption. Chloroplasts move toward relatively weak light for maximum photon capture, but move away from high-intensity light to avoid photodamage. Stomata, pores formed by two surrounding guard cells in epidermis, adjust their aperture in response to light, opening in the daytime to allow gas exchange but closing at night to minimize water loss. Phototropin was initially identified as a ∼120 kD plasma membrane protein that undergoes blue light–dependent phosphorylation in pea and other plants, including Arabidopsis. Shortly after, it was found that the light-dependent phosphorylation of this protein occurred at a much lower level in a phototropic-deficient Arabidopsis mutant, JK224, indicating its role in phototropism. Mutations in the NPH1 gene were found in different nph1 alleles, confirming its role in phototropism. Genes encoding phototropins have also been found in plant species such as rice, maize, oat, ice plant, and alga. Phot1 also mediates the phototropic response in roots. In a study of Arabidopsis rpt (root phototropism) mutants impaired in the negative curvature of roots in response to light, it was found that one of the rpt mutants, rpt1, was allelic to nph1/phot1, and both mutants were completely insensitive to both high- and low-intensity blue light with respect to root phototropism. Using a microbeam irradiation technique, it was shown in Arabidopsis that chloroplasts move toward blue light of relatively low intensity but move away from high-intensity blue light that can cause photodamage to the chloroplasts. The light sensitivity of chloroplast relocation responses is slightly lower in the phot1 mutant than in the wild type. It was also discovered that mutation of NPL1 did not affect chloroplast accumulation in low-intensity blue light. It was demonstrated that the phot1phot2 double mutant is completely insensitive to both low and high light in hypocotyl phototropism and chloroplast movement responses. It is clear now that these two phototropins mediate similar blue light responses, but they have different photosensitivities. In Arabidopsis, phot1 mediates the negative root curvature throughout a wide range of light intensities, and it can act alone to bring about the positive hypocotyl curvature response in low light. In high light, phot1 and phot2 act redundantly in mediating hypocotyl phototropism. On the other hand, phot2 is the major photoreceptor mediating chloroplast avoidance in high light, whereas phot1 and phot2 act redundantly in mediating chloroplast accumulation in low light.
Phototropins and Stomatal Opening
Stomata open in response to light, including blue and red light. Red light induces stomatal opening via photosynthesis in the mesophyll and guard cell chloroplasts and blue light as a signal induces stomatal opening. Phototropins expressed in guard cells act as major blue light receptors for stomatal opening. Blue light-induced stomatal opening is mediated through activation of a plasma membrane (PM)H+ pump, later identified as the PMH+-ATPase, in guard cells. The blue light activated pump provides driving force for stomatal opening concomitant with ion accumulation and cell volume increase in guard cells. It was found that stomatal opening in response to weak blue light as a signal requires background red light, indicating that red light has a synergistic effect on the blue light response in guard cells. When guard cells are irradiated by blue light, blue light-photoreceptor protein kinases, phototropins, are activated through autophosphorylation and initiate signaling for stomatal opening. Blue light induces autophosphorylation of two Ser residues in the kinase activation-loop of phototropin molecules, and phosphorylation is required for downstream signaling, probably through substrate recognition.
Source
Lin C. Blue light receptors and signal transduction. Plant Cell. 2002;14 Suppl(Suppl):S207-S225. doi:10.1105/tpc.000646