Photoreceptors allow living organisms to optimize perception of light in the natural environment and thus to gain information about their external world. Analyses performed in different organisms have revealed wonderful examples of structural modifications of the light‐sensing proteins themselves, as well as diversification of the signal pathways they use in relation with their evolutionary history and function. In different organisms, the same photoreceptor may have a very conserved role or may modulate different responses. Two different photoreceptors may be involved in the control of the same physiological response. Light signals are amongst the most important environmental cues regulating plant growth and development which is achieved through a suite of photoreceptor proteins. These photoreceptors can detect the presence, intensity, direction and color of light, and in turn utilize this information to direct their growth. In plants, many types of photoreceptors have been identified including phytochrome (phy), and cryptochrome (cry) and phototropin (phot) are known as major red/far-red and blue light receptors, respectively. Photoreceptors have photoreceptive domains binding chromophores to absorb light signals: cGMP-specific phosphodiesterases, adenylyl cyclases and FhlA (GAF) of phy, photolyase homologous region (PHR) of cry, and LOV (light, oxygen, or voltage) of phot bind tetrapyrrole and flavins as their chromophores, respectively. Photochemical reactions on the chromophore lead to conformational changes of the domain and regulate output domains. In Arabidopsis, two cryptochrome and two phototropin family members have been identified and characterized. Photoreceptor action regulates development throughout the lifecycle of plants, from seed germination through to architecture of the mature plant and the onset of reproduction.
Blue Light Sensing
Phototropins are the photoreceptors which are activated specifically by UV/blue wavelengths of light. The photoactivation of phototropins stimulates a range of processes that ultimately optimize the photosynthetic efficiency of plants, including phototropism. For instance, phototropins direct the movement of chloroplasts, which represent the heart of the photosynthetic machinery as their position within the cell can greatly affect the efficiency of energy production. Leaf positioning and expansion is also directed by the phototropins. Additionally, phototropins control the opening of stomata pores in the leaf epidermis, which regulate gaseous exchange. Stomatal opening is important for energy production, as it allows CO2 uptake for photosynthesis. Collectively, these responses serve to enhance the photosynthetic performance of plants and maximize their growth potential. Many plant species are able to track the movement of the sun by a process known as heliotropism. This photomovement response is also likely mediated by phototropins.
Arabidopsis contains two phototropins referred to as phot1 and phot2. Mutants of Arabidopsis lacking both phototropins lose their phototropic responsiveness. Phot1 and phot2 act to induce chloroplast accumulation movement to the upper cell surface under weak light conditions to maximize light capture for photosynthesis. By contrast, phot2 alone induces chloroplast avoidance movement to the cell sidewalls in bright light to increase mutual shading and prevent photodamage of the photosynthetic machinery under excess light.
The structure of plant phototropins can be separated into two parts: a N-terminal photosensory input region coupled to a C-terminal effector or output region that contains a classic serine/threonine kinase motif. The N-terminal region comprises two so-called LOV domains, each of which bind the vitamin-B derived cofactor flavin mononucleotide (FMN) as a blue light-absorbing chromophore. LOV domains exhibit protein sequence homology to motifs found in a diverse range of eukaryotic and prokaryotic proteins involved in sensing Light, Oxygen, or Voltage, hence the acronym LOV.The current view of phototropin receptor activation is that LOV2 functions as a repressor of the C-terminal kinase domain in the dark, and that this mode of repression is alleviated upon photoexcitation, resulting in receptor autophosphorylation throughout the protein. The photoexcitation of LOV2 leads to the displacement of an ( -helix from the surface of the domain. Unfolding of this ( -helix, designated J , results in the activation of the C-terminal kinase domain. Protein rearrangements within the central ( -sheet scaffold have been reported to play a role in propagating the photochemical signal generated within LOV2 domain to bring about protein changes at the surface, which are necessary for the activation of the C-terminal kinase domain and autophosphorylation of specific serine/threonine residues. Light-activated phototropin can return to its non-phosphorylated state upon incubation in darkness. This recovery process involves dephosphorylation of the receptor by an as yet unidentified protein phosphatase.In darkness, phototropins are typically associated with the plasma membrane, but a small fraction of the receptor pool is rapidly internalized (within minutes) upon blue light irradiation. One consequence of autophosphorylation may therefore be to promote receptor dissociation from the plasma membrane, and the desensitizing of the photosensory system analogous to other receptor kinase-based signaling systems.
Red Light sensing
Plants have developed a series of photoreceptors that allow them to sense light from the UV-B to the near far-red. The red to near farred region of the light spectrum is particularly rich in environmental information that is most important to plants. For example, changes in the seasons and the time of day and the shading from other plants are all indicated by changes in the ratio of red to far-red light. Red light also penetrates the ground further than light of shorter wavelengths and thereby gives a seedling an early indication that it is approaching the soil’s surface.
The red-light sensing system comprises a complex and intriguing signaling network in contrast to the linear amplification cascade of the mammalian rhodopsin-based light sensing systems. The photoreceptors that allow plants to monitor the red-to-far-red band of the spectrum are known as phytochromes. Phytochromes were the first plant photoreceptors to be identified and are found all across the plant kingdom. In most plants phytochromes exist as a Phytochromes can exist in two stable states. One of them is the red light absorbing form (Pr) with an absorption maximum at around small multi-gene family. Arabidopsis thaliana, a plant popular with geneticists, has five distinct phytochromes (phyA–phyE), which are differentially expressed in different plant tissues and during different stages of development. Plant phytochromes exist as dimers of a ~125 kDa polypeptide chain. Each monomer can be divided into different functional regions. The 60 kDa aminoterminal domain houses a covalently linked linear tetrapyrrole chromophore, while the carboxy-terminal region is responsible for the transduction of the light signal. This signal transduction region can itself be separated into two sub regions. The 30 kDa region immediately adjacent to the chromophore binding region contains two PAS domains, while the very carboxy-terminal domain has sequence similarity to two-component histidine kinases.
Phytochromes mediate responses during the entire life span of a plant, and respond to light intensities over a dynamic range of more than nine orders of magnitude. The best-studied phytochrome mediated responses are stimulated by light doses between 1 µmol m–2 (equivalent to a 0.1 second exposure of light under a dense plant canopy, or under a few millimeters of soil) and 1,000 µmol m–2 (one second of broad daylight). These responses are called low fluence responses. For most responses Pfr is believed to be the biologically active form. In plants, phytochrome is synthesized in the Pr form and will return to Pr from Pfr in a spontaneous yet very slow (hours to days) process. Exposing the Pr form to red light will lead to the conversion of Pr to Pfr. The reverse applies as well, exposure to far red light will convert the Pfr form back into the Pr form. This effect is known as reversibility and is considered a telltale sign for phytochrome-mediated LFRs. Phytochrome-mediated responses that are triggered by the dimmest light are called very low fluence responses (VLFR) and occur at photon doses as low as 0.1 nmol m (comparable to the light emitted by a flash from a firefly). At such low light levels, only approximately 0.01% of the total phytochrome is light activated. High irradiance responses (HIR) occur at the other end of the intensity spectrum. These responses, such as the induction of coloring in fruit skins or the inhibition of stem growth, need hours of direct sunlight to reach saturation. Under these high irradiance conditions, all phytochrome molecules in a cell would be expected to undergo continuous light-driven cycling between their two stable states.
In some plants species, including the fern Adiantum capillus-veneris, phototropism and chloroplast movement is induced by red light as well as blue. Adiantum contains a novel dual red/blue light-sensing photoreceptor known as neochrome, comprising of a red light-absorbing phytochrome photosensory domain fused to the N-terminus of an entire phototropin receptor. The presence of such a hybrid photoreceptor is proposed to enhance light sensitivity and aid the prevalence of species such as ferns in low light conditions typically found under the canopy of dense forests. Similarly, the photoactivation of red light-absorbing phytochromes is known to enhance blue light-induced phototropism in Arabidopsis. However, this phototropic enhancement results from an interaction between separate phytochrome and phototropin receptor systems that likely involves Phytochrome Kinase Substrate (PKS) proteins.Crosstalk between red and blue light sensing photoreceptors occurs at all stages of plant development. Although the exact nature of co-action has yet to be elucidated, it is accepted that blue light-mediated de-etiolation involves the interaction of both phytochrome and cryptochrome signaling. Comparisons of mutants deficient in multiple combinations of phyA, phyB and cry1 revealed numerous genetic interactions between these photoreceptors during seedling development. Physical interactions have been demonstrated between CRY1 and PHYA proteins in vitro and between cry2 and phyB photoreceptors in vivo.
Mutants deficient in cry2 displayed a longer period length in CAB2 :LUCIFERASE gene expression in white light, a phenotype not observed in red light and severely attenuated in blue light. Such observations suggest cry2 function to depend on the activation of phytochromes. This notion was supported by observations that supplementation of white light with FR wavelengths could abolish the late flowering phenotype of cry2 mutants. It is, however, possible that removal of phytochrome Pfr over-rides the cry2 regulation of flowering in a separate pathway mediated by light quality. In addition, fluorescence resonance energy transfer microscopy revealed phyB and cry2 to form light-dependent nuclear speckles. The translocation of phyB to the nucleus in red light is well established and suggests that the cellular compartmentalization of photoreceptors may be important in mediating light-induced physiological responses. Phytochromes C and D have also been reported to show functional interactions with cryptochromes. A role for phyC in blue light sensing was proposed following observations that phyC mutants displayed long hypocotyls in low fluence rates of blue light. Under these conditions, cry2 function has been shown to predominate in the regulation of hypocotyls extension. The hyposensitivity of phyC mutants to low fluence rates of blue light therefore suggests a possible functional interaction between phyC and cry2. A functional interaction between phyD and cryptochrome was reported following observations that the red light mediated inhibition of hypocotyl elongation following a white light pretreatment required the presence of either phyD or cry1.
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