Phototropism: a "simple" physiological response modulated by multiple interacting photosensory-response pathways

Phototropism is the process by which plants reorient growth of various organs, most notably stems, in response to lateral differences in light quantity and/or quality. The ubiquitous nature of the phototropic response in the plant kingdom implies that it provides some adaptive evolutionary advantage. Upon visual inspection it is tempting to surmise that phototropic curvatures result from a relatively simple growth response to a directional stimulus. However, detailed photophysiological, and more recently genetic and molecular, studies have demonstrated that phototropism is in fact regulated by complex interactions among several photosensory systems. At least two receptors, phototropin and a presently unidentified receptor, appear to mediate the primary photoreception of directional blue light cues in dark-grown plants. PhyB may also function as a primary receptor to detect lateral increases in far-red light in neighbor-avoidance responses of light-grown plants. Phytochromes (phyA and phyB at a minimum) also appear to function as secondary receptors to regulate adaptation processes that ultimately modulate the magnitude of curvature induced by primary photoperception. As a result of the interactions of these multiple photosensory systems plants are able to maximize the adaptive advantage of the phototropic response in ever changing light environments.     

Phytochrome-mediated inhibition of coleoptile growth in rice: age-dependency and action spectra

Phytochrome has been shown to be the major photoreceptor involved in the photo-inhibition of coleoptile growth in Japonica-type rice (Oryza sativa L.). We have characterized this typical photomorphogenetic response of rice using mutants deficient in phytochrome A (phyA) and phytochrome B (phyB) and with respect to age-dependency and action spectra. Seedlings were irradiated with a pulse of light 40 h or 80 h after germination (i.e. at an early or late developmental stage) and the final coleoptile length of these seedlings was determined. A saturating pulse of red light (R) had a stronger effect when it was given in the late stage than in the early stage. It was found that the photoinhibition is mediated by both the phyA and the phyB in the late stage but predominantly by phyB in the early stage. Consistent with many other reported responses, the photo-inhibition in the phyA mutant, which was observed in the early and late developmental stages and is thought to be mediated mainly by phyB, occurred in the low-fluence range (10(1)-10(3) micromol m(-2)) of R and was far-red-light (FR)-reversible; the photo-inhibition in the phyB mutant, which was observed in the late developmental stage and is thought to be mediated mainly by phyA, occurred in the very-low-fluence range (10(-2)-10(0) micromol m(-2)) and was FR-irreversible. The action spectra (350-800 nm at 50 nm intervals) obtained at the two developmental stages using phyA and phyB mutants indicated that both the phyB-mediated low-fluence response and the phyA-mediated very-low-fluence response have a major peak at 650 nm and a minor peak at 400 nm.     

Light quality influences indigo precursors production and seed germination in Isatis tinctoria L. and Isatis indigotica Fort

Isatis tinctoria L. and Isatis indigotica Fort. are biennial herbaceous plants belonging to the family of Cruciferae that are used as a source of natural indigo and show several morphological and genetic differences. Production of indigo (indigotin) precursors, indican (indoxyl beta-D glucoside) and isatan B (indoxyl ketogluconate), together with seed germination ability were compared in Isatis tinctoria and Isatis indigotica grown under six different light conditions (darkness, white, red, far red, blue, yellow light) at 25 degrees C. Light quality influenced both germination and production of indigo precursors in the two Isatis species. Different responsiveness to far red and blue light was observed. Indeed, a detrimental effect on germination by blue and far red light was found in I. tinctoria only. Different amounts of isatan B were produced under red and far red light in the two Isatis species. In I. tinctoria, the level of main indigo precursor isatan B was maximal under red light and minimal under far red light. Whereas in I. indigotica far red light promoted a large accumulation of isatan B. The photon fluence rate dependency for white and yellow light responses showed that the accumulation of indigo precursors was differently influenced in the two Isatis species. In particular, both white and yellow light enhanced above 40 micromol m(-2) s(-1) the production of isatan B in I. indigotica while only white light showed a photon fluence dependency in I. tinctoria. These results suggest a different role played by the labile and stable phytochrome species (phyA and phyB) in the isatan B production in I. tinctoria and I. indigotica. I. indigotica, whose germination percentage was not influenced by light quality, demonstrated higher germination capability compared with I. tinctoria. In fact, I. tinctoria showed high frequency of germination in darkness and under light sources that establish high phytochrome photoequilibrium (red, white and yellow light). Germination in I. tinctoria was negatively affected by far red and blue light. I. indigotica seeds appear to be indifferent to canopy-like light (far red). Our results provide further insights on the distinct behaviour of I. tinctoria and I. indigotica that belong to two different genetic clusters and different original environments.     

Two modes of the light-induced phytochrome A decline--with and without changes in the proportion of its isoforms (phyA' and phyA''): evidence from fluorescence investigations of mutant phyA-3D pea

Different modes of the phytochrome function are connected with its polymorphism, the major isoforms being phytochromes A and B (phyA and phyB). In its turn, phyA comprises two native species, phyA' and phyA', whose precise nature and functions remain obscure. With the use of in situ fluorescence spectroscopy, we investigated their properties in a mutant of pea, phyA-3D, characterized by exaggerated photoresponses and impaired photodestruction of phyA. The mutation is a substitution of alanine by valine at the position 194 in phyA. The phyA-3DphyB and phyB mutants were also investigated. In dark-grown plants, all the lines had the content and properties of the two phyA species very similar to the wild type. However, a considerably more intense reduction in [phyA] without changes in the phyA'/phyA' equilibrium was found in far-red grown mutant plants suggesting a hypersensitivity of phyA-3D with regard to its autoregulation. On the contrary, under red illumination, a higher stability of phyA-3D was observed confirming our earlier findings. This allows a conclusion that the A194V substitution in phyA-3D not only impairs its destruction but also enhances its signaling ability, suggesting a role of this locus in modulation of its activity.     

Phytochrome-dependent photomovement responses mediated by phototropin family proteins in cryptogam plants

In this review, we describe the regulation of photomovement responses by phototropin and phytochrome photoreceptors. The blue light receptor phototropin mediates various photomovement responses such as phototropism, chloroplast movement and stomatal opening. In cryptogamic plants including ferns, mosses and green alga, red as well as blue light mediates phototropism and chloroplast movement. The red/far-red light reversibility suggests the involvement of phytochrome in these responses. Thereby, plant growth is presumably promoted by coordinating these photomovements to capture efficiently light for photosynthesis.     

Fluorescence and photochemical characterization of phytochromes A and B in transgenic potato expressing Arabidopsis phytochrome B

Phytochrome in etiolated sprouts of wild type (WT) potato and its transgenic strains (DARA5 and DARA12) expressing Arabidopsis thaliana phytochrome B (phyB) was investigated using low-temperature (85 K) fluorescence spectroscopy and photochemistry. Phytochrome content, [Ptot], position of the Pr emission and excitation spectra, lambda(max), and extent of the Pr-->lumi-R, gamma1, and Pr-->Pfr, gamma2, phototransformations (at 85 and 273 K, respectively) were shown to vary in the transgenic lines and WT depending on tissue used (upper vs. lower parts of etiolated sprouts) and light-induced phytochrome depletion. Differences in the parameters between the transgenic lines and WT were detected which were interpreted in terms of the two phenomenological Pr types: a labile Pr' with gamma1 approximately 0.5 consisting of a major phytochrome A (phyA) fraction (phyA') and a relatively conserved Pr" with gamma1 = 0 comprising a minor phyA fraction (phyA") and phyB. Both DARA lines had higher [Pr"] as compared with WT in the lower parts of etiolated stems, especially after light-induced phytochrome depletion (residual phytochrome in DARA5 and DARA12 made up to one-third of its initial level vs. <5% in WT). These differences were associated with the expression of Arabidopsis phyB in the DARA lines and its higher light stability than that of phyA. Arabidopsis phyB expressed in potato was characterised by lambda(max) = 683/669 nm in the emission/excitation (absorption) spectra and gamma1 = 0. PhyB also revealed a relatively low gamma2 (approx. 0.5) and its early red drop as compared with the gamma2 wavelength dependence for phyA. This is believed to contribute to the lower signalling ability of phyB and to confine the region (red) of its physiological activity.     

Two native pools of phytochrome A in monocots: Evidence from fluorescence investigations of phytochrome mutants of rice

Fluorescence investigations of phytochrome (phy) in rice (Oryza sativa L. cv. Nipponbare) mutants deficient in phyA, phyB and phyA plus phyB were performed. Total content of the pigment (P(tot)) and its spectroscopic and photochemical characteristics were determined in different parts of the dark-grown and far-red light (FR)-grown coleoptiles. Spectroscopically, phyA in the phyB mutant was identical to phyA in the wild-type (WT) and the extent of the conversion from Pr to lumi-R at 85 K was the same for phyA in both lines and varied similarly, depending on the part of the coleoptile used. The latter finding proved that phyA in rice is heterogeneous and comprises two phyA populations, phyA' and phyA". Functional properties of phyA were also determined. In the dark the phyB mutant had a higher content of phyA, inactive protochlorophyllide (Pchlide633) and active protochlorophyllide (Pchlide655) than WT and its coleoptile was longer, indicating that phyB may affect the development of WT seedlings in the dark. Constant FR drastically reduced the content of phyA, Pchlide633 and Pchlide655 and brought about coleoptile shortening and appearance of the first leaf, whereas pulsed FR of equal fluence was less effective. This suggested that the reactions were primarily of the high irradiance responses type, which are likely to be mediated by phyA'. The effects on protochlorophyllide biosynthesis and growth responses type were more pronounced in the phyB mutant than in the WT seedlings, which can be connected with the higher phyA' content in the phyB mutant and/or phyB interference with its action in WT seedlings. In the phyA mutant induction of Pchlide633 and Pchlide655 biosynthesis was observed under constant FR, indicating that phyC may be responsible for this effect.     

PHOTOREGULATION OF ROOT: SHOOT RATIO IN SOYBEAN SEEDLINGS*

Abstract— The partitioning of plant growth between shoot and root has the potential to affect diverse physiological processes including water and nutrient uptake, nitrogen fixation, light interception, and interactions between plant and soil microorganisms. Root: shoot ratio is determined both by genetics and developmental status as well as by availability of water, nutrients and light. It is shown here that relative root growth was modulated by photomorphogenetic treatments designed to affect phytochrome (supplemental far-red radiation given either as an end-of-day treatment or continuously during the photoperiod) or blue light photoreceptors (blue light-deficient low pressure sodium lamps ± low irradiances of supplemental blue [i.e. 5% of total photon flux: 25 μ.mol m^?2 s^?1]). Photomorphogenetic control of root: shoot ratio was apparent within1–2 days when light treatments were initiated at emergence, and did not necessarily involve changes in net seedling growth. On the other hand, shortened daylength inhibited early seedling growth but had little effect on partitioning. Changes in relative root dry matter induced by supplemental far red radiation or blue light deficiency were similar to those caused by low irradiances, suggesting that phytochrome or blue light photoreceptors may be involved in regulating the partitioning of growth between shoot and root as a part of adaptation to vegetation shade. The influence of spectral quality on root: shoot ratio should be considered when comparing plants grown under different types of lighting or with different spacing.     

Cryptochrome signaling in plants

Cryptochromes are blue light receptors that mediate various light-induced responses in plants and animals. They share sequence similarity to photolyases, flavoproteins that catalyze the repair of UV light-damaged DNA, but do not have photolyase activity. Arabidopsis cryptochromes work together with the red/far-red light receptor phytochromes to regulate various light responses, including the regulation of cell elongation and photoperiodic flowering, and are also found to act together with the blue light receptor phototropins to mediate blue light regulation of stomatal opening. The signaling mechanism of Arabidopsis cryptochromes is mediated through negative regulation of COP1 by direct CRY-COP1 interaction through CRY C-terminal domain. Arabidopsis CRY dimerized through its N-terminal domain and dimerization of CRY is required for light activation of the photoreceptor activity. Recently, significant progresses have been made in our understanding of cryptochrome functions in other dicots such as pea and tomato and lower plants including moss and fern. This review will focus on recent advances in functional and mechanism characterization of cryptochromes in plants. It is not intended to cover every aspect of the field; readers are referred to other review articles for historical perspectives and a more comprehensive understanding of this photoreceptor.     

Selective Photoreceptor Gene Knock‐out Reveals a Regulatory Role for the Growth Behavior of Pseudomonas syringae

The plant pathogen Pseudomonas syringae (Ps) is a well‐established model organism for bacterial infection of plants. The genome sequences of two pathovars, pv. syringae and pv. tomato, revealed one gene encoding a blue and two genes encoding red/far red light‐sensing photoreceptors. Continuing former molecular characterization of the photoreceptor proteins, we here report selective photoreceptor gene disruption for pv. tomato aiming at identification of potentially regulatory functions of these photoreceptors. Transformation of Ps cells with linear DNA constructs yielded interposon mutations of the corresponding genes. Cell growth studies of the generated photoreceptor knock‐out mutants revealed their role in light‐dependent regulation of cell growth and motility. Disruption of the blue‐light (BL) receptor gene caused a growth deregulation, in line with an observed increased virulence of this mutant (Moriconi et al., Plant J., 2013, 76, 322). Bacterial phytochrome‐1 (BphP1) deletion mutant caused unaltered cell growth, but a stronger swarming capacity. Inactivation of its ortholog, BphP2, however, caused reduced growth and remarkably altered dendritic swarming behavior. Combined knock‐out of both bacteriophytochromes reproduced the swarming pattern observed for the BphP2 mutant alone. A triple knock‐out mutant showed a growth rate between that of the BL (deregulation) and the phytochrome‐2 mutant (growth reduction).     

Effect of light quality on the accumulation of photosynthetic pigments, proteins and mycosporine-like amino acids in the red alga Porphyra leucosticta (Bangiales, Rhodophyta)

The effect of different light qualities (white, blue, green, yellow and red light) on photosynthesis, measured as chlorophyll fluorescence, and the accumulation of photosynthetic pigments, proteins and the UV-absorbing mycosporine-like amino acids (MAAs) was studied in the red alga Porphyra leucosticta. Blue light promoted the highest accumulation of nitrogen metabolism derived compounds i.e., MAAs, phycoerythrin and proteins in previously N-starved algae after seven days culture in ammonium enriched medium. Similar results were observed in the culture under white light. In contrast, the lowest photosynthetic capacity i.e., lowest electron transport rate and lowest photosynthetic efficiency as well as the growth rate were found under blue light, while higher values were found in red and white lights. Blue light favored the accumulation of the MAAs porphyra-334, palythine and asterina-330 in P. leucosticta. However, white, green, yellow and red lights favored the accumulation of shinorine. The increase of porphyra-334, palythine and asterina-330 occurred in blue light simultaneous to a decrease in shinorine. The accumulation of MAAs and other nitrogenous compounds in P. leucosticta under blue light could not be attributed to photosynthesis and the action of a non-photosynthetic blue light photoreceptor is suggested. A non-photosynthetic photoreceptor could be also involved in the MAAs interconversion pathways in P. leucosticta.     

Photoreception and Phototropism in Phycomyces: Antagonistic Interactions between Far-UV, Blue, and Red Light

Abstract— Phototropism of the sporangiophore of the fungus Phycomyces is mediated by UV and blue light. Classical phototropism action spectra with maxima near 280, 370 and 450 nm indicate a flavin-like photoreceptor. Blue light mediates positive phototropism while far-UV light mediates negative phototropism. To better understand the mode of interaction of far-UV with blue light we performed phototropism experiments in which sporangio-phores were placed for 4 h between sources of 280 and 454 nm light coming from opposite directions. The fluence rates of the far-UV were chosen such that unilateral light alone elicited 90° of negative bending. For blue light, moderate fluence rates were applied that elicited about 40° bending. Under conditions of bilateral irradiation the blue light substantially reduced the far-UV elicited phototropism. In the presence of tonic red light the antagonism between far-UV and blue light was greatly reduced. Red light, which by itself is phototropically ineffective, also reduced phototropic bending elicited by either far-UV or blue light. These observations are taken as indications for the existence of a red light-absorbing intermediate of the blue-light receptor. Because the far-UV/ blue-light antagonism disappeared almost completely in the presence of tonic red light, the antagonism may occur at the level of this receptor intermediate.     

Phototropins and associated signaling: providing the power of movement in higher plants

Recent developments in phototropin biology have provided exciting new findings on the roles of these photoreceptor proteins in plants. Much of the recent work has focused on phototropin photochemistry and the structural alterations in both the chromophoric and peptide components of the molecule associated with light perception. In this review, specific aspects of phototropin action in higher plants will be discussed in the context of these new findings. Although, as their name suggests, phototropins play a key role in phototropic responses in plants, increasing evidence shows they have many other functions in plants. In this review, the roles of phototropins in additional plant "movement" responses will be addressed; in particular their roles in stomatal aperture control and chloroplast movements. In discussing these various movement responses special attention will be given to identified and hypothesized downstream signaling partners or events that enable the phototropins to selectively participate in any one or more of these responses in a given light condition.     

THE PHOTOBIOLOGY OF Paphiopedilum STOMATA: OPENING UNDER BLUE BUT NOT RED LIGHT

Abstract— The responses of stomata from Paphiopedilum harrisianum , Orchidaceae, to light and CO₂ were studied in epidermal peels. Stomatal opening under red light was indistinguishable from that in darkness, whereas blue light promoted opening above dark levels. The ineffectiveness of red light in causing stomatal opening was confirmed in the presence of 100 μ M KCN; average apertures in both darkness and red light were 53% of those measured in the absence of the inhibitor, whereas under blue irradiation, the KCN inhibition was only 30%, with average apertures two-fold of those measured under red light or darkness. Fluence rate response curves under blue light were typical of a single photoreceptor; removal of CO₂ increased aperture values without a significant light-CO₂ interaction. The lack of a stomatal red light response contrasts with results obtained in species with chlorophyllous stomata in which red light consistently causes stomatal opening, and suggests that the previously reported red light responses in stomata from intact Paphiopedilum leaves resulted from indirect effects, such as depletion of intercellular CO₂ by mesophyll photosynthesis. In isolation, Paphiopedilum stomata appear to rely on a blue light photosystem for their responses to light and fail to open under red light because of their lack of guard cell chloroplasts.     

Evidence for yellow light suppression of lettuce growth

Researchers studying plant growth under different lamp types often attribute differences in growth to a blue light response. Lettuce plants were grown in six blue light treatments comprising five blue light fractions (0, 2, 6% from high-pressure sodium [HPS] lamps and 6, 12, 26% from metal halide [MH] lamps). Lettuce chlorophyll concentration, dry mass, leaf area and specific leaf area under the HPS and MH 6% blue were significantly different, suggesting wavelengths other than blue and red affected plant growth. Results were reproducible in two replicate studies at each of two photosynthetic photon fluxes, 200 and 500 mumol m-2 s-1. We graphed the data against absolute blue light, phytochrome photoequilibrium, phototropic blue, UV, red:far red, blue:red, blue: far red and 'yellow' light fraction. Only the 'yellow' wavelength range (580-600 nm) explained the differences between the two lamp types.     

EFFECT OF BLUE LIGHT AND RED LIGHT ON THE CONTROL PEA LEAVES TO INCREASED FLUENCE RATES OF CHLOROPLAST ACCLIMATION OF LIGHT-GROWN PEA LEAVES TO INCREASED FLUENCE RATES

Abstract— We have investigated the possibility of the involvement of a blue light fluence-rate sensing photoreceptor in the light acclimation of chloroplast components in light-grown pea seedlings. Low lightgrown seedlings were acclimated for 2 days to either 20 or 200 μmol^m-2s-2 of white, blue-enriched, or broad-band red light. An increase in blue-enriched light fluence rate was more effective than that of red light in bringing about both inhibition of internode growth and the enhancement of the chlorophyll a/b ratio. Ribulose 1,5-bisphosphate carboxylase/oxygenase and cytochrome f protein levels, per unit cell, also increased more markedly (around two-fold) in response to an increase in blue light. The 23 kDa polypeptide of the oxygen-evolving complex and the light-harvesting chlorophyll d b protein of photosystem II apoprotein levels vaned under all wavelengths to a lesser extent, correlating with total protein levels or greening. These data are consistent with the hypothesis of a role for a blue photoreceptor in detecting low versus high fluence rate of light, and subsequently controlling the light acclimation responses. Nevertheless photosynthesis or other mechanisms of fluence-rate photoperception must also be involved.     

Phytochrome,the Visual Pigment of Plants

Plants require light for photosynthesis. In order to adapt to the light conditions in their particular habitat, they have developed various photoreceptor systems. Of these, phytochrome allows even two-color vision in the red/far-red region. The photoreceptor phytochrome is of interest not only to botanists, but also to natural product chemists, photochemists, biochemists, photobiologists, and recently molecular biologists. Despite numerous studies, there are still considerable gaps in our knowledge of this photoreceptor. This article first describes the basic structural studies of the tetrapyrrole chromophore and its photochemical cis–trans isomerization, which is the source of the chromoprotein's photochromism. In the section on the protein moiety, beside other topics, the domain structure of phytochrome and the conformational changes during phototransformation are discussed. Finally, the known phytochrome genes are used to derive phylogenetic relationships, and possible structure–function relationships are discussed.     

EFFECT OF BLUE/UV LIGHT ON ANTHOCYANIN SYNTHESIS IN TOMATO SEEDLINGS IN THE ABSENCE OF BULK CAROTENOIDS

Abstract Anthocyanin synthesis in the hypocotyl of tomato ( Lycopersicon esculentum ) seedlings responds strongly and specifically to blue/UV light while the response to red and far-red light, operating through phytochrome, is weak. The herbicide Norflurazon (SAN 9789) was used to inhibit synthesis of colored carotenoids almost completely without affecting growth and development measurably. Even though carotenoid content was reduced to less than 2% of normal and the fluence rate response function for blue and UV light was linear within the experimental range, Norflurazon treatment did not reduce seedling sensitivity toward blue/UV light. It was concluded that at least'bulk'carotenoids are not the photoreceptor chromophore of the blue/UV photoreceptor pigment.     

AN ACTION SPECTRUM IN THE BLUE and ULTRAVIOLET FOR PHOTOTROPISM IN ALFALFA*

Abstract Phototropism is a common property of plants, but it is not known if different species use the same photoreceptor for their response. We have determined fluence-response relations for phototropism in response to brief, broad-band blue irradiation for four plant species grown under red light (Amaranthus paniculatus, Linum usitatissimum, Vigna radiata and Medicago sativa) and compared these to ones previously obtained for Pisum sativum and Zea mays, grown under similar conditions. Curves for all species showed a bell-shaped dependence on fluence, a characteristic of first positive curvature as originally defined for the Avena coleoptile, and had a similar optimal fluence, near 3 H.mol m^?2. We have obtained an action spectrum in the blue and UV spectral regions for first positive phototropism of the hypocotyl of alfalfa grown under red light. Fluence-response curves at wavelengths between 300 and 500 nm were nearly identical in shape and magnitude; whereas below 300 nm, their slopes and maximum curvatures were reduced. The action spectrum showed that activity rose sharply at wavelengths below 500 nm, peaked at 450 nm with shoulders on either side of that peak, and had lesser peaks at 380 and, in the far ultraviolet, at 280 nm. This action spectrum was very similar to ones in the literature (obtained between 350 and 500 nm) for first and second positive phototropism of oat coleoptiles. We conclude that the same photoreceptor mediates phototropism in oat and alfalfa.