Chapter 23 Phototropism in higher plants

Air channels create a directional light signal to regulate hypocotyl phototropism

In plants, light direction is perceived by the phototropin photoreceptors, which trigger directional growth responses known as phototropism. The formation of a phototropin activation gradient across a photosensitive organ initiates this response. However, the optical tissue properties that functionally contribute to phototropism remain unclear. In this work, we show that intercellular air channels limit light transmittance through various organs in several species. Air channels enhance light scattering in Arabidopsis hypocotyls, thereby steepening the light gradient. This is required for an efficient phototropic response in Arabidopsis and Brassica. We identified an embryonically expressed ABC transporter required for the presence of air channels in seedlings and a structure surrounding them. Our work provides insights into intercellular air space development or maintenance and identifies a mechanism of directional light sensing in plants.

The phosphorylation status of NONPHOTOTROPIC HYPOCOTYL3 affects phot2-dependent phototropism in Arabidopsis

The blue light photoreceptors, phototropin 1 (phot1) and phot2, and their signal transducer, NONPHOTOTROPIC HYPOCOTYL3 (NPH3), are activators of the phototropic responses of Arabidopsis hypocotyls. In a recent study, we reported that the control of NPH3 phosphorylation at serine 7 (S7: or S5), S213, S223, S237, S467, S474 (or S476), and S722 (or S723) contributes to the photosensory adaptation of phot1 signaling during the phototropic response. Phosphomimetic NPH3^SE mutant and unphosphorylatable NPH3^SA mutant on those serine residues function efficiently under blue light conditions at fluence rates of 10^-5 µmol m^-2 s^-1 and 10^-3 µmol m^-2 s^-1 or more, respectively. We here demonstrate that phosphomimetic NPH3^SE, but not unphosphorylatable NPH3^SA, promotes phot2-dependent phototropism under blue light condition at 100 µmol m^-2 s^-1. This result suggests that phot1 negatively controls phot2 signaling through the dephosphorylation of NPH3 at those residues and that the hyperactivation of phot1- and phot2-NPH3 complexes does not occur at the same time under high intensity blue light. We hypothesize that the dephosphorylation of NPH3 on those serine residues suppresses both phot1 and phot2 signaling, which results in different impacts on phot1- and phot2-dependent hypocotyl phototropism due to the differences in the photosensitivity and activation levels of phot1 and phot2.

From dark to darkness, negative phototropism influences the support-tree location of the massive woody climber Hydrangea serratifolia (Hydrangeaceae) in a Chilean temperate rainforest

Climbing plants rely on suitable support to provide the light conditions they require in the canopy. Negative phototropism is a directional search behavior proposed to detect a support-tree, which indicates growth or movement away from light, based on light attenuation. In a Chilean temperate rainforest, we addressed whether the massive woody climber Hydrangea serratifolia (H. et A.) F. Phil (Hydrangeaceae) presents a support-tree location pattern influenced by light availability. We analyzed direction and light received in two groups of juvenile shoots: searching shoots (SS), with plagiotropic (creeping) growth vs. ascending shoots (AS), with orthotropic growth. We found that, in accordance with light attenuation, SS and AS used directional orientation to search and then ascend host trees. The light available to H. serratifolia searching shoots was less than that of the general forest understory; the directional orientation in both groups showed a significant deviation from a random distribution, with no circular statistical difference between them. Circular-linear regression indicated a relationship between directional orientations and light availability. Negative phototropism encodes the light environment's heterogeneous spatial and temporal information, guiding the shoot apex to the most shaded part of the support-tree base, the climbing start point.

Phosphorylation of NONPHOTOTROPIC HYPOCOTYL3 affects photosensory adaptation during the phototropic response

Photosensory adaptation, which can be classified as sensor or effector adaptation, optimizes the light sensing of living organisms by tuning their sensitivity to changing light conditions. During the phototropic response in Arabidopsis (Arabidopsis thaliana), the light-dependent expression controls of blue-light (BL) photoreceptor phototropin 1 (phot1) and its modulator ROOT PHOTOTROPISM2 (RPT2) are known as the molecular mechanisms underlying sensor adaptation. However, little is known about effector adaption in plant phototropism. Here, we show that control of the phosphorylation status of NONPHOTOTROPIC HYPOCOTYL3 (NPH3) leads to effector adaptation in hypocotyl phototropism. We generated unphosphorable and phosphomimetic NPH3 proteins on seven phosphorylation sites in the etiolated seedlings of Arabidopsis. Unphosphorable NPH3 showed a shortening of its retention time in the cytosol and caused an inability to adapt to very low fluence rates of BL (∼10-5 µmol m-2 s-1) during the phototropic response. In contrast, the phosphomimetic NPH3 proteins had a lengthened retention time in the cytosol and could not enable the adaptation to BL at fluence rates of 10-3 µmol m-2 s-1 or more. Our results indicate that the activation level of phot1 and the corresponding phosphorylation level of NPH3 determine the dissociation rate and the reassociation rate of NPH3 on the plasma membrane, respectively. These mechanisms may moderately maintain the active state of phot1 signaling across a broad range of BL intensities and contribute to the photosensory adaptation of phot1 signaling during the phototropic response in hypocotyls.

Determination of Auxin Flow During Phototropic Responses Using Fluorescent Auxin Analogs

Altered auxin distribution patterns and the formation of a lateral auxin gradient are often discussed in the context of phototropism. The DR5rev::GFP auxin reporter gene is commonly used in phototropism research in Arabidopsis. This study describes a fluorescent auxin analog, NBD-NAA, as an additional tool for the determination of auxin flow during phototropic responses.

Roles of AGCVIII Kinases in the Hypocotyl Phototropism of Arabidopsis Seedlings

Regulation of protein function by phosphorylation and dephosphorylation is an important mechanism in many cellular events. The phototropin blue-light photoreceptors, plant-specific AGCVIII kinases, are essential for phototropic responses. Members of the D6 PROTEIN KINASE (D6PK) family, representing a subfamily of the AGCVIII kinases, also contribute to phototropic responses, suggesting that possibly further AGCVIII kinases may potentially control phototropism. The present study investigates the functional roles of Arabidopsis (Arabidopsis thaliana) AGCVIII kinases in hypocotyl phototropism. We demonstrate that D6PK family kinases are not only required for the second but also for the first positive phototropism. In addition, we find that a previously uncharacterized AGCVIII protein, AGC1-12, is involved in the first positive phototropism and gravitropism. AGC1-12 phosphorylates serine residues in the cytoplasmic loop of PIN-FORMED 1 (PIN1) and shares phosphosite preferences with D6PK. Our work strongly suggests that the D6PK family and AGC1-12 are critical components for both hypocotyl phototropism and gravitropism, and that these kinases control tropic responses mainly through regulation of PIN-mediated auxin transport by protein phosphorylation.

Asymmetric Auxin Distribution is Not Required to Establish Root Phototropism in Arabidopsis

An asymmetric auxin distribution pattern is assumed to underlie the tropic responses of seed plants. It is unclear, however, whether this pattern is required for root negative phototropism. We here demonstrate that asymmetric auxin distribution is not required to establish root phototropism in Arabidopsis. Our detailed analyses of auxin reporter genes indicate that auxin accumulates on the irradiated side of roots in response to an incidental gravitropic stimulus caused by phototropic bending. Further, an agravitropic mutant showed a suppression of this accumulation with an enhancement of the phototropic response. In this context, our pharmacological and genetic analyses revealed that both polar auxin transport and auxin biosynthesis are critical for the establishment of root gravitropism, but not for root phototropism, and that defects in these processes actually enhance phototropic responses in roots. The auxin response factor double mutant arf7 arf19 and the auxin receptor mutant tir1 showed a slight reduction in phototropic curvatures in roots, suggesting that the transcriptional regulation by some specific ARF proteins and their regulators is at least partly involved in root phototropism. However, the auxin antagonist PEO-IAA [α-(phenylethyl-2-one)-indole-3-acetic acid] suppressed root gravitropism and enhanced root phototropism, suggesting that the TIR1/AFB auxin receptors and ARF transcriptional factors play minor roles in root phototropism. Taken together, we conclude from our current data that the phototropic response in Arabidopsis roots is induced by an unknown mechanism that does not require asymmetric auxin distribution and that the Cholodny-Went hypothesis probably does not apply to root phototropism.

Phytochrome A Mediates Blue-Light Enhancement of Second-Positive Phototropism in Arabidopsis

Hypocotyl phototropism of etiolated Arabidopsis seedlings is primarily mediated by the blue-light receptor kinase phototropin 1 (phot1). Phot1-mediated curvature to continuous unilateral blue light irradiation (0.5 μmol m(-2) s(-1)) is enhanced by overhead pre-treatment with red light (20 μmol m(-2) s(-1) for 15 min) through the action of phytochrome (phyA). Here, we show that pre-treatment with blue light is equally as effective in eliciting phototropic enhancement and is dependent on phyA. Although blue light pre-treatment was sufficient to activate early phot1 signaling events, phot1 autophosphorylation in vivo was not found to be saturated, as assessed by subsequently measuring phot1 kinase activity in vitro. However, enhancement effects by red and blue light pre-treatment were not observed at higher intensities of phototropic stimulation (10 μmol m(-2) s(-1)). Phototropic enhancement by red and blue light pre-treatments to 0.5 μmol m(-2) s(-1) unilateral blue light irradiation was also lacking in transgenic Arabidopsis where PHOT1 expression was restricted to the epidermis. Together, these findings indicate that phyA-mediated effects on phot1 signaling are restricted to low intensities of phototropic stimulation and originate from tissues other than the epidermis.

Phototropic solar tracking in sunflower plants: an integrative perspective

BACKGROUND

One of the best-known plant movements, phototropic solar tracking in sunflower (Helianthus annuus), has not yet been fully characterized. Two questions are still a matter of debate. (1) Is the adaptive significance solely an optimization of photosynthesis via the exposure of the leaves to the sun? (2) Is shade avoidance involved in this process? In this study, these concepts are discussed from a historical perspective and novel insights are provided.

SCOPE AND METHODS

Results from the primary literature on heliotropic growth movements led to the conclusion that these responses cease before anthesis, so that the flowering heads point to the East. Based on observations on 10-week-old plants, the diurnal East-West oscillations of the upper fifth of the growing stem and leaves in relation to the position of the sun (inclusive of nocturnal re-orientation) were documented, and photon fluence rates on the leaf surfaces on clear, cloudy and rainy days were determined. In addition, the light-response curve of net CO2 assimilation was determined on the upper leaves of the same batch of plants, and evidence for the occurrence of shade-avoidance responses in growing sunflower plants is summarized.

CONCLUSIONS

Only elongating, vegetative sunflower shoots and the upper leaves perform phototropic solar tracking. Photon fluence response and CO2 assimilation measurements cast doubt on the 'photosynthesis-optimization hypothesis' as the sole explanation for the evolution of these plant movements. We suggest that the shade-avoidance response, which maximizes light-driven CO2 assimilation, plays a major role in solar tracking populations of competing sunflower plants, and an integrative scheme of these growth movements is provided.

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.

PINOID functions in root phototropism as a negative regulator

The PINOID (PID) family, which belongs to AGCVIII kinases, is known to be involved in the regulation of auxin efflux transporter PIN-formed (PIN) proteins through changes in the phosphorylation status. Recently, we demonstrated that the PID family is necessary for phytochrome-mediated phototropic enhancement in Arabidopsis hypocotyls and that the downregulation of PID expression by red-light pretreatment results in the promotion of the PIN-mediated auxin gradient during phototropic responses. However, whether PID participates in root phototropism in Arabidopsis seedlings has not been well studied. Here, we demonstrated that negative root phototropic responses are enhanced in the pid quadruple mutant and are severely impaired in transgenic plants expressing PID constitutively. The results indicate that the PID family functions in a negative root phototropism as a negative regulator. On the other hand, analysis with PID fused to a yellow fluorescent protein, VENUS, showed that unilateral blue-light irradiation causes a lower accumulation of PID proteins on the shaded side than on the irradiated side. This result suggests that the blue-light-mediated asymmetrical distribution of PID proteins may be one of the critical responses in phototropin-mediated signals during a negative root phototropism. Alternatively, such a transverse gradient of PID proteins may result from gravitropic stimulation produced by phototropic bending.

PINOID AGC kinases are necessary for phytochrome-mediated enhancement of hypocotyl phototropism in Arabidopsis

Several members of the AGCVIII kinase subfamily, which includes PINOID (PID), PID2, and WAVY ROOT GROWTH (WAG) proteins, have previously been shown to phosphorylate PIN-FORMED (PIN) auxin transporters and control the auxin flow in plants. PID has been proposed as a key component of the phototropin signaling pathway that induces phototropic responses, although the responses were not significantly impaired in the pid single and pid wag1 wag2 triple mutants. This raises questions about the functional roles of the PID family in phototropic responses. Here, we investigated hypocotyl phototropism in the pid pid2 wag1 wag2 quadruple mutant in detail to clarify the roles of the PID family in Arabidopsis (Arabidopsis thaliana). The pid quadruple mutants exhibited moderate responses in continuous light-induced phototropism with a decrease in growth rates of hypocotyls and normal responses in pulse-induced phototropism. However, they showed serious defects in enhancements of pulse-induced phototropic curvatures and lateral fluorescent auxin transport by red light pretreatment. Red light pretreatment significantly reduced the expression level of PID, and the constitutive expression of PID prevented pulse-induced phototropism, irrespective of red light pretreatment. This suggests that the PID family plays a significant role in phytochrome-mediated phototropic enhancement but not the phototropin signaling pathway. Red light treatment enhanced the intracellular accumulation of PIN proteins in response to the vesicle-trafficking inhibitor brefeldin A in addition to increasing their expression levels. Taken together, these results suggest that red light preirradiation enhances phototropic curvatures by up-regulation of PIN proteins, which are not being phosphorylated by the PID family.

Phototropism: translating light into directional growth

Phototropism allows plants to align their photosynthetic tissues with incoming light. The direction of incident light is sensed by the phototropin family of blue light photoreceptors (phot1 and phot2 in Arabidopsis), which are light-activated protein kinases. The kinase activity of phototropins and phosphorylation of residues in the activation loop of their kinase domains are essential for the phototropic response. These initial steps trigger the formation of the auxin gradient across the hypocotyl that leads to asymmetric growth. The molecular events between photoreceptor activation and the growth response are only starting to be elucidated. In this review, we discuss the major steps leading from light perception to directional growth concentrating on Arabidopsis. In addition, we highlight links that connect these different steps enabling the phototropic response.

Genetic control of biosynthesis and transport of riboflavin and flavin nucleotides and construction of robust biotechnological producers

Riboflavin [7,8-dimethyl-10-(1'-d-ribityl)isoalloxazine, vitamin B₂] is an obligatory component of human and animal diets, as it serves as the precursor of flavin coenzymes, flavin mononucleotide, and flavin adenine dinucleotide, which are involved in oxidative metabolism and other processes. Commercially produced riboflavin is used in agriculture, medicine, and the food industry. Riboflavin synthesis starts from GTP and ribulose-5-phosphate and proceeds through pyrimidine and pteridine intermediates. Flavin nucleotides are synthesized in two consecutive reactions from riboflavin. Some microorganisms and all animal cells are capable of riboflavin uptake, whereas many microorganisms have distinct systems for riboflavin excretion to the medium. Regulation of riboflavin synthesis in bacteria occurs by repression at the transcriptional level by flavin mononucleotide, which binds to nascent noncoding mRNA and blocks further transcription (named the riboswitch). In flavinogenic molds, riboflavin overproduction starts at the stationary phase and is accompanied by derepression of enzymes involved in riboflavin synthesis, sporulation, and mycelial lysis. In flavinogenic yeasts, transcriptional repression of riboflavin synthesis is exerted by iron ions and not by flavins. The putative transcription factor encoded by SEF1 is somehow involved in this regulation. Most commercial riboflavin is currently produced or was produced earlier by microbial synthesis using special selected strains of Bacillus subtilis, Ashbya gossypii, and Candida famata. Whereas earlier RF overproducers were isolated by classical selection, current producers of riboflavin and flavin nucleotides have been developed using modern approaches of metabolic engineering that involve overexpression of structural and regulatory genes of the RF biosynthetic pathway as well as genes involved in the overproduction of the purine precursor of riboflavin, GTP.

Phototropic leaf movements and photosynthetic performance in an amphibious fern, Marsilea quadrifolia

Diurnal phototropism has not been reported in ferns. In this study we found that the four leaflets of the amphibious fern Marsilea quadrifolia are capable of adjusting their leaflet angle and leaflet azimuth in response to changes in the position of the sun's direct beam, exhibiting more diaphototropic movements (orienting the plane of the lamina perpendicular to incident light) in the morning and late afternoon, and more paraphototropic movements (orienting the plane of the lamina parallel to incident light) at noon. In addition, by cutting off the leaflet lamina and covering portions of leaflets with black tape, the junction between the leaflet and petiole was found to be responsible for light reception. Among the light spectrum investigated, blue light was the most effective at inducing diaphototropism. The role of diurnal phototropism in enhancing carbon return and ameliorating photoinhibition was also evaluated. It was concluded that diurnal phototropic leaf movement represents one of the plastic responses enabling this amphibious fern to grow under terrestrial conditions.

The ABC subfamily B auxin transporter AtABCB19 is involved in the inhibitory effects of N-1-naphthyphthalamic acid on the phototropic and gravitropic responses of Arabidopsis hypocotyls

N-1-Naphthylphthalamic acid (NPA) causes the abnormal growth and development of plants by suppressing polar auxin transport. The mechanisms underlying this inhibition, however, have remained elusive. In Arabidopsis, we show that a defect in the ABC subfamily B auxin transporter AtABCB19 suppresses the inhibitory effects of NPA on hypocotyl phototropism and gravitropism, but not on hypocotyl elongation. Expression analysis using the auxin reporter gene DR5:GUS further suggests that NPA partially inhibits the asymmetric distribution of auxin in an AtABCB19-dependent manner. These data thus suggest that AtABCB19 plays an important role in the inhibitory effects of NPA on hypocotyl tropism induced by auxin.

Where and how does phototropin transduce light signals in the cell?

Light plays pivotal roles as an important environmental signal in plant growth and development. In Arabidopsis, phototropin 1 (phot1) and 2 (phot2) are the photoreceptors that mediate phototropism, chloroplast relocation, stomatal opening and leaf flattening, in response to blue light. However, little is known about how phototropins transduce the signals after the light is perceived. Changes induced by blue light in terms of intracellular localization patterns of phot2 in Arabidopsis were examined. Phot2 distributed uniformly in the plasma membrane under dark conditions. Upon irradiation with blue light, some of the phot2 associated with the Golgi apparatus. It was also shown that the kinase domain, but not the photosensory domain, is required for a plasma membrane and Golgi localization. Furthermore a kinase fragment, lacking the photosensory domain, constitutively triggered physiological responses in planta. Thus, the plasma membrane and the Golgi apparatus appear to be the most likely sites for the initial step of phot2 signal transduction. The Golgi apparatus facilitates vesicle trafficking and delivery of membrane proteins to the required locations in the cell. Therefore, this study implicates the regulation of vesicle trafficking by the Golgi apparatus as a mechanism by which phot2 elicits its cellular responses.

Redox properties of LOV domains: chemical versus photochemical reduction, and influence on the photocycle

LOV (light-oxygen-voltage-sensitive) domains comprise the light-sensitive parts of many blue light photoreceptor proteins. Photoexcitation of the chromophore flavin mononucleotide (FMN) in these LOV domains leads to formation of a covalent adduct between FMN and a cysteine residue. So far, the electronically excited singlet and triplet states of FMN have been identified as the only intermediates in the photocycles of LOV domains from several organisms. Since many flavoproteins are redox-active, however, the photocycles of LOV domains might involve other redox states of FMN, and might be controlled by the external redox potential. Here we report on the redox properties of the LOV1 domain from phototropin of the green alga Chlamydomonas reinhardtii. By equilibrium-redox spectropotentiometry a redox potential [E(fq/fhq) (flavoquinone/flavohydroquinone)] of -290 mV vs. the normal hydrogen electrode (NHE) was determined for the wild-type domain (LOV1-wt). A similar value of -280 mV was found for the mutant LOV1-C57G, in which the photoreactive cysteine is replaced by glycine. The recovery kinetics (photoadduct-->ground state) in the photocycle of LOV1-wt are not influenced by a redox potential in the range between +500 and -260 mV versus NHE. No flavosemiquinone could be generated by chemical reduction with sodium dithionite. However, photoreduction of LOV1-C57G with EDTA leads exclusively to the flavosemiquinone. This semiquinone is stable against disproportionation, and the photoreduction is not mediated by free FMN.

The C-terminal kinase fragment of Arabidopsis phototropin 2 triggers constitutive phototropin responses

Phototropins mediate various blue-light responses such as phototropism, chloroplast relocation, stomatal opening and leaf flattening in plants. Phototropins are hydrophilic chromoproteins that are mainly bound to the plasma membrane. One of two phototropins in Arabidopsis thaliana, phot2, associates with the Golgi apparatus in a light-dependent manner. In this study, we analyzed the biological activities of the N-terminal photosensory and C-terminal kinase domains of phot2. For this purpose, these domains were fused to green fluorescent protein (GFP) and ectopically expressed in the wild-type and a phot1 phot2 double mutant of Arabidopsis. The kinase domain fused to GFP (P2CG) was localized to the plasma membrane and the Golgi apparatus, whereas the photosensory domain fused to GFP (P2NG) was uniformly localized in the cytosol. Hence, the kinase domain rather than the photosensory domain is responsible for the membrane association. Interestingly, the P2CG plants exhibited constitutive blue-light responses even in dark conditions, i.e. stomata were open and chloroplasts were in the avoidance position. By contrast, P2CG with a mutation that abolishes the kinase activity (P2C[D720/N]G) failed to exhibit these responses. phot2 kinase is therefore suggested to be correctly localized to functional sites in the cell and to trigger light signal transduction through its kinase activity. In contrast to P2CG, P2NG did not affect the phot2 responses, except for partial inhibition of the phototropic response caused by the endogenous phototropins.

Phototropins and blue light-dependent calcium signaling in higher plants

Plants have several kinds of photoreceptors, which regulate growth and development. Recent investigations using Arabidopsis thaliana revealed that the newly found blue light receptor phototropins mediate phototropism, chloroplast relocation, stomatal opening, rapid inhibition of hypocotyl elongation and leaf expansion. Several physiological studies suggest that one of the intermediates in phototropin signaling is cytosolic Ca2+. Studies using phototropin mutants have demonstrated that phototropins induce an increase in cytosolic Ca2+ concentration. However, the function of Ca2+ in the phototropin-mediated signaling process remains largely unknown. This review presents findings about phototropin-mediated calcium mobilization and the involvement of calcium in blue light-dependent plant responses.

Phototropic bending of non-elongating and radially growing woody stems results from asymmetrical xylem formation

Active phototropic bending of non-elongating and radially growing portion of stems (woody stems) has not been previously documented, whereas negative gravitropic bending is well known. We found phototropic bending in woody stems and searched for the underlying mechanism. We inclined 1-year-old Quercus crispula Blume seedlings and unilaterally illuminated them from a horizontal direction perpendicular to ('normal' illumination) or parallel to ('parallel' illumination) the inclination azimuth. With normal illumination, active phototropic bending and xylem formation could be evaluated separately from the negative gravitropic response and vertical deflection resulting from the weight of the seedlings. One-year-old stems with normal illumination bent significantly, with asymmetrical xylem formation towards the illuminated upper surface and side of the stem, whereas those with parallel illumination showed non-significant lateral bending, with asymmetrical xylem formation only on the upper side. A mechanical model was built on the assumption that a bending moment resulted from the asymmetrical xylem formation during phototropic bending of the woody stems. The model fitted the relationship between the observed spatial distributions of the xylem and the observed lateral bending, and thus supported the hypothesis that phototropic bending of woody stems results from asymmetrical xylem formation, as such occurs during gravitropism.

Phototropin blue-light receptors

Phototropins are blue-light receptors controlling a range of responses that serve to optimize the photosynthetic efficiency of plants. These include phototropism, light-induced stomatal opening, and chloroplast movements in response to changes in light intensity. Since the isolation of the Arabidopsis PHOT1 gene in 1997, phototropins have been identified in ferns and mosses where their physiological functions appear to be conserved. Arabidopsis contains two phototropins, phot1 and phot2, that exhibit overlapping functions in addition to having unique physiological roles. Phototropins are light-activated serine/threonine protein kinases. Light sensing by the phototropins is mediated by a repeated motif at the N-terminal region of the protein known as the LOV domain. Photoexcitation of the LOV domain results in receptor autophosphorylation and an initiation of phototropin signaling. Here we summarize the photochemical and biochemical events underlying phototropin activation in addition to the current knowledge of the molecular mechanisms associated with photoreceptor signaling.

Stem phototropism of trees: a possible significant factor in determining stem inclination on forest slopes

BACKGROUND AND AIMS

The main stems of trees on forest slopes incline down the slope to various extents that are characteristic of the species. The inclination has been explained as an active response to a horizontally asymmetrical light environment, but the contributing physiological mechanisms are unknown. The present study tested the hypothesis that stem phototropism, gravitropism, or a combination of the two determines the inclination of tree stems on forest slopes.

METHODS

Cryptomeria japonica, Pinus densiflora, Quercus myrsinaefolia and Q. serrata were studied. Measurements were made of stem inclination of mature trees on forest slopes in uniform plantations of each species, and changes in stem inclination of potted seedlings in response to illumination treatments (unilateral or overhead) and inclination treatments (artificially inclined or erect). Indices of phototropic and gravitropic responsiveness were evaluated for each species, calculated from the change in stem inclination in response to artificial inclination with unilateral or overhead illumination.

KEY RESULTS

Stem inclination on forest slopes varied significantly among species: Q. serrata inclined most in the down-slope direction, C. japonica inclined the least, and P. densiflora and Q. myrsinaefolia were intermediate. The change in stem inclination of seedlings in each treatment varied significantly among species. One-year-old stems of Q. serrata and 2-year-old stems of Q. myrsinaefolia bent toward the light source. Interspecific variation in the change in stem inclination in response to the unilateral illumination or that in the index of phototropic responsiveness was strongly correlated with the variation in stem inclination on forest slopes.

CONCLUSIONS

The orientation of woody stems that have finished elongation can be actively controlled by phototropism. Interspecific variation in phototropic responsiveness of trees is a possible significant determinant of interspecific variation in stem inclination on forest slopes.

Blue light-induced association of phototropin 2 with the Golgi apparatus

Phototropins 1 and 2 (phot1 and phot2) function as blue light (BL) photoreceptors for phototropism, chloroplast relocation, stomatal opening and leaf flattening in Arabidopsis thaliana. Phototropin consists of two functional domains, the N-terminal photosensory domain and the C-terminal Ser/Thr kinase domain. However, little is known about the signal transduction pathway that links the photoreceptors and the physiological responses downstream of BL perception. To understand the mechanisms by which phot2 initiates these responses, we transformed the phot1phot2 double mutant of Arabidopsis with constructs encoding translationally fused phot2:green fluorescent protein (P2G). P2G was fully functional for the phot2-specific physiological responses in these transgenic plants. It localized strongly to the plasma membrane and weakly to the cytoplasm in the dark. Upon illumination with BL, punctate P2G staining was formed within a few minutes in addition to the constitutive plasma membrane staining. This punctate distribution pattern matched well with that of the Golgi-localized KAM1DeltaC:mRFP. Brefeldin A (BFA), an inhibitor of vesicle trafficking, induced accumulation of P2G around the perinuclear region even in darkness, but the punctate pattern was not observed. After treatment of these cells with BL, P2G exhibited the punctate distribution pattern that matched with that of the Golgi marker. Hence, the light-dependent association of P2G with the Golgi apparatus was BFA-insensitive. A structure/function analysis indicated that the kinase domain was essential for the Golgi localization of phot2. The BL-induced Golgi localization of phot2 may be one of important signaling steps in the phot2 signal transduction pathway.

Toward understanding the ecological functions of tropisms: interactions among and effects of light on tropisms

Tropisms of higher plants have been investigated for well over a century. Only recently, however, we have begun to establish their mechanisms firmly, mainly thanks to the availability of mutants and genome sequence information. For example, the starch-statolith hypothesis is now best supported as the main mechanism by which plants perceive gravity direction. Phototropins have been identified as the photoreceptors for the major blue-light-sensitive phototropism. Investigations have been extended to elucidate the relationships among tropisms and the controlling roles played by environmental factors, such as light. We are now finding examples in which phototropic and hydrotropic responses are modified through the environmental control of counteracting gravitropism. We are also finding that seedlings generally become phototropically competent only after phytochrome is activated. Such results are providing insights into how plants use tropisms to achieve adaptive growth movements.

Overexpression of constitutive differential growth 1 gene, which encodes a RLCKVII-subfamily protein kinase, causes abnormal differential and elongation growth after organ differentiation in Arabidopsis

To better understand genetic regulation of differential growth of plant organs, a dominant and semidwarf mutant, constitutive differential growth 1-Dominant (cdg1-D), was isolated utilizing the technique of activation tagging. cdg1-D showed pleiotropic phenotype including dwarfism, exaggerated leaf epinasty, and twisted or spiral growth in hypocotyl, inflorescence stem, and petiole. Hypocotyls of cdg1-D were longer than those of wild type under light conditions. The phenotype was caused by activation tagging of CDG1 gene that encodes a receptor-like cytoplasmic kinase of RLCKVII subfamily. When treated with high concentrations of brassinolide, light-grown wild-type seedlings showed long hypocotyls and strong leaf epinasty as observed in cdg1-D seedlings. Treatment of cdg1-D with brassinazole, a specific inhibitor of brassinosteroid (BR) biosynthesis, did not rescue the mutant phenotype. Gene expression of CONSTITUTIVE PHOTOMORPHOGENESIS AND DWARFISM involved in BR biosynthesis and phyB ACTIVATION-TAGGED SUPPRESSOR1 that inactivates BR was repressed and induced, respectively, in cdg1-D plants, suggesting constitutive activation of BR signaling in the mutant. CDG1 was expressed at a very low level in all the organs of the wild type tested. We isolated two independent intragenic suppressors of cdg1-D. However, they showed normal morphology and responded to BR in a similar manner to wild type. Taken together, CDG1 gene may interfere with signal transduction of BR when overexpressed, but is not an essential factor for it in the wild type.

Two distinct signaling pathways participate in auxin-induced swelling of pea epidermal protoplasts

Protoplast swelling was used to investigate auxin signaling in the growth-limiting stem epidermis. The protoplasts of epidermal cells were isolated from elongating internodes of pea (Pisum sativum). These protoplasts swelled in response to auxin, providing the clearest evidence that the epidermis can directly perceive auxin. The swelling response to the natural auxin IAA showed a biphasic dose response curve but that to the synthetic auxin 1-naphthalene acetic acid (NAA) showed a simple bell-shaped dose response curve. The responses to IAA and NAA were further analyzed using antibodies raised against ABP1 (auxin-binding protein 1), and their dependency on extracellular ions was investigated. Two signaling pathways were resolved for IAA, an ABP1-dependent pathway and an ABP1-independent pathway that is much more sensitive to IAA than the former. The response by the ABP1 pathway was eliminated by anti-ABP1 antibodies, had a higher sensitivity to NAA, and did not depend on extracellular Ca(2+). In contrast, the response by the non-ABP1 pathway was not affected by anti-ABP1 antibodies, had no sensitivity to NAA, and depended on extracellular Ca(2+). The swelling by either pathway required extracellular K(+) and Cl(-). The auxin-induced growth of pea internode segments showed similar response patterns, including the occurrence of two peaks in the dose response curve for IAA and the difference in Ca(2+) requirements. It is suggested that two signaling pathways participate in auxin-induced internode growth and that the non-ABP1 pathway is more likely to be involved in the control of growth by constitutive concentrations of endogenous auxin.

Blue light regulates an auxin-induced K+-channel gene in the maize coleoptile

Auxin redistribution along gravistimulated maize coleoptiles causes differential expression of the auxin-induced K+-channel gene ZMK1 (Zea mays K+ channel 1) and precedes the curvature response. To evaluate the role of ZMK1 during phototropism, we here investigated blue light-stimulated coleoptiles. Four hours of blue light stimulation resulted in phototropic bending (23 degrees ). Rotation on a clinostat, at nominally "zero" gravity, and simultaneous stimulation with unidirectional blue light, however, resulted in up to 51 degrees bending toward the light. Differential ZMK1 transcription reached a maximum after 90 min of blue light stimulation under gravity, whereas ZMK1 expression remained asymmetric for at least 180 min in photostimulated coleoptiles on a clinostat. We therefore conclude that the stronger phototropic bending under nominally "zero" gravity results from prolonged differential expression of ZMK1. Under both conditions, asymmetric expression of ZMK1 could be superimposed on the lateral auxin gradient across the coleoptile tip, whereas the gene for the blue light receptor phototropin 1 (PHOT1), expressed in the tip only, was not differentially regulated in response to blue light. The activation of the two different receptors eliciting the photo- and gravitropic response of the coleoptile thus feeds into a common signaling pathway, resulting in auxin redistribution in the coleoptile tip and finally in differential transcription of ZMK1. In the process of signal integration, gravity transduction restricts the magnitude of the blue light-inducible ZMK1 gradient. The spatial and temporal distribution of ZMK1 transcripts and thus differential K+ uptake in both flanks of the coleoptile seem to limit the stimulus-induced bending of this sensory organ.

Blue light inactivates plasma membrane H(+)-ATPase in pulvinar motor cells of Phaseolus vulgaris L

Unilateral blue light irradiation induces bending of pulvini of Phaseolus vulgaris towards the source of light. The pulvinar bending is caused by a decrease in turgor pressure of motor cells that are irradiated with blue light. Decrease in the turgor pressure is caused by the net efflux of K(+) and counter anions, accompanying membrane depolarization. In the present study the effect of blue light on the activity of plasma membrane H(+)-ATPase was studied in relation to the membrane depolarization. The activity of the plasma membrane H(+)-ATPase was measured using protoplast suspensions prepared from laminar pulvini from primary leaves. A pulse of blue light under continuous red light irradiation induced both a transient increase in the external pH and transient inhibition of the vanadate-sensitive ATPase. Continuous blue light irradiation under continuous red light irradiation induced both a sustained increase in the external pH and sustained inhibition of the vanadate-sensitive ATPase. These results show that blue light inhibits the activity of the plasma membrane H(+)-ATPase. Inactivation of the plasma membrane H(+)-ATPase supports the membrane depolarization induced by the blue light irradiation.

Blue-light-dependent osmoregulation in protoplasts of Phaseolus vulgaris Pulvini

Blue light was found to induce shrinkage of the protoplasts isolated from first-leaf lamina pulvini of 18-day-old Phaseolus vulgaris. The response was transient following pulse stimulation, while it was sustainable during continuous stimulation. No apparent difference was found between flexor and extensor protoplasts. Protoplasts of the petiolar segment located close to the pulvinus showed no detectable response. In the plants used, the pulvinus was fully matured and the petiole was ceasing its elongation growth. When younger, 12-day-old, plants were used, however, the petiolar protoplasts did respond to blue light. The pulse-induced response was similar to that in pulvinar protoplasts, although the response to continuous stimulation was transient and differed from that in pulvinar protoplasts. No shrinkage was induced in pulvinar protoplasts when the far-red-light-absorbing form of phytochrome was absent for a period before blue-light stimulation, indicating that the blue-light responsiveness is strictly controlled by phytochrome. Inhibitors of anion channels and H(+)-ATPase abolished the shrinking response, supporting the view that protoplasts shrink by extruding ions. The response of pulvinar protoplasts is probably involved in the blue-light-induced, turgor-based movement of pulvini. The blue-light responding system in pulvini is suggested to have evolved from that functioning in other growing organs.

Auxin- and abscisic acid-dependent osmoregulation in protoplasts of Phaseolus vulgaris pulvini

Protoplasts isolated from the laminar pulvinus of Phaseolus vulgaris and bathed in a medium containing KCl as the major salt were found to swell in response to IAA and to shrink in response to ABA. The protoplasts of flexor cells and those of extensor cells responded similarly. The results indicate that the cellular content of osmotic solutes is enhanced by IAA and reduced by ABA. The IAA-induced swelling was abolished when either the K(+) or the Cl(-) of the bathing medium was replaced by an impermeant ion or when the medium was adjusted to neutral pH (instead of pH 6). The response was inhibited by vanadate. It is concluded that the swelling is caused by enhanced influxes of K(+) and Cl(-), which probably occur through K(+) channels and Cl(-)/H(+) symporters, respectively. The ABA-induced shrinking was inhibited by 5-nitro-2-(3-phenylpropylamino)-benzoic acid, an anion-channel inhibitor, suggesting that it is caused by Cl(-) efflux through anion channels and charge-balancing K(+) efflux through outward-rectifying K(+) channels. It appears that the two plant hormones act on pulvinar motor cells to regulate their turgor pressure, as they do in stomatal guard cells. The findings are discussed in relation to the pulvinar movements induced by environmental stimuli.