Plant phototropic growth

Phototropins Mediate Chloroplast Movement in Phalaenopsis aphrodite (Moth Orchid)

Chloroplast movement is important for plants to avoid photodamage and to perform efficient photosynthesis. Phototropins are blue light receptors in plants that function in chloroplast movement, phototropism, stomatal opening, and they also affect plant growth and development. In this study, full-length cDNAs of two PHOTOTROPIN genes, PaPHOT1 and PaPHOT2, were cloned from a moth orchid Phalaenopsis aphrodite, and their functions in chloroplast movement were investigated. Phylogenetic analysis showed that PaPHOT1 and PaPHOT2 orthologs were highly similar to PHOT1 and PHOT2 of the close relative Phalaenopsis equestris, respectively, and clustered with monocots PHOT1 and PHOT2 orthologs, respectively. Phalaenopsis aphrodite expressed a moderate level of PaPHOT1 under low blue light of 5 μmol�m-2�s-1 (BL5) and a high levels of PaPHOT1 at >BL100. However, PaPHOT2 was expressed at low levels at <BL50 but expressed at high levels at > BL100. Analysis of light-induced chloroplast movements using the SPAD method indicated that orchid accumulated chloroplasts at <BL10. The chloroplast avoidance response was detectable at >BL25 and significant chloroplast avoidance movement was observed at >BL100. Virus-induced gene silencing of PaPHOTs in orchids showed decreased gene expression of PaPHOTs and reduced both chloroplast accumulation and avoidance responses. Heterologous expression of PaPHOT1 in Arabidopsis phot1phot2 double mutant recovered chloroplast accumulation response at BL5, but neither PaPHOT1 nor PaPHOT2 was able to restore mutant chloroplast avoidance at BL100. Overall, this study showed that phototropins mediate chloroplast movement in Phalaenopsis orchid is blue light-dependent but their function is slightly different from Arabidopsis which might be due to gene evolution.

Is the Responsiveness to Light Related to the Differences in Stem Straightness among Populations of Pinus pinaster?

Stem straightness is related to wood quality and yield. Although important genetic differences in stem straightness among the natural populations of Pinus pinaster are well established, the main drivers of these differences are not well known. Since the responses of trees to light are key ecological features that induce stem curvature, we hypothesized that populations with better straightness should exhibit lower photomorphogenetic and phototropic sensitivity. We compared three populations to identify the main processes driven by primary and secondary growth that explain their differences in response to light. One-year-old seedlings were grown under two treatments—direct sunlight and lateral light plus shade—for a period of 5 months. The length and the leaning of the stems were measured weekly. The asymmetry of radial growth and compression wood (CW) formation were analyzed in cross-sections. We found differences among the populations in photomorphogenetic and phototropic reactions. However, the population with straighter stems was not characterized by reduced sensitivity to light. Photo(gravi)tropic responses driven by primary growth and gravitropic responses driven by secondary growth explained the kinetics of the stem leaning and CW pattern. Asymmetric radial growth and CW formation did not contribute to the phototropic reactions.

Engineering the phototropin photocycle improves photoreceptor performance and plant biomass production

A key challenge for plant molecular biologists is to increase plant yield by altering photosynthetic productivity to secure food, energy, and environmental sustainability. In the model plant Arabidopsis thaliana, the plasma-membrane–associated phototropin kinases, phot1 and phot2, are activated by blue light and play important roles in regulating several responses that optimize photosynthetic efficiency. However, little effort has been made to target these pathways to increase plant growth. Here, we demonstrate that modifying the photocycle of phot1 and phot2 increases their sensitivity to light. Plants with these engineered phototropins exhibit more rapid and robust chloroplast movement responses and improved leaf positioning and expansion, leading to improved biomass accumulation under light-limiting conditions.

The ability to enhance photosynthetic capacity remains a recognized bottleneck to improving plant productivity. Phototropin blue light receptors (phot1 and phot2) optimize photosynthetic efficiency in Arabidopsis thaliana by coordinating multiple light-capturing processes. In this study, we explore the potential of using protein engineering to improve photoreceptor performance and thereby plant growth. We demonstrate that targeted mutagenesis can decrease or increase the photocycle lifetime of Arabidopsis phototropins in vitro and show that these variants can be used to reduce or extend the duration of photoreceptor activation in planta. Our findings show that slowing the phototropin photocycle enhanced several light-capturing responses, while accelerating it reduced phototropin’s sensitivity for chloroplast accumulation movement. Moreover, plants engineered to have a slow-photocycling variant of phot1 or phot2 displayed increased biomass production under low-light conditions as a consequence of their improved sensitivity. Together, these findings demonstrate the feasibility of engineering photoreceptors to manipulate plant growth and offer additional opportunities to enhance photosynthetic competence, particularly under suboptimal light regimes.

Measurement of Phototropism of the Sporangiophore of Phycomyces blakesleeanus

The giant sporangiophore, fruiting body, of the fungus Phycomyces blakesleeanus is a single cell that grows guided by several environmental signals, including light. The phototropic response has been investigated in detail. Three proteins, the components of a photoreceptor and transcription factor complex and a regulator of the signal transduction protein Ras, participate in the signal transduction pathway. We describe the basic methods for characterizing phototropic bending and the correlated elongation and rotation responses of the sporangiophore.

A Simple Procedure to Observe Phototropic Responses in the Red Seaweed Pyropia yezoensis

The marine red seaweed Pyropia yezoensis exhibits phototropic responses in gametophyte and conchosporangia phases, but not in sporophytes. These responses are easily monitored with a simple culturing box that has one side open to allow for unilateral light irradiation within an incubator. Confirmation of phototropic responses is achieved by changing the direction of unilateral light irradiation via rotation of the culture dishes clockwise 90°.

Physiological Characterization of Phototropism in Arabidopsis Seedlings

To date, many mutants have been isolated from dicot plants, including Arabidopsis thaliana, and the physiological roles of the isolated genes have been identified. Molecular genetic analyses have usually been conducted in the model plant Arabidopsis to identify blue-light photoreceptors and key signaling components in phototropic responses. Despite these investigations, several molecular mechanisms involved in phototropism remain unknown, possibly because detailed physiological analyses have not been conducted properly in the isolated mutants. This chapter describes an approach for the detailed investigation of hypocotyl and root phototropism in Arabidopsis seedlings. The information provided here is expected to facilitate the analysis of phototropic responses in other plant species.

The LNK Gene Family: At the Crossroad between Light Signaling and the Circadian Clock

Light signaling pathways interact with the circadian clock to help organisms synchronize physiological and developmental processes to periodic environmental cycles. The plant photoreceptors responsible for clock resetting have been characterized, but signaling components that link the photoreceptors to the clock remain to be identified. Members of the family of NIGHT LIGHT–INDUCIBLE AND CLOCK-REGULATED (LNK) genes play key roles linking light regulation of gene expression to the control of daily and seasonal rhythms in Arabidopsis thaliana. Particularly, LNK1 and LNK2 were shown to control circadian rhythms, photomorphogenic responses, and photoperiod-dependent flowering time. Here we analyze the role of the four members of the LNK family in Arabidopsis in these processes. We found that depletion of the closely related LNK3 and LNK4 in a lnk1;lnk2 mutant background affects circadian rhythms, but not other clock-regulated processes such as flowering time and seedling photomorphogenesis. Nevertheless, plants defective in all LNK genes (lnkQ quadruple mutants) display developmental alterations that lead to increased rosette size, biomass, and enhanced phototropic responses. Our work indicates that members of the LNK family have both distinctive and partially overlapping functions, and are an essential link to orchestrate light-regulated developmental processes.

Multiple Pathways in the Control of the Shade Avoidance Response

To detect the presence of neighboring vegetation, shade-avoiding plants have evolved the ability to perceive and integrate multiple signals. Among them, changes in light quality and quantity are central to elicit and regulate the shade avoidance response. Here, we describe recent progresses in the comprehension of the signaling mechanisms underlying the shade avoidance response, focusing on Arabidopsis, because most of our knowledge derives from studies conducted on this model plant. Shade avoidance is an adaptive response that results in phenotypes with a high relative fitness in individual plants growing within dense vegetation. However, it affects the growth, development, and yield of crops, and the design of new strategies aimed at attenuating shade avoidance at defined developmental stages and/or in specific organs in high-density crop plantings is a major challenge for the future. For this reason, in this review, we also report on recent advances in the molecular description of the shade avoidance response in crops, such as maize and tomato, and discuss their similarities and differences with Arabidopsis.

Photoreceptor-mediated regulation of the COP1/SPA E3 ubiquitin ligase

Plants have evolved specific photoreceptors that capture informational cues from sunlight. The phytochrome, cryptochrome, and UVR8 photoreceptors perceive red/far-red, blue/UV-A, and UV-B light, respectively, and control overlapping photomorphogenic responses important for plant growth and development. A major repressor of such photomorphogenic responses is the E3 ubiquitin ligase formed by CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) and SUPPRESSOR OF PHYA-105 (SPA) proteins, which acts by regulating the stability of photomorphogenesis-promoting transcription factors. The direct interaction of light-activated photoreceptors with the COP1/SPA complex represses its activity via nuclear exclusion of COP1, disruption of the COP1-SPA interaction, and/or SPA protein degradation. This process enables plants to integrate different light signals at the level of the COP1/SPA complex to enact appropriate photomorphogenic responses according to the light environment.

Location Matters: Canopy Light Responses over Spatial Scales

Plants use light as a signal to determine neighbour proximity in dense vegetation. Far-red (FR) light reflected from neighbour plants elicits an array of growth responses throughout the plant. Recently, various light quality-induced signals have been discovered that travel between organs and tissue layers. These signals share upstream and downstream components, but can have opposing effects on cell growth. The question is how plants can coordinate these spatial signals into various growth responses in remote tissues. This coordination allows plants to adapt to the environment, and understanding the underlying mechanisms could allow precision engineering of crops. To achieve this understanding, plant photobiology research will need to focus increasingly on spatial signalling at the whole-plant level.

Arabidopsis petiole torsions induced by lateral light or externally supplied auxin require microtubule-associated TORTIFOLIA1/SPIRAL2

Although rather inconspicuous, movements are an important adaptive trait of plants. Consequently, light- or gravity-induced movements leading to organ bending have been studied intensively. In the field, however, plant movements often result in organ twisting rather than bending. This study investigates the mechanism of light- or gravity-induced twisting movements, coined "helical tropisms." Because certain Arabidopsis cell expansion mutants show organ twisting under standard growth conditions, we here investigated how the right-handed helical growth mutant tortifolia1/spiral2 (tor1) responds when stimulated to perform helical tropisms. When leaves were illuminated from the left, tor1 was capable of producing left-handed petiole torsions, but these occurred at a reduced rate. When light was applied from right, tor1 plants rotated their petioles much faster than the wild-type. Applying auxin to the lateral-distal side of wild-type petioles produced petiole torsions in which the auxinated flank was consistently turned upwards. This kind of movement was not observed in tor1 mutants when auxinated to produce left-handed movements. Investigating auxin transport in twisting petioles based on the DR5-marker suggested that auxin flow was apical-basal rather than helical. While cortical microtubules of excised wild-type petioles oriented transversely when stimulated with auxin, those of tor1 were largely incapable of reorientation. Together, our results show that tor1 is a tropism mutant and suggest a mechanism in which auxin and microtubules both contribute to helical tropisms.

The Arabidopsis SAL1-PAP Pathway: A Case Study for Integrating Chloroplast Retrograde, Light and Hormonal Signaling in Modulating Plant Growth and Development?

Plant growth and development are dependent on chloroplast development and function. Constitutive high level accumulation of a chloroplast stress signal, 3'-phosphoadenosine-5'-phosphate (PAP), confers drought tolerance to plants, but slow downs and alters plant growth and development. PAP, a by-product of sulfur metabolism, is maintained at very low levels by the SAL1 phosphatase during vegetative growth of Arabidopsis and accumulates in rosettes during drought and excess light. Eight independent forward genetic screens in Arabidopsis identified SAL1 as the regulator of multiple phenotypes related to stress responses, hormonal signaling and/or perception. In this perspective article, we collate all the sal1 phenotypes published in the past two decades, and distill the different pathways affected. Our meta-analysis of publicly available sal1 microarray data coupled to preliminary hormonal treatment and profiling results on sal1 indicate that homeostasis and responses to multiple hormones in sal1 are altered during rosette growth, suggesting a potential connection between SAL1-PAP stress retrograde pathway and hormonal signaling. We propose the SAL1-PAP pathway as a case study for integrating chloroplast retrograde signaling, light signaling and hormonal signaling in plant growth and morphogenesis.

Blue-Light Receptors for Optogenetics

Sensory photoreceptors underpin light-dependent adaptations of organismal physiology, development, and behavior in nature. Adapted for optogenetics, sensory photoreceptors become genetically encoded actuators and reporters to enable the noninvasive, spatiotemporally accurate and reversible control by light of cellular processes. Rooted in a mechanistic understanding of natural photoreceptors, artificial photoreceptors with customized light-gated function have been engineered that greatly expand the scope of optogenetics beyond the original application of light-controlled ion flow. As we survey presently, UV/blue-light-sensitive photoreceptors have particularly allowed optogenetics to transcend its initial neuroscience applications by unlocking numerous additional cellular processes and parameters for optogenetic intervention, including gene expression, DNA recombination, subcellular localization, cytoskeleton dynamics, intracellular protein stability, signal transduction cascades, apoptosis, and enzyme activity. The engineering of novel photoreceptors benefits from powerful and reusable design strategies, most importantly light-dependent protein association and (un)folding reactions. Additionally, modified versions of these same sensory photoreceptors serve as fluorescent proteins and generators of singlet oxygen, thereby further enriching the optogenetic toolkit. The available and upcoming UV/blue-light-sensitive actuators and reporters enable the detailed and quantitative interrogation of cellular signal networks and processes in increasingly more precise and illuminating manners.

Phot2-regulated relocation of NPH3 mediates phototropic response to high-intensity blue light in Arabidopsis thaliana

Two redundant blue-light receptors, known as phototropins (phot1 and phot2), influence a variety of physiological responses, including phototropism, chloroplast positioning, and stomatal opening in Arabidopsis thaliana. Whereas phot1 functions in both low- and high-intensity blue light (HBL), phot2 functions primarily in HBL. Here, we aimed to elucidate phot2-specific functions by screening for HBL-insensitive mutants among mutagenized Arabidopsis phot1 mutants. One of the resulting phot2 signaling associated (p2sa) double mutants, phot1 p2sa2, exhibited phototropic defects that could be restored by constitutively expressing NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3), indicating that P2SA2 was allelic to NPH3. It was observed that NPH3-GFP signal mainly localized to and clustered on the plasma membrane in darkness. This NPH3 clustering on the plasma membrane was not affected by mutations in genes encoding proteins that interact with NPH3, including PHOT1, PHOT2 and ROOT PHOTOTROPISM 2 (RPT2). However, the HBL irradiation-mediated release of NPH3 proteins into the cytoplasm was inhibited in phot1 mutants and enhanced in phot2 and rpt2-2 mutants. Furthermore, HBL-induced hypocotyl phototropism was enhanced in phot1 mutants and inhibited in the phot2 and rpt2-2 mutants. Our findings indicate that phot1 regulates the dissociation of NPH3 from the plasma membrane, whereas phot2 mediates the stabilization and relocation of NPH3 to the plasma membrane to acclimate to HBL.

A phosphorylation switch turns a positive regulator of phototropism into an inhibitor of the process

Phototropins are light-activated protein kinases, which contribute to photosynthesis optimization both through enhancement of photon absorption when light is limiting and avoidance responses in high light. This duality is in part endowed by the presence of phototropins with different photosensitivity (phot1 and phot2). Here we show that phot1, which senses low light to promote positive phototropism (growth towards the light), also limits the response in high light. This response depends in part on phot1-mediated phosphorylation of Phytochrome Kinase Substrate 4 (PKS4). This light-regulated phosphorylation switch changes PKS4 from a phototropism enhancer in low light to a factor limiting the process in high light. In such conditions phot1 and PKS4 phosphorylation prevent phototropic responses to shallow light gradients and limit phototropism in a natural high light environment. Hence, by modifying PKS4 activity in high light the phot1-PKS4 regulon enables appropriate physiological adaptations over a range of light intensities.

Light conditions modify plant growth and development via photoreceptors such as phototropins. Here the authors show that while phot1 promotes phototropism under low light, it can act to suppress phototropism in high-light environments through phosphorylation of PKS4.

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.

Coping with 'Dark Sides of the Sun' through Photoreceptor Signaling

Plants grow in constantly changing environments, including highly variable light intensities. Sunlight provides the energy that drives photosynthesis and is thus of the utmost importance for plant growth and the generation of oxygen, which the majority of life on Earth depends on. However, exposure to either insufficient or excess levels of light can have detrimental effects and cause light stress. Whereas exposure to insufficient light limits photosynthetic activity, resulting in 'energy starvation', exposure to excess light can damage the photosynthetic apparatus. Furthermore, strong sunlight is associated with high levels of potentially damaging UV-B radiation. Different classes of photoreceptors play important roles in coping with the negative aspects of sunlight, for which specific mechanisms are emerging that are reviewed here.

Growth-mediated plant movements: hidden in plain sight

While fast plant movements are spectacular but rare, almost all plants exhibit relatively slow, growth-mediated tropic movements that are key to their survival in the natural world. In this brief review, we discuss recent insights into the molecular mechanisms underlying phototropism, gravitropism, hydrotropism, and autostraightening. Careful molecular genetic and physiological studies have helped confirm the importance of lateral auxin gradients in gravitropic and phototropic responses. However, auxin signaling does not explain all tropisms: recent work has shown that abscisic acid signaling mediates root hydrotropism and has implicated mechanosensing in autostraightening, the organ straightening process recently modeled as a proprioceptive response. The interactions between distinct tropic signaling pathways and other internal and external sensory processes are also now being untangled.

Auxin-mediated developmental control in the moss Physcomitrella patens

The signalling molecule auxin regulates many fundamental aspects of growth and development in plants. We review and discuss what is known about auxin-regulated development in mosses, with special emphasis on the model species Physcomitrella patens. It is well established that mosses and other early diverging plants produce and respond to auxin. By sequencing the P. patens genome, it became clear that it encodes many core proteins important for auxin homeostasis, perception, and signalling, which have also been identified in flowering plants. This suggests that the auxin molecular network was present in the last common ancestor of flowering plants and mosses. Despite fundamental differences in their life cycles, key processes such as organ initiation and outgrowth, branching, tropic responses, as well as cell differentiation, division, and expansion appear to be regulated by auxin in the two lineages. This knowledge paves the way for studies aimed at a better understanding of the origin and evolution of auxin function and how auxin may have contributed to the evolution of land plants.

Functional Analysis of MAX2 in Phototropins-Mediated Cotyledon Flattening in Arabidopsis

Phototropins (phot1 and phot2) are blue-light receptors that control cotyledon flattening and positioning under strong light; however, their functional redundancy restricts our understanding of the specific roles of phot2. To identify the factors responsible for phot2-dependent cotyledon flattening and growth, we screened for light-insensitive mutants among mutagenized phot1 mutants in Arabidopsis thaliana. The double mutant phot1 lea1 (leaf expansion associated 1), which is defective in cotyledon flattening and positioning but not the phototropic response was selected. This mutant phenotype could be alleviated by constitutively expressing MORE AXILLARY GROWTH 2 (MAX2), indicating that LEA1 was allelic to MAX2. The max2 mutants (max2-2 and max2-3) are defective in cotyledon flattening, which is similar to that of the phot1 phot2 mutants. Moreover, the amounts of MAX2 transcripts are inhibited in leaves of phot1 mutant. However, the additional disruption of PHOT1 gene in max2-2 or max2-3 did not affect their phenotype, including MAX2-mediated inhibition of hypocotyl elongation. By contrast, phototropins-mediated hypocotyl phototropism was not regulated by MAX2. Together, these results suggest that cotyledon flattening was mediated by both phototropins and MAX2 signaling, but the relationship between two pathways need further study.

Molecular mechanisms and ecological function of far-red light signalling

Land plants possess the ability to sense and respond to far-red light (700-760 nm), which serves as an important environmental cue. Due to the nature of far-red light, it is not absorbed by chlorophyll and thus is enriched in canopy shade and will also penetrate deeper into soil than other visible wavelengths. Far-red light responses include regulation of seed germination, suppression of hypocotyl growth, induction of flowering and accumulation of anthocyanins, which depend on one member of the phytochrome photoreceptor family, phytochrome A (phyA). Here, we review the current understanding of the underlying molecular mechanisms of how plants sense far-red light through phyA and the physiological responses to this light quality. Light-activated phytochromes act on two primary pathways within the nucleus; suppression of the E3 ubiquitin ligase complex CUL4/DDB1^COP1/SPA and inactivation of the PHYTOCHROME INTERACTING FACTOR (PIF) family of bHLH transcription factors. These pathways integrate with other signal transduction pathways, including phytohormones, for tissue and developmental stage specific responses. Unlike other phytochromes that mediate red-light responses, phyA is transported from the cytoplasm to the nucleus in far-red light by the shuttle proteins FAR-RED ELONGATED HYPOCOTYL 1 (FHY1) and FHY1-LIKE (FHL). However, additional mechanisms must exist that shift the action of phyA to far-red light; current hypotheses are discussed.

Development of transgenic crops based on photo-biotechnology

The phenotypes associated with plant photomorphogenesis such as the suppressed shade avoidance response and de-etiolation offer the potential for significant enhancement of crop yields. Of many light signal transducers and transcription factors involved in the photomorphogenic responses of plants, this review focuses on the transgenic overexpression of the photoreceptor genes at the uppermost stream of the signalling events, particularly phytochromes, crytochromes and phototropins as the transgenes for the genetic engineering of crops with improved harvest yields. In promoting the harvest yields of crops, the photoreceptors mediate the light regulation of photosynthetically important genes, and the improved yields often come with the tolerance to abiotic stresses such as drought, salinity and heavy metal ions. As a genetic engineering approach, the term photo-biotechnology has been coined to convey the idea that the greater the photosynthetic efficiency that crop plants can be engineered to possess, the stronger the resistance to biotic and abiotic stresses. Development of GM crops based on photoreceptor transgenes (mainly phytochromes, crytochromes and phototropins) is reviewed with the proposal of photo-biotechnology that the photoreceptors mediate the light regulation of photosynthetically important genes, and the improved yields often come with the added benefits of crops' tolerance to environmental stresses.

Plant dual-specificity tyrosine phosphorylation-regulated kinase optimizes light-regulated growth and development in Arabidopsis

Light controls vegetative and reproductive development of plants. For a plant, sensing the light input properly ensures coordination with the ever-changing environment. Previously, we found that LIGHT-REGULATED WD1 (LWD1) and LWD2 regulate the circadian clock and photoperiodic flowering. Here, we identified Arabidopsis YET ANOTHER KINASE1 (AtYAK1), an evolutionarily conserved protein and a member of dual-specificity tyrosine phosphorylation-regulated kinases (DYRKs), as an interacting protein of LWDs. Our study revealed that AtYAK1 is an important regulator for various light responses, including the circadian clock, photomorphogenesis and reproductive development. AtYAK1 could antagonize the function of LWDs in regulating the circadian clock and photoperiodic flowering. By examining phenotypes of atyak1, we found that AtYAK1 regulated light-induced period-length shortening and photomorphogenic development. Moreover, AtYAK1 mediated plant fertility especially under inferior light conditions including low light and short-day length. This study discloses a new regulator connecting environmental light to plant growth.

Plant Science View on Biohybrid Development

Biohybrid consists of a living organism or cell and at least one engineered component. Designing robot-plant biohybrids is a great challenge: it requires interdisciplinary reconsideration of capabilities intimate specific to the biology of plants. Envisioned advances should improve agricultural/horticultural/social practice and could open new directions in utilization of plants by humans. Proper biohybrid cooperation depends upon effective communication. During evolution, plants developed many ways to communicate with each other, with animals, and with microorganisms. The most notable examples are: the use of phytohormones, rapid long-distance signaling, gravity, and light perception. These processes can now be intentionally re-shaped to establish plant-robot communication. In this article, we focus on plants physiological and molecular processes that could be used in bio-hybrids. We show phototropism and biomechanics as promising ways of effective communication, resulting in an alteration in plant architecture, and discuss the specifics of plants anatomy, physiology and development with regards to the bio-hybrids. Moreover, we discuss ways how robots could influence plants growth and development and present aims, ideas, and realized projects of plant-robot biohybrids.

Functional characterization of a constitutively active kinase variant of Arabidopsis phototropin 1

Phototropins (phots) are plasma membrane–associated serine/threonine kinases that coordinate a range of processes linked to optimizing photosynthetic efficiency in plants. These photoreceptors contain two light-, oxygen-, or voltage-sensing (LOV) domains within their N terminus, with each binding one molecule of flavin mononucleotide as a UV/blue light–absorbing chromophore. Although phots contain two LOV domains, light-induced activation of the C-terminal kinase domain and subsequent receptor autophosphorylation is controlled primarily by the A′α-LOV2-Jα photosensory module. Mutations that disrupt interactions between the LOV2 core and its flanking helical segments can uncouple this mode of light regulation. However, the impact of these mutations on phot function in Arabidopsis has not been explored. Here we report that histidine substitution of Arg-472 located within the A′α-helix of Arabidopsis phot1 constitutively activates phot1 kinase activity in vitro without affecting LOV2 photochemistry. Expression analysis of phot1 R472H in the phot-deficient mutant confirmed that it is autophosphorylated in darkness in vivo but unable to initiate phot1 signaling in the absence of light. Instead, we found that phot1 R472H is poorly functional under low-light conditions but can restore phototropism, chloroplast accumulation, stomatal opening, and leaf positioning and expansion at higher light intensities. Our findings suggest that Arabidopsis can adapt to the elevated phosphorylation status of the phot1 R472H mutant in part by reducing its stability, whereas the activity of the mutant under high-light conditions can be attributed to additional increases in LOV2-mediated photoreceptor autophosphorylation.

A New Facet of Vitamin B12: Gene Regulation by Cobalamin-Based Photoreceptors

Living organisms sense and respond to light, a crucial environmental factor, using photoreceptors, which rely on bound chromophores such as retinal, flavins, or linear tetrapyrroles for light sensing. The discovery of photoreceptors that sense light using 5'-deoxyadenosylcobalamin, a form of vitamin B12 that is best known as an enzyme cofactor, has expanded the number of known photoreceptor families and unveiled a new biological role of this vitamin. The prototype of these B12-dependent photoreceptors, the transcriptional repressor CarH, is widespread in bacteria and mediates light-dependent gene regulation in a photoprotective cellular response. CarH activity as a transcription factor relies on the modulation of its oligomeric state by 5'-deoxyadenosylcobalamin and light. This review surveys current knowledge about these B12-dependent photoreceptors, their distribution and mode of action, and the structural and photochemical basis of how they orchestrate signal transduction and control gene expression.

FRUITFULL controls SAUR10 expression and regulates Arabidopsis growth and architecture

MADS-domain transcription factors are well known for their roles in plant development and regulate sets of downstream genes that have been uncovered by high-throughput analyses. A considerable number of these targets are predicted to function in hormone responses or responses to environmental stimuli, suggesting that there is a close link between developmental and environmental regulators of plant growth and development. Here, we show that the Arabidopsis MADS-domain factor FRUITFULL (FUL) executes several functions in addition to its noted role in fruit development. Among the direct targets of FUL, we identified SMALL AUXIN UPREGULATED RNA 10 (SAUR10), a growth regulator that is highly induced by a combination of auxin and brassinosteroids and in response to reduced R:FR light. Interestingly, we discovered that SAUR10 is repressed by FUL in stems and inflorescence branches. SAUR10 is specifically expressed at the abaxial side of these branches and this localized activity is influenced by hormones, light conditions and by FUL, which has an effect on branch angle. Furthermore, we identified a number of other genes involved in hormone pathways and light signalling as direct targets of FUL in the stem, demonstrating a connection between developmentally and environmentally regulated growth programs.

Temporal and spatial changes in gene expression, metabolite accumulation and phytohormone content in rice seedlings grown under drought stress conditions

In order to analyze the molecular mechanisms underlying the responses of plants to different levels of drought stress, we developed a soil matric potential (SMP)-based irrigation system that precisely controls soil moisture. Using this system, rice seedlings were grown under three different drought levels, denoted Md1, Md2 and Md3, with SMP values set to -9.8, -31.0 and -309.9 kPa, respectively. Although the Md1 treatment did not alter the visible phenotype, the Md2 treatment caused stomatal closure and shoot growth retardation (SGR). The Md3 treatment markedly induced SGR, without inhibition of photosynthesis. More severe drought (Sds) treatment, under which irrigation was terminated, resulted in the wilting of leaves and inhibition of photosynthesis. Metabolome analysis revealed the accumulation of primary sugars under Md3 and Sds and of most amino acids under Sds. The starch content was increased under Md3 and decreased under Sds. Transcriptome data showed that the expression profiles of associated genes supported the observed changes in photosynthesis and metabolites, suggesting that the time lag from SGR to inhibition of photosynthesis might lead to the accumulation of photosynthates under Md3, which can be used as osmolytes under Sds. To gain further insight into the observed SGR, transcriptome and hormonome analyses were performed in specific tissues. The results showed specific decreases in indole-3-acetic acid (IAA) and cytokinin levels in Md2-, Md3- and Sds-treated shoot bases, though the expression levels of hormone metabolism-related genes were not reflected in IAA and cytokinin contents. These observations suggest that drought stress affects the distribution or degradation of cytokinin and IAA molecules.

Differential TOR activation and cell proliferation in Arabidopsis root and shoot apexes

The developmental plasticity of plants relies on the remarkable ability of the meristems to integrate nutrient and energy availability with environmental signals. Meristems in root and shoot apexes share highly similar molecular players but are spatially separated by soil. Whether and how these two meristematic tissues have differential activation requirements for local nutrient, hormone, and environmental cues (e.g., light) remain enigmatic in photosynthetic plants. Here, we report that the activation of root and shoot apexes relies on distinct glucose and light signals. Glucose energy signaling is sufficient to activate target of rapamycin (TOR) kinase in root apexes. In contrast, both the glucose and light signals are required for TOR activation in shoot apexes. Strikingly, exogenously applied auxin is able to replace light to activate TOR in shoot apexes and promote true leaf development. A relatively low concentration of auxin in the shoot and high concentration of auxin in the root might be responsible for this distinctive light requirement in root and shoot apexes, because light is required to promote auxin biosynthesis in the shoot. Furthermore, we reveal that the small GTPase Rho-related protein 2 (ROP2) transduces light-auxin signal to activate TOR by direct interaction, which, in turn, promotes transcription factors E2Fa,b for activating cell cycle genes in shoot apexes. Consistently, constitutively activated ROP2 plants stimulate TOR in the shoot apex and cause true leaf development even without light. Together, our findings establish a pivotal hub role of TOR signaling in integrating different environmental signals to regulate distinct developmental transition and growth in the shoot and root.

Functional characterization of Arabidopsis phototropin 1 in the hypocotyl apex

Phototropin (phot1) is a blue light-activated plasma membrane-associated kinase that acts as the principal photoreceptor for shoot phototropism in Arabidopsis in conjunction with the signalling component Non-Phototropic Hypocotyl 3 (NPH3). PHOT1 is uniformly expressed throughout the Arabidopsis hypocotyl, yet decapitation experiments have localized the site of light perception to the upper hypocotyl. This prompted us to investigate in more detail the functional role of the hypocotyl apex, and the regions surrounding it, in establishing phototropism. We used a non-invasive approach where PHOT1-GFP (P1-GFP) expression was targeted to the hypocotyl apex of the phot-deficient mutant using the promoters of CUP-SHAPED COTYLEDON 3 (CUC3) and AINTEGUMENTA (ANT). Expression of CUC3::P1-GFP was clearly visible at the hypocotyl apex, with weaker expression in the cotyledons, whereas ANT::P1-GFP was specifically targeted to the developing leaves. Both lines showed impaired curvature to 0.005 μmol m^-2  sec^-1 unilateral blue light, indicating that regions below the apical meristem are necessary for phototropism. Curvature was however apparent at higher fluence rates. Moreover, CUC3::P1-GFP partially or fully complemented petiole positioning, leaf flattening and chloroplast accumulation, but not stomatal opening. Yet, tissue analysis of NPH3 de-phosphorylation showed that CUC3::P1-GFP and ANT::P1-GFP mis-express very low levels of phot1 that likely account for this responsiveness. Our spatial targeting approach therefore excludes the hypocotyl apex as the site for light perception for phototropism and shows that phot1-mediated NPH3 de-phosphorylation is tissue autonomous and occurs more prominently in the basal hypocotyl.

Leaf structural traits of tropical woody species resistant to cement dust

Cement industries located nearby limestone outcrops in Brazil have contributed to the coating of cement dust over native plant species. However, little is known about the extent of the response of tropical woody plants to such environmental pollutant particularly during the first stages of plant development and establishment. This work focused on the investigation of possible alterations in leaf structural and ultrastructural traits of 5-month-old Guazuma ulmifolia Lam. (Malvaceae), 6-month-old Myracrodruon urundeuva Allemão (Anacardiaceae), and 9-month-old Trichilia hirta L. (Meliaceae) challenged superficially with cement dust during new leaf development. Leaf surface of plants, the soil or both (leaf plus soil), were treated (or not) for 60 days, under controlled conditions, with cement dust at 2.5 or 5.0 mg cm(-2). After exposure, no significant structural changes were observed in plant leaves. Also, no plant death was recorded by the end of the experiment. There was also some evidence of localized leaf necrosis in G. ulmifolia and T. hirta, leaf curling in M. urundeuva and T. hirta, and bulges formation on epidermal surface of T. hirta, after cement dust contact with plant shoots. All species studied exhibited stomata obliteration while T. hirta, in particular, presented early leaf abscission, changes in cellular relief, and organization and content of midrib cells. No significant ultrastructural alterations were detected under the experimental conditions studied. Indeed, mesophyll cells presented plastids with intact membrane systems. The high plant survival rates, together with mild morphoanatomic traits alterations in leaves, indicate that G. ulmifolia is more resistant to cement dust pollutant, followed by M. urundeuva and T. hirta. Thus, the three plant species are promising for being used to revegetate areas impacted by cement industries activities.

SPA proteins: SPAnning the gap between visible light and gene expression

In this review we focus on the role of SPA proteins in light signalling and discuss different aspects, including molecular mechanisms, specificity, and evolution. The ability of plants to perceive and respond to their environment is key to their survival under ever-changing conditions. The abiotic factor light is of particular importance for plants. Light provides plants energy for carbon fixation through photosynthesis, but also is a source of information for the adaptation of growth and development to the environment. Cryptochromes and phytochromes are major photoreceptors involved in control of developmental decisions in response to light cues, including seed germination, seedling de-etiolation, and induction of flowering. The SPA protein family acts in complex with the E3 ubiquitin ligase COP1 to target positive regulators of light responses for degradation by the 26S proteasome to suppress photomorphogenic development in darkness. Light-activated cryptochromes and phytochromes both repress the function of COP1, allowing accumulation of positive photomorphogenic factors in light. In this review, we highlight the role of the SPA proteins in this process and discuss recent advances in understanding how SPAs link light-activation of photoreceptors and downstream signaling.

Regulation of polar auxin transport by protein and lipid kinases

The directional transport of auxin, known as polar auxin transport (PAT), allows asymmetric distribution of this hormone in different cells and tissues. This system creates local auxin maxima, minima, and gradients that are instrumental in both organ initiation and shape determination. As such, PAT is crucial for all aspects of plant development but also for environmental interaction, notably in shaping plant architecture to its environment. Cell to cell auxin transport is mediated by a network of auxin carriers that are regulated at the transcriptional and post-translational levels. Here we review our current knowledge on some aspects of the 'non-genomic' regulation of auxin transport, placing an emphasis on how phosphorylation by protein and lipid kinases controls the polarity, intracellular trafficking, stability, and activity of auxin carriers. We describe the role of several AGC kinases, including PINOID, D6PK, and the blue light photoreceptor phot1, in phosphorylating auxin carriers from the PIN and ABCB families. We also highlight the function of some receptor-like kinases (RLKs) and two-component histidine kinase receptors in PAT, noting that there are probably RLKs involved in co-ordinating auxin distribution yet to be discovered. In addition, we describe the emerging role of phospholipid phosphorylation in polarity establishment and intracellular trafficking of PIN proteins. We outline these various phosphorylation mechanisms in the context of primary and lateral root development, leaf cell shape acquisition, as well as root gravitropism and shoot phototropism.

Light-Mediated Hormonal Regulation of Plant Growth and Development

Light is crucial for plant life, and perception of the light environment dictates plant growth, morphology, and developmental changes. Such adjustments in growth and development in response to light conditions are often established through changes in hormone levels and signaling. This review discusses examples of light-regulated processes throughout a plant's life cycle for which it is known how light signals lead to hormonal regulation. Light acts as an important developmental switch in germination, photomorphogenesis, and transition to flowering, and light cues are essential to ensure light capture through architectural changes during phototropism and the shade avoidance response. In describing well-established links between light perception and hormonal changes, we aim to give insight into the mechanisms that enable plants to thrive in variable light environments.

Evolutionary aspects of plant photoreceptors

Plant photoreceptors link environmental light cues with physiological responses, determining how individual plants complete their life cycles. Structural and functional evolution of photoreceptors has co-occurred as plants diversified and faced the challenge of new light environments, during the transition of plants to land and as substantial plant canopies evolved. Large-scale comparative sequencing projects allow us for the first time to document photoreceptor evolution in understudied clades, revealing some surprises. Here we review recent progress in evolutionary studies of three photoreceptor families: phytochromes, phototropins and neochromes.

Organ-specific regulation of growth-defense tradeoffs by plants

Plants grow while also defending themselves against phylogenetically unrelated pathogens. Because defense and growth are both costly programs, a plant's success in colonizing resource-scarce environments requires tradeoffs between the two. Here, we summarize efforts aimed at understanding how plants use iterative tradeoffs to modulate differential organ growth when defenses are elicited. First, we focus on shoots to illustrate how light, in conjunction with the growth hormone gibberellin (GA) and the defense hormone jasmonic acid (JA), act to finely regulate defense and growth programs in this organ. Second, we expand on the regulation of growth-defense trade-offs in the root, a less well-studied topic despite the critical role of this organ in acquiring resources in an environment deeply entrenched with disparate populations of microbes.

The impact of abiotic factors on cellulose synthesis

As sessile organisms, plants require mechanisms to sense and respond to changes in their environment, including both biotic and abiotic factors. One of the most common plant adaptations to environmental changes is differential regulation of growth, which results in growth either away from adverse conditions or towards more favorable conditions. As cell walls shape plant growth, this differential growth response must be accompanied by alterations to the plant cell wall. Here, we review the impact of four abiotic factors (osmotic conditions, ionic stress, light, and temperature) on the synthesis of cellulose, an important component of the plant cell wall. Understanding how different abiotic factors influence cellulose production and addressing key questions that remain in this field can provide crucial information to cope with the need for increased crop production under the mounting pressures of a growing world population and global climate change.

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.

How and why do root apices sense light under the soil surface?

Light can penetrate several centimeters below the soil surface. Growth, development and behavior of plant roots are markedly affected by light despite their underground lifestyle. Early studies provided contrasting information on the spatial and temporal distribution of light-sensing cells in the apical region of root apex and discussed the physiological roles of plant hormones in root responses to light. Recent biological and microscopic advances have improved our understanding of the processes involved in the sensing and transduction of light signals, resulting in subsequent physiological and behavioral responses in growing root apices. Here, we review current knowledge of cellular distributions of photoreceptors and their signal transduction pathways in diverse root tissues and root apex zones. We are discussing also the roles of auxin transporters in roots exposed to light, as well as interactions of light signal perceptions with sensing of other environmental factors relevant to plant roots.