Mechanism of the Pulvinus-Driven Leaf Movement: An Overview.

Leaf movement is a manifestation of plant response to the changing internal and external environment, aiming to optimize plant growth and development. Leaf movement is usually driven by a specialized motor organ, the pulvinus, and this movement is associated with different changes in volume and expansion on the two sides of the pulvinus. Blue light, auxin, GA, H+-ATPase, K+, Cl-, Ca2+, actin, and aquaporin collectively influence the changes in water flux in the tissue of the extensor and flexor of the pulvinus to establish a turgor pressure difference, thereby controlling leaf movement. However, how these factors regulate the multicellular motility of the pulvinus tissues in a species remains obscure. In addition, model plants such as Medicago truncatula, Mimosa pudica, and Samanea saman have been used to study pulvinus-driven leaf movement, showing a similarity in their pulvinus movement mechanisms. In this review, we summarize past research findings from the three model plants, and using Medicago truncatula as an example, suggest that genes regulating pulvinus movement are also involved in regulating plant growth and development. We also propose a model in which the variation of ion flux and water flux are critical steps to pulvinus movement and highlight questions for future research.

Nyctinastic movement in legumes: Developmental mechanisms, factors and biological significance.

In legumes, a common phenomenon known as nyctinastic movement is observed. This movement involves the horizontal expansion of leaves during the day and relative vertical closure at night. Nyctinastic movement is driven by the pulvinus, which consists of flexor and extensor motor cells. The turgor pressure difference between these two cell types generates a driving force for the bending and deformation of the pulvinus. This review focuses on the developmental mechanisms of the pulvinus, the factors affecting nyctinastic movement, and the biological significance of this phenomenon in legumes, thus providing a reference for further research on nyctinastic movement.

An ecological perspective on water shedding from leaves.

Water shedding from leaves is a complex process depending on multiple leaf traits interacting with rain, wind, and air humidity, and with the entire plant and surrounding vegetation. Here, we synthesize current knowledge of the physics of water shedding with implications for plant physiology and ecology. We argue that the drop retention angle is a more meaningful parameter to characterize the water-shedding capacity of leaves than the commonly measured static contact angle. The understanding of the mechanics of water shedding is largely derived from laboratory experiments on artificial rather than natural surfaces, often on individual aspects such as surface wettability or drop impacts. In contrast, field studies attempting to identify the adaptive value of leaf traits linked to water shedding are largely correlative in nature, with inconclusive results. We make a strong case for taking the hypothesis-driven experimental approach of biomechanical laboratory studies into a real-world field setting to gain a comprehensive understanding of leaf water shedding in a whole-plant ecological and evolutionary context.

Multidimensional Gene Regulatory Landscape of Motor Organ Pulvinus in the Model Legume Medicago truncatula.

Nyctinastic leaf movement of Fabaceae is driven by the tiny motor organ pulvinus located at the base of the leaf or leaflet. Despite the increased understanding of the essential role of ELONGATED PETIOLULE1 (ELP1)/PETIOLE LIKE PULVINUS (PLP) orthologs in determining pulvinus identity in legumes, key regulatory components and molecular mechanisms underlying this movement remain largely unclear. Here, we used WT pulvinus and the equivalent tissue in the elp1 mutant to carry out transcriptome and proteome experiments. The omics data indicated that there are multiple cell biological processes altered at the gene expression and protein abundance level during the pulvinus development. In addition, comparative analysis of different leaf tissues provided clues to illuminate the possible common primordium between pulvinus and petiole, as well as the function of ELP1. Furthermore, the auxin pathway, cell wall composition and chloroplast distribution were altered in elp1 mutants, verifying their important roles in pulvinus development. This study provides a comprehensive insight into the motor organ of the model legume Medicago truncatula and further supplies a rich dataset to facilitate the identification of novel players involved in nyctinastic movement.

Mechanics of Reversible Deformation during Leaf Movement and Regulation of Pulvinus Development in Legumes.

Plant cell deformation is a mechanical process that is driven by differences in the osmotic pressure inside and outside of the cell and is influenced by cell wall properties. Legume leaf movements result from reversible deformation of pulvinar motor cells. Reversible cell deformation is an elastic process distinct from the irreversible cell growth of developing organs. Here, we begin with a review of the basic mathematics of cell volume changes, cell wall function, and the mechanics of bending deformation at a macro scale. Next, we summarize the findings of recent molecular genetic studies of pulvinar development. We then review the mechanisms of the adaxial/abaxial patterning because pulvinar bending deformation depends on the differences in mechanical properties and physiological responses of motor cells on the adaxial versus abaxial sides of the pulvinus. Intriguingly, pulvini simultaneously encompass morphological symmetry and functional asymmetry along the adaxial/abaxial axis. This review provides an introduction to leaf movement and reversible deformation from the perspective of mechanics and molecular genetics.

The Conserved and Specific Roles of the LUX ARRHYTHMO in Circadian Clock and Nodulation.

LUX ARRHYTHMO (LUX) plays a key role in circadian rhythms and flowering. Here, we identified the MtLUX gene which is the putative ortholog of LUX in Medicago truncatula. The roles of MtLUX, in both the nodulation belowground and leaf movement aboveground, were investigated by characterizing a loss-of-function mtlux mutant. MtLUX was required for the control of flowering time under both long-day and short-day conditions. Further investigations showed that the early flowering in the mtlux mutant was correlated with the elevated expression level of the MtFTa1 gene but in a CO-like independent manner. MtLUX played a conserved role in the regulatory interactions with MtLHY, MtTOC1, and MtPRR genes, which is similar to those in other species. Meanwhile, the unexpected functions of MtLUX were revealed in nodule formation and nyctinastic leaf movement, probably through the indirect regulation in MtLHY. Its participation in nodulation is of interest in the context of functional conservation and the neo-functionalization of the products of LUX orthologs.

The geometry of the compound leaf plays a significant role in the leaf movement of Medicago truncatula modulated by mtdwarf4a.

In most legumes, two typical features found in leaves are diverse compound forms and the pulvinus-driven nyctinastic movement. Many genes have been identified for leaf-shape determination, but the underlying nature of leaf movement as well as its association with the compound form remains largely unknown. Using forward-genetic screening and whole-genome resequencing, we found that two allelic mutants of Medicago truncatula with unclosed leaflets at night were impaired in MtDWARF4A (MtDWF4A), a gene encoding a cytochrome P450 protein orthologous to Arabidopsis DWARF4. The mtdwf4a mutant also had a mild brassinosteroid (BR)-deficient phenotype bearing pulvini without significant deficiency in organ identity. Both mtdwf4a and dwf4 could be fully rescued by MtDWF4A, and mtdwf4a could close their leaflets at night after the application of exogenous 24-epi-BL. Surgical experiments and genetic analysis of double mutants revealed that the failure to exhibit leaf movement in mtdwf4a is a consequence of the physical obstruction of the overlapping leaflet laminae, suggesting a proper geometry of leaflets is important for their movement in M. truncatula. These observations provide a novel insight into the nyctinastic movement of compound leaves, shedding light on the importance of open space for organ movements in plants.

Neighbor detection at the leaf tip adaptively regulates upward leaf movement through spatial auxin dynamics.

Vegetation stands have a heterogeneous distribution of light quality, including the red/far-red light ratio (R/FR) that informs plants about proximity of neighbors. Adequate responses to changes in R/FR are important for competitive success. How the detection and response to R/FR are spatially linked and how this spatial coordination between detection and response affects plant performance remains unresolved. We show in Arabidopsis thaliana and Brassica nigra that localized FR enrichment at the lamina tip induces upward leaf movement (hyponasty) from the petiole base. Using a combination of organ-level transcriptome analysis, molecular reporters, and physiology, we show that PIF-dependent spatial auxin dynamics are key to this remote response to localized FR enrichment. Using computational 3D modeling, we show that remote signaling of R/FR for hyponasty has an adaptive advantage over local signaling in the petiole, because it optimizes the timing of leaf movement in response to neighbors and prevents hyponasty caused by self-shading.

Photosynthetic opportunity cost and energetic cost of a rapid leaf closure behavior in Mimosa pudica.

PREMISE OF THE STUDY: The rapid leaf movement of Mimosa pudica is expected to be costly because of energetic trade-offs with other processes such as growth and reproduction. Here, we assess the photosynthetic opportunity cost and energetic cost of the unique leaf closing behavior of M. pudica. METHODS: In the greenhouse, we employed novel touch-stimulation machines to expose plants to one of three treatments: (1) untouched control plants; (2) plants touch-stimulated to close their leaves during the day to incur energetic costs associated with leaf movement and reduced photosynthesis; (3) plants touched at night to assess the effects of touch alone. M. pudica is nyctinastic and closes its leaves at night; thus, touching at night does not impart additional costs. We directly assessed costs by comparing physical traits. Leaf re-opening response was measured to assess the potential for plant behavioral plasticity to impact photosynthetic opportunity costs. KEY RESULTS: The cost of rapid leaf closure behavior was expressed as a 47% reduction in reproductive biomass accounting for the effect of touch. Touch itself changed physical traits such as biomass, with touched plants being generally bigger. Plants touched at night re-opened their leaflets 26% quicker than plants touched during the day. CONCLUSIONS: We reason that the reproductive allocation costs incurred by M. pudica can be attributed to a combination of photosynthetic opportunity cost and the energetic cost associated with increased stimulation of leaf movement and that behavioral plasticity has the potential to alter photosynthetic opportunity costs.

High-resolution time-resolved imaging of in vitro Arabidopsis rosette growth.

Although quantitative characterization of growth phenotypes is of key importance for the understanding of essential networks driving plant growth, the majority of growth-related genes are still being identified based on qualitative visual observations and/or single-endpoint quantitative measurements. We developed an in vitro growth imaging system (IGIS) to perform time-resolved analysis of rosette growth. In this system, Arabidopsis plants are grown in Petri dishes mounted on a rotating disk, and images of each plate are taken on an hourly basis. Automated image analysis was developed in order to obtain several growth-related parameters, such as projected rosette area, rosette relative growth rate, compactness and stockiness, over time. To illustrate the use of the platform and the resulting data, we present the results for the growth response of Col-0 plants subjected to three mild stress conditions. Although the reduction in rosette area was relatively similar at 19 days after stratification, the time-lapse analysis demonstrated that plants react differently to salt, osmotic and oxidative stress. The rosette area was altered at various time points during development, and leaf movement and shape parameters were also affected differently. We also used the IGIS to analyze in detail the growth behavior of mutants with enhanced leaf size. Analysis of several growth-related parameters over time in these mutants revealed several specificities in growth behavior, underlining the high complexity of leaf growth coordination. These results demonstrate that time-resolved imaging of in vitro rosette growth generates a better understanding of growth phenotypes than endpoint measurements.

Sensitive Timing: A Reappraisal of Chronobiology's Foundational Texts.

The origin of experimental chronobiology can be traced to observations made in the 18th and 19th centuries on the sensitive plant Mimosa, which were described in two seminal reports: Jean-Jacques d'Ortous de Mairan's "Observation Botanique" (A Botanical Observation) and Augustin Pyramus de Candolle's "Du sommeil des feuilles" (On the sleep of leaves). Both report observations of the striking daily closing and opening of Mimosa leaves in controlled environments. This review presents translations of both texts with the aim of staying as faithful as possible to the original French texts. We also present the historical context in which these texts were written and link them to subsequent experiments that aimed at testing the veracity of their central conclusions. In particular, we definitely establish that Mairan himself presented his work to the French Royal Academy of Sciences, while the published report of his observation was authored by Fontenelle, the Secretary of the Academy. In addition, we offer a translation of Mairan's own presentation, based on the hand-written minutes of the academy. Finally, we discuss the decades of work on plant rhythms that laid the foundation for modern experimental chronobiology, including translations and discussion of the insightful and prescient reports by Charles François de Cisternay Dufay, Henri Louis Duhamel du Monceau, Johann Gottfried Zinn, and Wilhelm Pfeffer, which describe their efforts to reproduce and extend Mairan's pioneering observations.

An outward-rectifying plant K+ channel SPORK2 exhibits temperature-sensitive ion-transport activity.

Temperature sensing is critical for the survival of living organisms.1,2 Thermosensitive transient receptor-potential (TRP) cation channels function as thermosensors in mammals.2,3,4,5,6 In contrast to animals, land plants lack TRP genes.7,8,9 Previous patch-clamp studies in plant cells suggested the presence of ion channels whose activities are related to temperature, implying the presence of TRP-like channels.10,11,12,13,14 However, the molecular entities of such temperature-sensitive ion channels were still unknown in land plants. In this study, we observed that the unique rainfall-induced leaf-folding movement of the legume tree Samanea saman15 was temperature-sensitive by using a rainfall-mimicking assay. Chilling-induced leaf folding in S. saman was shown to be related to the swelling of the motor cells16,17 at the base of the leaflet. This swelling suggested involvement of temperature-sensitive inactivation of K+ currents, independent of fluctuations in ion channel gene expression in motor cells. These findings led us to examine the temperature sensitivity of an outward-rectifying K+ channel, SPORK2, which was reported as an ion channel responsible for the nyctinastic (circadian-rhythmic) leaf movement of S. saman.18 We also discovered that SPORK2 exhibits temperature-sensitive K+ transport activity in the Xenopus oocyte expression system. Using chimeric channels, we showed that two domains of SPORK2 regulated the temperature sensitivity. Furthermore, heterologously expressed SPORK2 in Arabidopsis guard cells induced temperature-dependent stomatal closure. Therefore, SPORK2 is an ion channel in land plants with temperature-sensitive ion-transport activity that functions similarly to mammalian TRP channels. Our current findings advance the molecular understanding of temperature-sensing mechanisms in plants.

Local light signaling at the leaf tip drives remote differential petiole growth through auxin-gibberellin dynamics.

Although plants are immobile, many of their organs are flexible to move in response to environmental cues. In dense vegetation, plants detect neighbors through far-red light perception with their leaf tip. They respond remotely, with asymmetrical growth between the abaxial and adaxial sides of the leafstalk, the petiole. This results in upward movement that brings the leaf blades into better lit zones of the canopy. The plant hormone auxin is required for this response, but it is not understood how non-differential leaf tip-derived auxin can remotely regulate movement. Here, we show that remote signaling of far-red light promotes auxin accumulation in the abaxial petiole. This local auxin accumulation is facilitated by reinforcing an intrinsic directionality of the auxin transport protein PIN3 on the petiole endodermis, as visualized with a PIN3-GFP line. Using an auxin biosensor, we show that auxin accumulates in all cell layers from endodermis to epidermis in the abaxial petiole, upon far-red light signaling in the remote leaf tip. In the petiole, auxin elicits a response to both auxin itself as well as a second growth promoter; gibberellin. We show that this dual regulation is necessary for hyponastic leaf movement in response to light. Our data indicate that gibberellin is required to permit cell growth, whereas differential auxin accumulation determines which cells can grow. Our results reveal how plants can spatially relay information about neighbor proximity from their sensory leaf tips to the petiole base, thus driving adaptive growth.

Heterologous expression of flowering locus T promotes flowering but does not affect diurnal movement in the legume Lotus japonicus.

Flowering locus T (FT) is known to promote flowering in response to photoperiodic conditions and has recently been shown to contribute to other phenomenon, such as diurnal stomatal movement. In legumes, FTs are classified into three subtypes, though the role of each subtype is not well defined. It has been reported that when FT of Lotus japonicus (LjFT) is heterologously expressed in Arabidopsis, LjFT functions as a mobile florigen to promote flowering, similar to Arabidopsis FT (AtFT). In this study, we expressed AtFT in L. japonicus using the SUC2 promoter and showed that heterologous expression of AtFT was able to promote flowering in the plant. We also showed that AtFT expression does not affect stomatal closing nor nyctinastic leaf movement. These findings contribute to our understanding of flower development and have potential application to breeding or plant biotechnology.

Rhythmic Leaf and Cotyledon Movement Analysis.

The first descriptions of circadian rhythms were of the rhythmic leaf movements of plants. Rhythmic leaf movements offer a sensitive, noninvasive, nondestructive, and non-transgenic assay of plant circadian rhythms that can be readily automated, greatly facilitating genetic studies. Rhythmic leaf movement is particularly useful for the assessment of standing variation in clock function and can be readily applied to a diverse array of dicotyledonous plants, including both wild species and domesticated crops.

The Adaptive Power of Ammophila arenaria: Biomimetic Study, Systematic Observation, Parametric Design and Experimental Tests with Bimetal.

The aim of our study was to apply a biomimetic approach, inspired by the Ammophila arenaria. This organism possesses a reversible leaf opening and closing mechanism that responds to water and salt stress (hydronastic movement). We adopted a problem-based biomimetic methodology in three stages: (i) two observation studies; (ii) how to abstract and develop a parametric model to simulate the leaf movement; and (iii) experiments with bimetal, a smart material that curls up when heated. We added creases to the bimetal active layer in analogy to the position of bulliform cells. These cells determine the leaf-closing pattern. The experiments demonstrated that creases influence and can change the direction of the bimetal natural movement. Thus, it is possible to replicate the Ammophila arenaria leaf-rolling mechanism in response to temperature variation and solar radiation in the bimetal. In future works, we will be able to propose responsive facade solutions based on these results.

Circadian leaf movements facilitate overtopping of neighbors.

Many plants exhibit circadian clock-driven leaf movements whereby the leaves are raised during the day to achieve a relatively high angle during the evening, before lowering late in the night. Such leaf movements were first recorded over 2000 years ago but there is still much debate as to their purpose. We investigated whether such leaf movements within Arabidopsis, a ruderal rosette plant, can aid in overtopping leaves of neighboring plants. Wild type and circadian clock mutant plants were grown in an alternating grid system so that their leaves would meet as the plants grew. Experiments were performed using day lengths that matched the endogenous rhythm of either wild type or mutant. Plants grown in a day length shorter than their endogenous rhythm were consistently overtopped by plants which were in synchrony with the day night cycle, demonstrating a clear overtopping advantage resulting from circadian leaf movement rhythms. Furthermore, we found that this leaf overtopping as a result of correctly synchronized circadian leaf movements is additive to leaf overtopping due to shade avoidance. Curiously, this did not apply to plants grown in a day length longer than their endogenous period. Plants grown in a day length longer than their endogenous period were able to adapt their leaf rhythms and suffered no overtopping disadvantage. Crucially, our results show that, in a context-dependent manner, circadian clock-driven leaf movements in resonance with the external light/dark cycle can facilitate overtopping of the leaves of neighboring plants.

Diaheliotropic leaf movement enhances leaf photosynthetic capacity and photosynthetic light and nitrogen use efficiency via optimising nitrogen partitioning among photosynthetic components in cotton (Gossypium hirsutum L.).

Phototropic leaf movement of plants is an effective mechanism for adapting to light conditions. Light is the major driver of plant photosynthesis. Leaf N is also an important limiting factor on leaf photosynthetic potential. Cotton (Gossypium hirsutum L.) exhibits diaheliotropic leaf movement. Here, we compared the long-term photosynthetic acclimation of fixed leaves (restrained) and free leaves (allowed free movement) in cotton. The fixed leaves and free leaves were used for determination of PAR, leaf chlorophyll concentration, leaf N content and leaf gas exchange. The measurements were conducted under clear sky conditions at 0, 7, 15 and 30 days after treatment (DAT). The results showed that leaf N allocation and partitioning among different components of the photosynthetic apparatus were significantly affected by diaheliotropic leaf movement. Diaheliotropic leaf movement significantly increased light interception per unit leaf area, which in turn affected leaf mass per area (LMA), leaf N content (NA ) and leaf N allocation to photosynthesis (NP ). In addition, cotton leaves optimised leaf N allocation to the photosynthetic apparatus by adjusting leaf mass per area and NA in response to optimal light interception. In the presence of diaheliotropic leaf movement, cotton leaves optimised their structural tissue and photosynthetic characteristics, such as LMA, NA and leaf N allocation to photosynthesis, so that leaf photosynthetic capacity was maximised to improve the photosynthetic use efficiency of light and N under high light conditions.

Geographic Variation of Plant Circadian Clock Function in Natural and Agricultural Settings.

The increasing demand for improved agricultural production will require more efficient breeding for traits that maintain yield under heterogeneous environments. The internal circadian oscillator is essential for perceiving and coordinating environmental cues such as day length, temperature, and abiotic stress responses within physiological processes. To investigate the contribution of the circadian clock to local adaptability, we have analyzed circadian period by leaf movement in natural populations of Mimulus guttatus and domesticated cultivars of Glycine max. We detected consistent variation in circadian period along a latitudinal gradient in annual populations of the wild plant and the selectively bred crop, and this provides novel evidence of natural and artificial selection for circadian performance. These findings provide new support that the circadian clock acts as a central regulator of plant adaptability and further highlight the potential of applying circadian clock gene variation to marker-assisted breeding programs in crops.

The plant leaf movement analyzer (PALMA): a simple tool for the analysis of periodic cotyledon and leaf movement in Arabidopsis thaliana.

BACKGROUND: The analysis of circadian leaf movement rhythms is a simple yet effective method to study effects of treatments or gene mutations on the circadian clock of plants. Currently, leaf movements are analysed using time lapse photography and subsequent bioinformatics analyses of leaf movements. Programs that are used for this purpose either are able to perform one function (i.e. leaf tip detection or rhythm analysis) or their function is limited to specific computational environments. We developed a leaf movement analysis tool-PALMA-that works in command line and combines image extraction with rhythm analysis using Fast Fourier transformation and non-linear least squares fitting. RESULTS: We validated PALMA in both simulated time series and in experiments using the known short period mutant sensitivity to red light reduced 1 (srr1-1). We compared PALMA with two established leaf movement analysis tools and found it to perform equally well. Finally, we tested the effect of reduced iron conditions on the leaf movement rhythms of wild type plants. Here, we found that PALMA successfully detected period lengthening under reduced iron conditions. CONCLUSIONS: PALMA correctly estimated the period of both simulated and real-life leaf movement experiments. As a platform-independent console-program that unites both functions needed for the analysis of circadian leaf movements it is a valid alternative to existing leaf movement analysis tools.