Amyloplasts and vacuolar membrane dynamics in the living graviperceptive cell of the Arabidopsis inflorescence stem

Cellular and physiological functions of SGR family in gravitropic response in higher plants

BACKGROUND

In plants, gravity directs bidirectional growth; it specifies upward growth of shoots and downward growth of roots. Due to gravity, roots establish robust anchorage and shoot, which enables to photosynthesize. It sets optimum posture and develops plant architecture to efficiently use resources like water, nutrients, CO2, and gaseous exchange. Hence, gravitropism is crucial for crop productivity as well as for the growth of plants in challenging climate. Some SGR members are known to affect tiller and shoot angle, organ size, and inflorescence stem in plants.

AIM OF REVIEW

Although the SHOOT GRAVITROPISM (SGR) family plays a key role in regulating the fate of shoot gravitropism, little is known about its function compared to other proteins involved in gravity response in plant cells and tissues. Moreover, less information on the SGR family's physiological activities and biochemical responses in shoot gravitropism is available. This review scrutinizes and highlights the recent developments in shoot gravitropism and provides an outlook for future crop development, multi-application scenarios, and translational research to improve agricultural productivity.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Plants have evolved multiple gene families specialized in gravitropic responses, of which the SGR family is highly significant. The SGR family regulates the plant's gravity response by regulating specific physiological and biochemical processes such as transcription, cell division, amyloplast sedimentation, endodermis development, and vacuole formation. Here, we analyze the latest discoveries in shoot gravitropism with particular attention to SGR proteins in plant cell biology, cellular physiology, and homeostasis. Plant cells detect gravity signals by sedimentation of amyloplast (starch granules) in the direction of gravity, and the signaling cascade begins. Gravity sensing, signaling, and auxin redistribution (organ curvature) are the three components of plant gravitropism. Eventually, we focus on the role of multiple SGR genes in shoot and present a complete update on the participation of SGR family members in gravity.

Shoot gravitropism and organ straightening cooperate to arrive at a mechanically favorable shape in Arabidopsis

Gravitropism is the plant organ bending in response to gravity, while a straightening mechanism prevents bending beyond the gravitropic set-point angle. The promotion and prevention of bending occur simultaneously around the inflorescence stem tip. How these two opposing forces work together and what part of the stem they affect are unknown. To understand the mechanical forces involved, we rotated wild type and organ-straightening-deficient mutant (myosin xif xik) Arabidopsis plants to a horizontal position to initiate bending. The mutant stems started to bend before the wild-type stems, which led us to hypothesize that the force preventing bending was weaker in mutant. We modeled the wild-type and mutant stems as elastic rods, and evaluated two parameters: an organ-angle-dependent gravitropic-responsive parameter (β) and an organ-curvature-dependent proprioceptive-responsive parameter (γ). Our model showed that these two parameters were lower in mutant than in wild type, implying that, unexpectedly, both promotion and prevention of bending are weak in mutant. Subsequently, finite element method simulations revealed that the compressive stress in the middle of the stem was significantly lower in wild type than in mutant. The results of this study show that myosin-XIk-and-XIf-dependent organ straightening adjusts the stress distribution to achieve a mechanically favorable shape.

Gravity sensing and responses in the coordination of the shoot gravitropic setpoint angle

Gravity is one of the fundamental environmental cues that affect plant development. Indeed, the plant architecture in the shoots and roots is modulated by gravity. Stems grow vertically upward, whereas lateral organs, such as the lateral branches in shoots, tend to grow at a specific angle according to a gravity vector known as the gravitropic setpoint angle (GSA). During this process, gravity is sensed in specialised gravity-sensing cells named statocytes, which convert gravity information into biochemical signals, leading to asymmetric auxin distribution and driving asymmetric cell division/expansion in the organs to achieve gravitropism. As a hypothetical offset mechanism against gravitropism to determine the GSA, the anti-gravitropic offset (AGO) has been proposed. According to this concept, the GSA is a balance of two antagonistic growth components, that is gravitropism and the AGO. Although the nature of the AGO has not been clarified, studies have suggested that gravitropism and the AGO share a common gravity-sensing mechanism in statocytes. This review discusses the molecular mechanisms underlying gravitropism as well as the hypothetical AGO in the control of the GSA.

Are cyclic plant and animal behaviours driven by gravimetric mechanical forces?

The celestial mechanics of the Sun, Moon, and Earth dominate the variations in gravitational force that all matter, live or inert, experiences on Earth. Expressed as gravimetric tides, these variations are pervasive and have forever been part of the physical ecology with which organisms evolved. Here, we first offer a brief review of previously proposed explanations that gravimetric tides constitute a tangible and potent force shaping the rhythmic activities of organisms. Through meta-analysis, we then interrogate data from three study cases and show the close association between the omnipresent gravimetric tides and cyclic activity. As exemplified by free-running cyclic locomotor activity in isopods, reproductive effort in coral, and modulation of growth in seedlings, biological rhythms coincide with temporal patterns of the local gravimetric tide. These data reveal that, in the presumed absence of rhythmic cues such as light and temperature, local gravimetric tide is sufficient to entrain cyclic behaviour. The present evidence thus questions the phenomenological significance of so-called free-run experiments.

Looking at mechanobiology through an evolutionary lens

Mechanical forces were arguably among the first stimuli to be perceived by cells, and they continue to shape the evolution of all organisms. Great strides have been made in recent years in the field of plant cell and molecular mechanobiology, in part owing to focused efforts on key model systems. Here, we propose to enrich such work through evolutionary mechanobiology, or 'evo-mechano', and describe three major themes that could drive research in this area. We use plastid evo-mechano as a case study, describing how plastids from different lineages perceive their mechanical environments, how their mechanical properties vary across lineages, and their distinct roles in graviperception. Finally, we argue that future research into the biomechanical properties and mechanobiological signaling mechanisms that have been elaborated by green species over the past 1.5 billion years will help us understand both the universal and the unique adaptations of plants to their physical environment.

Role of the Earth's Motions in Plant Orientation - Planetary Mechanism

According to the proposed theory, the starch-rich particles (statoliths) help the plant to convert the signals from Earth's motions into the signals necessary for the plant to perceive its orientation relative to the gravity vector while moving freely because of inertia in the sensory cells (statocytes) of roots and stems. Motions of the Earth are never constant, which, in particular, refers to the so-called polar motions and oscillations of the planet's rotation axis. Statoliths at any given moment move in the cytoplasmic liquid of statocytes due to inertial motion initiated by the action of the Earth's movements, maintaining the trajectory set by the previous movement of the oscillating planet. Unlike statoliths, the walls of a statocyte move in space along with the entire plant and with the Earth, in strict accordance with the current direction of motion of the planet's axis. This leads to the inevitable collision of statoliths with the statocytic wall/membrane. Cytoplasmic liquid, as a substance that is not able to maintain its shape, does not interfere with the inertial motions of the statoliths and collision with the wall of the statocyte. By striking the membrane, statoliths cause the release of ions and other factors at the impact site, which further participate in the gravitropic process. Pressure of the sediment of statoliths at the bottom of the statocyte, as well as position of this sediment, are not the defining factors of gravitropism.

EARLY STARVATION 1 Is a Functionally Conserved Protein Promoting Gravitropic Responses in Plants by Forming Starch Granules

Starch granules in the endodermis of plant hypocotyls act as statoliths that promote hypocotyl negative gravitropism-the directional growth of hypocotyls against gravity-in the dark. To identify the molecular components that regulate hypocotyl negative gravitropism, we performed a mutagenesis screen and isolated reduced gravitropic 1 (rgv1) mutants that lack starch granules in their hypocotyl endodermis and show reduced hypocotyl negative gravitropism in the dark. Using whole genome sequencing, we identified three different rgv1 mutants that are allelic to the previously reported early starvation 1 mutant, which is rapidly depleted of starch just before the dawn. ESV1 orthologs are present in starch-producing green organisms, suggesting ESV1 is a functionally conserved protein necessary for the formation of starch granules. Consistent with this, we found that liverwort and rice ESV1 can complement the Arabidopsis ESV1 mutant phenotype for both starch granules and hypocotyl negative gravitropism. To further investigate the function of ESV1 in other plants, we isolated rice ESV1 mutants and found that they show reduced levels of starch in their leaves and loosely packed starch granules in their grains. Both Arabidopsis and rice ESV1 mutants also lack starch granules in root columella and show reduced root gravitropism. Together, these results indicate ESV1 is a functionally conserved protein that promotes gravitropic responses in plants via its role in starch granule formation.

LAZY Gene Family in Plant Gravitropism

Adapting to the omnipresent gravitational field was a fundamental basis driving the flourishing of terrestrial plants on the Earth. Plants have evolved a remarkable capability that not only allows them to live and develop within the Earth's gravity field, but it also enables them to use the gravity vector to guide the growth of roots and shoots, in a process known as gravitropism. Triggered by gravistimulation, plant gravitropism is a highly complex, multistep process that requires many organelles and players to function in an intricate coordinated way. Although this process has been studied for several 100 years, much remains unclear, particularly the early events that trigger the relocation of the auxin efflux carrier PIN-FORMED (PIN) proteins, which presumably leads to the asymmetrical redistribution of auxin. In the past decade, the LAZY gene family has been identified as a crucial player that ensures the proper redistribution of auxin and a normal tropic response for both roots and shoots upon gravistimulation. LAZY proteins appear to be participating in the early steps of gravity signaling, as the mutation of LAZY genes consistently leads to altered auxin redistribution in multiple plant species. The identification and characterization of the LAZY gene family have significantly advanced our understanding of plant gravitropism, and opened new frontiers of investigation into the novel molecular details of the early events of gravitropism. Here we review current knowledge of the LAZY gene family and the mechanism modulated by LAZY proteins for controlling both roots and shoots gravitropism. We also discuss the evolutionary significance and conservation of the LAZY gene family in plants.

An Integrative Model of Plant Gravitropism Linking Statoliths Position and Auxin Transport

Gravity is a major cue for the proper growth and development of plants. The response of plants to gravity implies starch-filled plastids, the statoliths, which sediments at the bottom of the gravisensing cells, the statocytes. Statoliths are assumed to modify the transport of the growth hormone, auxin, by acting on specific auxin transporters, PIN proteins. However, the complete gravitropic signaling pathway from the intracellular signal associated to statoliths to the plant bending is still not well-understood. In this article, we build on recent experimental results showing that statoliths do not act as gravitational force sensor, but as position sensor, to develop a bottom-up theory of plant gravitropism. The main hypothesis of the model is that the presence of statoliths modifies PIN trafficking close to the cell membrane. This basic assumption, coupled with auxin transport and growth in an idealized tissue made of a one-dimensional array of cells, recovers several major features of the gravitropic response of plants. First, the model provides a new interpretation for the response of a plant to a steady stimulus, the so-called sine-law of plant gravitropism. Second, it predicts the existence of a gravity-independent memory process as observed recently in experiments studying the response to transient stimulus. The model suggests that the timescale of this process is associated to PIN turnover, calling for new experimental studies.

Micromanipulation of amyloplasts with optical tweezers in Arabidopsis stems

Intracellular sedimentation of highly dense, starch-filled amyloplasts toward the gravity vector is likely a key initial step for gravity sensing in plants. However, recent live-cell imaging technology revealed that most amyloplasts continuously exhibit dynamic, saltatory movements in the endodermal cells of Arabidopsis stems. These complicated movements led to questions about what type of amyloplast movement triggers gravity sensing. Here we show that a confocal microscope equipped with optical tweezers can be a powerful tool to trap and manipulate amyloplasts noninvasively, while simultaneously observing cellular responses such as vacuolar dynamics in living cells. A near-infrared (λ=1064 nm) laser that was focused into the endodermal cells at 1 mW of laser power attracted and captured amyloplasts at the laser focus. The optical force exerted on the amyloplasts was theoretically estimated to be up to 1 pN. Interestingly, endosomes and trans-Golgi network were trapped at 30 mW but not at 1 mW, which is probably due to lower refractive indices of these organelles than that of the amyloplasts. Because amyloplasts are in close proximity to vacuolar membranes in endodermal cells, their physical interaction could be visualized in real time. The vacuolar membranes drastically stretched and deformed in response to the manipulated movements of amyloplasts by optical tweezers. Our new method provides deep insights into the biophysical properties of plant organelles in vivo and opens a new avenue for studying gravity-sensing mechanisms in plants.

The change of gravity vector induces short-term phosphoproteomic alterations in Arabidopsis

Plants can sense the gravitational force. When plants perceive a change in this natural force, they tend to reorient their organs with respect to the direction of the gravity vector, i.e., the shoot stem curves up. In the present study, we performed a 4C quantitative phosphoproteomics to identify those altered protein phosphosites resulting from 150 s of reorientation of Arabidopsis plants on earth. A total of 5556 phosphopeptides were identified from the gravistimulated Arabidopsis. Quantification based on the ¹⁵N-stable isotope labeling in Arabidopsis (SILIA) and computational analysis of the extracted ion chromatogram (XIC) of phosphopeptides showed eight and five unique PTM peptide arrays (UPAs) being up- and down-regulated, respectively, by gravistimulation. Among the 13 plant reorientation-responsive protein groups, many are related to the cytoskeleton dynamic and plastid movement. Interestingly, the most gravistimulation-responsive phosphosites are three serine residues, S350, S376, and S410, of a blue light receptor Phototropin 1 (PHOT1). The immunoblots experiment confirmed that the change of gravity vector indeed affected the phosphorylation level of S410 in PHOT1. The functional role of PHOT1 in gravitropic response was further validated with gravicurvature measurement in the darkness of both the loss-of-function double mutant phot1phot2 and its complementary transgenic plant PHOT1/phot1phot2. SIGNIFICANCE: The organs of sessile organisms, plants, are able to move in response to environmental stimuli, such as gravity vector, touch, light, water, or nutrients, which is termed tropism. For instance, the bending of plant shoots to the light source is called phototropism. Since all plants growing on earth are continuously exposed to the gravitational field, plants receive the mechanical signal elicited by the gravity vector change and convert it into plant morphogenesis, growth, and development. Past studies have resulted in various hypotheses for gravisensing, but our knowledge about how the signal of gravity force is transduced in plant cells is still minimal. In the present study, we performed a SILIA-based 4C quantitative phosphoproteomics on 150-s gravistimulated Arabidopsis seedlings to explore the phosphoproteins involved in the gravitropic response. Our data demonstrated that such a short-term reorientation of Arabidopsis caused changes in phosphorylation of cytoskeleton structural proteins like Chloroplast Unusual Positioning1 (CHUP1), Patellin3 (PATL3), and Plastid Movement Impaired2 (PMI2), as well as the blue light receptor Phototropin1 (PHOT1). These results suggested that protein phosphorylation plays a crucial role in gravisignaling, and two primary tropic responses of plants, gravitropism and phototropism, may share some common components and signaling pathways. We expect that the phosphoproteins detected from this study will facilitate the subsequent molecular and cellular studies on the mechanism underlying the signal transduction in plant gravitropic response.

Switching the Direction of Stem Gravitropism by Altering Two Amino Acids in AtLAZY1

From germination to flowering, gravity influences plant growth and development. A rice (Oryza sativa) mutant with a distinctly prostrate growth habit led to the discovery of a gene category that participates in the shaping of plant form by gravity. Each so-called LAZY gene includes five short regions of conserved sequence. The importance of each of these regions in the LAZY1 gene of Arabidopsis (Arabidopsis thaliana; AtLAZY1) was tested by mutating each region and measuring how well transgenic expression of the resulting protein variant rescued the large inflorescence branch angle of an atlazy1 mutant. The effect of each alteration on subcellular localization was also determined. Region I was required for AtLAZY1 to reside at the plasma membrane, which is necessary for its function. Mutating region V severely disrupted function without affecting subcellular localization. Regions III and IV could be mutated without large impact on function or localization. Altering region II with two conservative amino acid substitutions (L92A/I94A) had the profound effect of switching shoot gravity responses from negative (upward bending) to positive (downward bending), resulting in a "weeping" inflorescence phenotype. Mechanical weakness of the stem was ruled out as an explanation for the downward bending. Instead, experiments demonstrated that the L92A/I94A change to AtLAZY1 reversed the auxin gradient normally established across stems by the gravity-sensing mechanism. This discovery opens up new avenues for studying how auxin gradients form across organs and new approaches for engineering plant architecture for agronomic and other practical purposes.

Settling for Less: Do Statoliths Modulate Gravity Perception?

Plants orientate their growth either towards (in roots) or away from (in shoots) the Earth's gravitational field. While we are now starting to understand the molecular architecture of these gravity response pathways, the gravity receptor remains elusive. This perspective looks at the biology of statoliths and suggests it is conceivable that their immediate environment may be tuned to modulate the strength of the gravity response. It then suggests how mutant screens could use this hypothesis to identify the gravity receptor.

The Superiority of the Otolith System

BACKGROUND

The peripheral vestibular end organ is considered to consist of semi-circular canals (SCC) for detection of angular accelerations and the otoliths for detection of linear accelerations. However, otoliths being phylogenetically the oldest part of the vestibular sensory organs are involved in detection of all motions.

SUMMARY

This study elaborates on this property of the otolith organ, as this concept can be of importance for the currently designed vestibular implant devices. Key Message: The analysis of the evolution of the inner ear and examination of clinical examples shows the robustness of the otolith system and inhibition capacity of the SCC. The otolith system must be considered superior to the SCC system as illustrated by evolution, clinical evidence, and physical principles.

Vacuolar membrane structures and their roles in plant-pathogen interactions

Short review focussing on the role and targeting of vacuolar substructure in plant immunity and pathogenesis. Plants lack specialized immune cells, therefore each plant cell must defend itself against invading pathogens. A typical plant defense strategy is the hypersensitive response that results in host cell death at the site of infection, a process largely regulated by the vacuole. In plant cells, the vacuole is a vital organelle that plays a central role in numerous fundamental processes, such as development, reproduction, and cellular responses to biotic and abiotic stimuli. It shows divergent membranous structures that are continuously transforming. Recent technical advances in visualization and live-cell imaging have significantly altered our view of the vacuolar structures and their dynamics. Understanding the active nature of the vacuolar structures and the mechanisms of vacuole-mediated defense responses is of great importance in understanding plant-pathogen interactions. In this review, we present an overview of the current knowledge about the vacuole and its internal structures, as well as their role in plant-microbe interactions. There is so far limited information on the modulation of the vacuolar structures by pathogens, but recent research has identified the vacuole as a possible target of microbial interference.

The Rice Actin-Binding Protein RMD Regulates Light-Dependent Shoot Gravitropism

Light and gravity are two key determinants in orientating plant stems for proper growth and development. The organization and dynamics of the actin cytoskeleton are essential for cell biology and critically regulated by actin-binding proteins. However, the role of actin cytoskeleton in shoot negative gravitropism remains controversial. In this work, we report that the actin-binding protein Rice Morphology Determinant (RMD) promotes reorganization of the actin cytoskeleton in rice (Oryza sativa) shoots. The changes in actin organization are associated with the ability of the rice shoots to respond to negative gravitropism. Here, light-grown rmd mutant shoots exhibited agravitropic phenotypes. By contrast, etiolated rmd shoots displayed normal negative shoot gravitropism. Furthermore, we show that RMD maintains an actin configuration that promotes statolith mobility in gravisensing endodermal cells, and for proper auxin distribution in light-grown, but not dark-grown, shoots. RMD gene expression is diurnally controlled and directly repressed by the phytochrome-interacting factor-like protein OsPIL16. Consequently, overexpression of OsPIL16 led to gravisensing and actin patterning defects that phenocopied the rmd mutant. Our findings outline a mechanism that links light signaling and gravity perception for straight shoot growth in rice.

Gravity sensing and signal conversion in plant gravitropism

Plant organs control their growth orientation in response to gravity. Within gravity-sensing cells, the input (gravity sensing) and signal conversion (gravity signalling) progress sequentially. The cells contain a number of high-density, starch-accumulating amyloplasts, which sense gravity when they reposition themselves by sedimentation to the bottom of the cell when the plant organ is re-orientated. This triggers the next step of gravity signalling, when the physical signal generated by the sedimentation of the amyloplasts is converted into a biochemical signal, which redirects auxin transport towards the lower flank of the plant organ. This review focuses on recent advances in our knowledge of the regulatory mechanisms that underlie amyloplast sedimentation and the system by which this is perceived, and on recent progress in characterising the factors that play significant roles in gravity signalling by which the sedimentation is linked to the regulation of directional auxin transport. Finally, we discuss the contribution of gravity signalling factors to the mechanisms that control the gravitropic set-point angle.

Lessons from optical tweezers: quantifying organelle interactions, dynamics and modelling subcellular events

Optical tweezers enable users to physically trap organelles and move them laterally within the plant cell. Recent advances have highlighted physical interactions between functionally related organelle pairs, such as ER-Golgi and peroxisome-chloroplast, and have shown how organelle positioning affects plant growth. Quantification of these processes has provided insight into the force components which ultimately drive organelle movement and positioning in plant cells. Application of optical tweezers has therefore revolutionised our understanding of plant organelle dynamics.

Ungerminated Rice Grains Observed by Femtosecond Pulse Laser Second-Harmonic Generation Microscopy

As a demonstration that second-order nonlinear optical microscopy is a powerful tool for rice grain science, we observed second-harmonic generation (SHG) images of amylose-free glutinous rice and amylose-containing nonglutinous rice grains. The images obtained from SHG microscopy and photographs of the iodine-stained starch granules indicate that the distribution of starch types in the embryo-facing endosperm region (EFR) depends on the type of rice and suggests that glucose, maltose, or both are localized on the testa side of the embryo. In the testa side of the embryo, crystallized glucose or maltose are judged to be detected by SHG. These monosaccharides and disaccharides play an important role, as they trigger energy in the initial stage of germination. These results confirm SHG microscopy is a good method to monitor the distribution of such sugars and amylopectin in the embryo and its neighboring regions of rice grains.

Both gravistimulation onset and removal trigger an increase of cytoplasmic free calcium in statocytes of roots grown in microgravity

Gravity is a permanent environmental signal guiding plant growth and development. Gravity sensing in plants starts with the displacement of starch-filled plastids called statoliths, ultimately leading to auxin redistribution and organ curvature. While the involvement in gravity sensing of several actors such as calcium is known, the effect of statolith displacement on calcium changes remains enigmatic. Microgravity is a unique environmental condition offering the opportunity to decipher this link. In this study, roots of Brassica napus were grown aboard the International Space Station (ISS) either in microgravity or in a centrifuge simulating Earth gravity. The impact of short simulated gravity onset and removal was measured on statolith positioning and intracellular free calcium was assessed using pyroantimonate precipitates as cytosolic calcium markers. Our findings show that a ten-minute onset or removal of gravity induces very low statolith displacement, but which is, nevertheless, associated with an increase of the number of pyroantimonate precipitates. These results highlight that a change in the cytosolic calcium distribution is triggered in absence of a significant statolith displacement.

Loss of a highly conserved sterile alpha motif domain gene (WEEP) results in pendulous branch growth in peach trees

Plant shoots typically grow upward in opposition to the pull of gravity. However, exceptions exist throughout the plant kingdom. Most conspicuous are trees with weeping or pendulous branches. While such trees have long been cultivated and appreciated for their ornamental value, the molecular basis behind the weeping habit is not known. Here, we characterized a weeping tree phenotype in Prunus persica (peach) and identified the underlying genetic mutation using a genomic sequencing approach. Weeping peach tree shoots exhibited a downward elliptical growth pattern and did not exhibit an upward bending in response to 90° reorientation. The causative allele was found to be an uncharacterized gene, Ppa013325, having a 1.8-Kb deletion spanning the 5' end. This gene, dubbed WEEP, was predominantly expressed in phloem tissues and encodes a highly conserved 129-amino acid protein containing a sterile alpha motif (SAM) domain. Silencing WEEP in the related tree species Prunus domestica (plum) resulted in more outward, downward, and wandering shoot orientations compared to standard trees, supporting a role for WEEP in directing lateral shoot growth in trees. This previously unknown regulator of branch orientation, which may also be a regulator of gravity perception or response, provides insights into our understanding of how tree branches grow in opposition to gravity and could serve as a critical target for manipulating tree architecture for improved tree shape in agricultural and horticulture applications.

Gravisensors in plant cells behave like an active granular liquid

Plants are able to sense and respond to minute tilt from the vertical direction of the gravity, which is key to maintain their upright posture during development. However, gravisensing in plants relies on a peculiar sensor made of microsize starch-filled grains (statoliths) that sediment and form tiny granular piles at the bottom of the cell. How such a sensor can detect inclination is unclear, as granular materials like sand are known to display flow threshold and finite avalanche angle due to friction and interparticle jamming. Here, we address this issue by combining direct visualization of statolith avalanches in plant cells and experiments in biomimetic cells made of microfluidic cavities filled with a suspension of heavy Brownian particles. We show that, despite their granular nature, statoliths move and respond to the weakest angle, as a liquid clinometer would do. Comparison between the biological and biomimetic systems reveals that this liquid-like behavior comes from the cell activity, which agitates statoliths with an apparent temperature one order of magnitude larger than actual temperature. Our results shed light on the key role of active fluctuations of statoliths for explaining the remarkable sensitivity of plants to inclination. Our study also provides support to a recent scenario of gravity perception in plants, by bridging the active granular rheology of statoliths at the microscopic level to the macroscopic gravitropic response of the plant.

Feeling stretched or compressed? The multiple mechanosensitive responses of wood formation to bending

Background and Aims

Trees constantly experience wind, perceive resulting mechanical cues, and modify their growth and development accordingly. Previous studies have demonstrated that multiple bending treatments trigger ovalization of the stem and the formation of flexure wood in gymnosperms, but ovalization and flexure wood have rarely been studied in angiosperms, and none of the experiments conducted so far has used multidirectional bending treatments at controlled intensities. Assuming that bending involves tensile and compressive strain, we hypothesized that different local strains may generate specific growth and wood differentiation responses.

Methods

Basal parts of young poplar stems were subjected to multiple transient controlled unidirectional bending treatments during 8 weeks, which enabled a distinction to be made between the wood formed under tensile or compressive flexural strains. This set-up enabled a local analysis of poplar stem responses to multiple stem bending treatments at growth, anatomical, biochemical and molecular levels.

Key Results

In response to multiple unidirectional bending treatments, poplar stems developed significant cross-sectional ovalization. At the tissue level, some aspects of wood differentiation were similarly modulated in the compressed and stretched zones (vessel frequency and diameter of fibres without a G-layer), whereas other anatomical traits (vessel diameter, G-layer formation, diameter of fibres with a G-layer and microfibril angle) and the expression of fasciclin-encoding genes were differentially modulated in the two zones.

Conclusions

This work leads us to propose new terminologies to distinguish the 'flexure wood' produced in response to multiple bidirectional bending treatments from wood produced under transient tensile strain (tensile flexure wood; TFW) or under transient compressive strain (compressive flexure wood; CFW). By highlighting similarities and differences between tension wood and TFW and by demonstrating that plants could have the ability to discriminate positive strains from negative strains, this work provides new insight into the mechanisms of mechanosensitivity in plants.

Update on Myosin Motors: Molecular Mechanisms and Physiological Functions

Recent progress has revealed aspects of the molecular mechanisms that allow myosin motors to carry outtheir physiological functions.

A new scenario for gravity detection in plants: the position sensor hypothesis

The detection of gravity plays a fundamental role during the growth and evolution of plants. Although progress has been made in our understanding of the molecular, cellular and physical mechanisms involved in the gravity detection, a coherent scenario consistent with all the observations is still lacking. In this special issue article, we discuss recent experiments showing that the response to inclination of shoots is independent of the gravity intensity, meaning that the gravity sensor detects an inclination and not a force. This result questions some of the commonly accepted hypotheses and leads to propose a new 'position sensor hypothesis'. The implications of this new scenario are discussed in light of the different observations available in the literature.

Gravireceptors in eukaryotes-a comparison of case studies on the cellular level

We have selected five evolutionary very different biological systems ranging from unicellular protists via algae and higher plants to human cells showing responses to the gravity vector of the Earth in order to compare their graviperception mechanisms. All these systems use a mass, which may either by a heavy statolith or the whole content of the cell heavier than the surrounding medium to operate on a gravireceptor either by exerting pressure or by pulling on a cytoskeletal element. In many cases the receptor seems to be a mechanosensitive ion channel activated by the gravitational force which allows a gated ion flux across the membrane when activated. This has been identified in many systems to be a calcium current, which in turn activates subsequent elements of the sensory transduction chain, such as calmodulin, which in turn results in the activation of ubiquitous enzymes, gene expression activation or silencing. Naturally, the subsequent responses to the gravity stimulus differ widely between the systems ranging from orientational movement and directed growth to physiological reactions and adaptation to the environmental conditions.

Inhibition of cell polarity establishment in stomatal asymmetric cell division using the chemical compound bubblin

Stem-cell polarization is a crucial step in asymmetric cell division, which is a universal system for generating cellular diversity in multicellular organisms. Several conventional genetics studies have attempted to elucidate the mechanisms underlying cell polarization in plants, but it remains largely unknown. In plants, stomata, which are valves for gas exchange, are generated through several rounds of asymmetric divisions. In this study, we identified and characterized a chemical compound that affects stomatal stem-cell polarity. High-throughput screening for bioactive molecules identified a pyridine-thiazole derivative, named bubblin, which induced stomatal clustering in Arabidopsis epidermis. Bubblin perturbed stomatal asymmetric division, resulting in the generation of two identical daughter cells. Both cells continued to express the stomatal-fate determinant SPEECHLESS, and then differentiated into mispatterned stomata. Bubblin-treated cells had a defect in the polarized localization of BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL), which is required for asymmetric cell fate determination. Our results suggest that bubblin induces stomatal lineage cells to divide without BASL-dependent pre-mitotic establishment of polarity. Bubblin is a potentially valuable tool for investigating cell polarity establishment in stomatal asymmetric division.

Wortmannin-induced vacuole fusion enhances amyloplast dynamics in Arabidopsis zigzag1 hypocotyls

Gravitropism in Arabidopsis shoots depends on the sedimentation of amyloplasts in the endodermis, and a complex interplay between the vacuole and F-actin. Gravity response is inhibited in zigzag-1 (zig-1), a mutant allele of VTI11, which encodes a SNARE protein involved in vacuole fusion. zig-1 seedlings have fragmented vacuoles that fuse after treatment with wortmannin, an inhibitor of phosphatidylinositol 3-kinase, and underscore a role of phosphoinositides in vacuole fusion. Using live-cell imaging with a vertical stage microscope, we determined that young endodermal cells below the apical hook that are smaller than 70 μm in length are the graviperceptive cells in dark-grown hypocotyls. This result was confirmed by local wortmannin application to the top of zig-1 hypocotyls, which enhanced shoot gravitropism in zig-1 mutants. Live-cell imaging of zig-1 hypocotyl endodermal cells indicated that amyloplasts are trapped between juxtaposed vacuoles and their movement is severely restricted. Wortmannin-induced fusion of vacuoles in zig-1 seedlings increased the formation of transvacuolar strands, enhanced amyloplast sedimentation and partially suppressed the agravitropic phenotype of zig-1 seedlings. Hypergravity conditions at 10 g were not sufficient to displace amyloplasts in zig-1, suggesting the existence of a physical tether between the vacuole and amyloplasts. Our results overall suggest that vacuole membrane remodeling may be involved in regulating the association of vacuoles and amyloplasts during graviperception.

PIF1 Regulates Plastid Development by Repressing Photosynthetic Genes in the Endodermis

Mutations in Phytochrome Interacting Factors (PIFs) induce a conversion of the endodermal amyloplasts necessary for gravity sensing to plastids with developed thylakoids accompanied by abnormal activation of photosynthetic genes in the dark. In this study, we investigated how PIFs regulate endodermal plastid development by performing comparative transcriptome analysis. We show that both endodermal expression of PIF1 and global expression of the PIF quartet induce transcriptional changes in genes enriched for nuclear-encoded photosynthetic genes such as LHCA and LHCB. Among the 94 shared differentially expressed genes identified from the comparative transcriptome analysis, only 14 genes are demonstrated to be direct targets of PIF1, and most photosynthetic genes are not. Using a co-expression analysis, we identified a direct target of PIF, whose expression pattern shows a strong negative correlation with many photosynthetic genes. We have named this gene REPRESSOR OF PHOTOSYNTHETIC GENES1 (RPGE1). Endodermal expression of RPGE1 rescued the elevated expression of photosynthetic genes found in the pif quadruple (pifQ) mutant and partly restored amyloplast development and hypocotyl negative gravitropism. Taken together, our results indicate that RPGE1 acts downstream of PIF1 in the endodermis to repress photosynthetic genes and regulate plastid development.

The role of Arabidopsis Actin-Related Protein 3 in amyloplast sedimentation and polar auxin transport in root gravitropism

Gravitropism is vital for shaping directional plant growth in response to the forces of gravity. Signals perceived in the gravity-sensing cells can be converted into biochemical signals and transmitted. Sedimentation of amyloplasts in the columella cells triggers asymmetric auxin redistribution in root tips, leading to downward root growth. The actin cytoskeleton is thought to play an important role in root gravitropism, although the molecular mechanism has not been resolved. DISTORTED1 (DIS1) encodes the ARP3 subunit of the Arabidopsis Actin-Related Protein 2/3 (ARP2/3) complex, and the ARP3/DIS1 mutant dis1-1 showed delayed root curvature after gravity stimulation. Microrheological analysis revealed that the high apparent viscosity within dis1-1 central columella cells is closely associated with abnormal movement trajectories of amyloplasts. Analysis using a sensitive auxin input reporter DII-VENUS showed that asymmetric auxin redistribution was reduced in the root tips of dis1-1, and the actin-disrupting drug Latrunculin B increased the asymmetric auxin redistribution. An uptake assay using the membrane-selective dye FM4-64 indicated that endocytosis was decelerated in dis1-1 root epidermal cells. Treatment and wash-out with Brefeldin A, which inhibits protein transport from the endoplasmic reticulum to the Golgi apparatus, showed that cycling of the auxin-transporter PIN-FORMED (PIN) proteins to the plasma membrane was also suppressed in dis1-1 roots. The results reveal that ARP3/DIS1 acts in root gravitropism by affecting amyloplast sedimentation and PIN-mediated polar auxin transport through regulation of PIN protein trafficking.

Isolation of New Gravitropic Mutants under Hypergravity Conditions

Forward genetics is a powerful approach used to link genotypes and phenotypes, and mutant screening/analysis has provided deep insights into many aspects of plant physiology. Gravitropism is a tropistic response in plants, in which hypocotyls and stems sense the direction of gravity and grow upward. Previous studies of gravitropic mutants have suggested that shoot endodermal cells in Arabidopsis stems and hypocotyls are capable of sensing gravity (i.e., statocytes). In the present study, we report a new screening system using hypergravity conditions to isolate enhancers of gravitropism mutants, and we also describe a rapid and efficient genome mapping method, using next-generation sequencing (NGS) and single nucleotide polymorphism (SNP)-based markers. Using the endodermal-amyloplast less 1 (eal1) mutant, which exhibits defective development of endodermal cells and gravitropism, we found that hypergravity (10 g) restored the reduced gravity responsiveness in eal1 hypocotyls and could, therefore, be used to obtain mutants with further reduction in gravitropism in the eal1 background. Using the new screening system, we successfully isolated six ene (enhancer of eal1) mutants that exhibited little or no gravitropism under hypergravity conditions, and using NGS and map-based cloning with SNP markers, we narrowed down the potential causative genes, which revealed a new genetic network for shoot gravitropism in Arabidopsis.

Arabidopsis Myosins XI1, XI2, and XIK Are Crucial for Gravity-Induced Bending of Inflorescence Stems

Myosins and actin filaments in the actomyosin system act in concert in regulating cell structure and dynamics and are also assumed to contribute to plant gravitropic response. To investigate the role of the actomyosin system in the inflorescence stem gravitropism, we used single and multiple mutants affecting each of the 17 Arabidopsis myosins of class VIII and XI. We show that class XI but not class VIII myosins are required for stem gravitropism. Simultaneous loss of function of myosins XI1, XI2, and XIK leads to impaired gravitropic bending that is correlated with altered growth, stiffness, and insufficient sedimentation of gravity sensing amyloplasts in stem endodermal cells. The gravitropic defect of the corresponding triple mutant xi1 xi2 xik could be rescued by stable expression of the functional XIK:YFP in the mutant background, indicating a role of class XI myosins in this process. Altogether, our results emphasize the critical contributions of myosins XI in stem gravitropism of Arabidopsis.

An ABC transporter B family protein, ABCB19, is required for cytoplasmic streaming and gravitropism of the inflorescence stems

A significant feature of plant cells is the extensive motility of organelles and the cytosol, which was originally defined as cytoplasmic streaming. We suggested previously that a three-way interaction between plant-specific motor proteins myosin XIs, actin filaments, and the endoplasmic reticulum (ER) was responsible for cytoplasmic streaming. (1) Currently, however, there are no reports of molecular components for cytoplasmic streaming other than the actin-myosin-cytoskeleton and ER-related proteins. In the present study, we found that elongated cells of inflorescence stems of Arabidopsis thaliana exhibit vigorous cytoplasmic streaming. Statistical analysis showed that the maximal velocity of plastid movements is 7.26 µm/s, which is much faster than the previously reported velocities of organelles. Surprisingly, the maximal velocity of streaming in the inflorescence stem cells was significantly reduced to 1.11 µm/s in an Arabidopsis mutant, abcb19-101, which lacks ATP BINDING CASSETTE SUBFAMILY B19 (ABCB19) that mediates the polar transport of the phytohormone auxin together with PIN-FORMED (PIN) proteins. Polar auxin transport establishes the auxin concentration gradient essential for plant development and tropisms. Deficiency of ABCB19 activity eventually caused enhanced gravitropic responses of the inflorescence stems and abnormally flexed inflorescence stems. These results suggest that ABCB19-mediated auxin transport plays a role not only in tropism regulation, but also in cytoplasmic streaming.

Morphometric analyses of petioles of seedlings grown in a spaceflight experiment

Gravity is a constant unidirectional stimulus on Earth, and gravitropism in plants involves three phases: perception, transduction, and response. In shoots, perception takes place within the endodermis. To investigate the cellular machinery of perception in microgravity, we conducted a spaceflight study with Arabidopsis thaliana seedlings, which were grown in microgravity in darkness using the Biological Research in Canisters (BRIC) hardware during space shuttle mission STS-131. In the 14-day-old etiolated plants, we studied seedling development and the morphological parameters of the endodermal cells in the petiole. Seedlings from the spaceflight experiment (FL) were compared to a ground control (GC), which both were in the BRIC flight hardware. In addition, to assay any potential effects from growth in spaceflight hardware, we performed another control by growing seedlings in Petri dishes in standard laboratory conditions (termed the hardware control, HC). Seed germination was significantly lower in samples grown in flight hardware (FL, GC) compared to the HC. In terms of cellular parameters of endodermal cells, the greatest differences also were between seedlings grown in spaceflight hardware (FL, GC) compared to those grown outside of this hardware (HC). Specifically, the endodermal cells were significantly smaller in seedlings grown in the BRIC system compared to those in the HC. However, a change in the shape of the cell, suggesting alterations in the cell wall, was one parameter that appears to be a true microgravity effect. Taken together, our results suggest that caution must be taken when interpreting results from the increasingly utilized BRIC spaceflight hardware system and that it is important to perform additional ground controls to aid in the analysis of spaceflight experiments.

Electron probe X-ray microanalysis studies on the distribution change of intra- and extracellular calcium in the elongation zone of horizontally reoriented soybean roots

To clarify the contribution of Ca to the gravitropic response, quantitative X-ray microanalyses were performed on cryosections of roots of soybean seedlings reoriented horizontally from their original vertical orientation. After reorientation, the roots bent gradually toward the ground at the elongation zone. The concentrations of Ca in the cell walls, cytoplasmic matrices and central vacuoles of cortical cells were measured in the upper and lower halves of the elongation zone at 0, 30, 60 and 120 min after reorientation. The Ca concentration did not significantly change in the cytoplasmic matrices or vacuoles. Additionally, the Ca concentration did not change significantly in cell walls at 30 min after reorientation; however, beyond 30 min, this concentration significantly increased gradually in the lower half of the elongation zone and decreased in the upper half of the elongation zone, indicating a typical asymmetrical distribution of Ca. These results suggest that Ca moves apoplastically in soybean roots to produce an asymmetrical Ca distribution in the elongation zone, which contributes to root curvature. The possible role of Ca in accelerating or repressing the effect of auxin is also discussed in this study.

Strategies of seedlings to overcome their sessile nature: auxin in mobility control

Plants are sessile organisms that are permanently restricted to their site of germination. To compensate for their lack of mobility, plants evolved unique mechanisms enabling them to rapidly react to ever changing environmental conditions and flexibly adapt their postembryonic developmental program. A prominent demonstration of this developmental plasticity is their ability to bend organs in order to reach the position most optimal for growth and utilization of light, nutrients, and other resources. Shortly after germination, dicotyledonous seedlings form a bended structure, the so-called apical hook, to protect the delicate shoot meristem and cotyledons from damage when penetrating through the soil. Upon perception of a light stimulus, the apical hook rapidly opens and the photomorphogenic developmental program is activated. After germination, plant organs are able to align their growth with the light source and adopt the most favorable orientation through bending, in a process named phototropism. On the other hand, when roots and shoots are diverted from their upright orientation, they immediately detect a change in the gravity vector and bend to maintain a vertical growth direction. Noteworthy, despite the diversity of external stimuli perceived by different plant organs, all plant tropic movements share a common mechanistic basis: differential cell growth. In our review, we will discuss the molecular principles underlying various tropic responses with the focus on mechanisms mediating the perception of external signals, transduction cascades and downstream responses that regulate differential cell growth and consequently, organ bending. In particular, we highlight common and specific features of regulatory pathways in control of the bending of organs and a role for the plant hormone auxin as a key regulatory component.

Regulation of organ straightening and plant posture by an actin-myosin XI cytoskeleton

Plants are able to bend nearly every organ in response to environmental stimuli such as gravity and light(1,2). After this first phase, the responses to stimuli are restrained by an independent mechanism, or even reversed, so that the organ will stop bending and attain its desired posture. This phenomenon of organ straightening has been called autotropism(3) and autostraightening(4) and modelled as proprioception(5). However, the machinery that drives organ straightening and where it occurs are mostly unknown. Here, we show that the straightening of inflorescence stems is regulated by an actin-myosin XI cytoskeleton in specialized immature fibre cells that are parallel to the stem and encircle it in a thin band. Arabidopsis mutants defective in myosin XI (specifically XIf and XIk) or ACTIN8 exhibit hyperbending of stems in response to gravity, an effect independent of the physical properties of the shoots. The actin-myosin XI cytoskeleton enables organs to attain their new position more rapidly than would an oscillating series of diminishing overshoots in environmental stimuli. We propose that the long actin filaments in elongating fibre cells act as a bending tensile sensor to perceive the organ's posture and trigger the straightening system.

Modulation of modeled microgravity on radiation-induced bystander effects in Arabidopsis thaliana

Both space radiation and microgravity have been demonstrated to have inevitable impact on living organisms during space flights and should be considered as important factors for estimating the potential health risk for astronauts. Therefore, the question whether radiation effects could be modulated by microgravity is an important aspect in such risk evaluation. Space particles at low dose and fluence rate, directly affect only a fraction of cells in the whole organism, which implement radiation-induced bystander effects (RIBE) in cellular response to space radiation exposure. The fact that all of the RIBE experiments are carried out in a normal gravity condition bring forward the need for evidence regarding the effect of microgravity on RIBE. In the present study, a two-dimensional rotation clinostat was adopted to demonstrate RIBE in microgravity conditions, in which the RIBE was assayed using an experimental system of root-localized irradiation of Arabidopsis thaliana (A. thaliana) plants. The results showed that the modeled microgravity inhibited significantly the RIBE-mediated up-regulation of expression of the AtRAD54 and AtRAD51 genes, generation of reactive oxygen species (ROS) and transcriptional activation of multicopy P35S:GUS, but made no difference to the induction of homologous recombination by RIBE, showing divergent responses of RIBE to the microgravity conditions. The time course of interaction between the modeled microgravity and RIBE was further investigated, and the results showed that the microgravity mainly modulated the processes of the generation or translocation of the bystander signal(s) in roots.

Live cell imaging of cytoskeletal and organelle dynamics in gravity-sensing cells in plant gravitropism

Plants sense gravity and change their morphology/growth direction accordingly (gravitropism). The early process of gravitropism, gravity sensing, is supposed to be triggered by sedimentation of starch-filled plastids (amyloplasts) in statocytes such as root columella cells and shoot endodermal cells. For several decades, many scientists have focused on characterizing the role of the amyloplasts and observed their intracellular sedimentation in various plants. Recently, it has been discovered that the complex sedimentary movements of the amyloplasts are created not only by gravity but also by cytoskeletal/organelle dynamics, such as those of actin filaments and the vacuolar membrane. Thus, to understand how plants sense gravity, we need to analyze both amyloplast movements and their regulatory systems in statocytes. We have developed a vertical-stage confocal microscope that allows multicolor fluorescence imaging of amyloplasts, actin filaments and vacuolar membranes in vertically oriented plant tissues. We also developed a centrifuge microscope that allows bright-field imaging of amyloplasts during centrifugation. These microscope systems provide new insights into gravity-sensing mechanisms in Arabidopsis.

Variation in stem morphology and movement of amyloplasts in white spruce grown in the weightless environment of the International Space Station

One-year-old white spruce (Picea glauca) seedlings were studied in microgravity conditions in the International Space Station (ISS) and compared with seedlings grown on Earth. Leaf growth was clearly stimulated in space whereas data suggest a similar trend for the shoots. Needles on the current shoots of ground-based seedlings were more inclined towards the stem base than those of seedlings grown in the ISS. Amyloplasts sedimented in specialized cells of shoots and roots in seedlings grown on Earth while they were distributed at random in similar cells of seedlings tested in the ISS. In shoots, such amyloplasts were found in starch sheath cells located between leaf traces and cortical cells whereas in roots they were constituents of columella cells of the cap. Nuclei were regularly observed just above the sedimented amyloplasts in both organs. It was also frequent to detect vacuoles with phenolic compounds and endoplasmic reticulum (ER) close to the sedimented amyloplasts. The ER was mainly observed just under these amyloplasts. Thus, when amyloplasts sediment, the pressure exerted on the ER, the organelle that can for instance secrete proteins destined for the plasma membrane, might influence their functioning and play a role in signaling pathways involved in gravity-sensing white spruce cells.

Stochastic processes in gravitropism

In this short review we focus on the role of noise in gravitropism of plants - the reorientation of plants according to the direction of gravity. We briefly introduce the conventional picture of static gravisensing in cells specialized in sensing. This model hinges on the sedimentation of statoliths (high in density and mass relative to other organelles) to the lowest part of the sensing cell. We then present experimental observations that cannot currently be understood within this framework. Lastly we introduce some current alternative models and directions that attempt to incorporate and interpret these experimental observations, including: (i) dynamic sensing, where gravisensing is suggested to be enhanced by stochastic events due to thermal and mechanical noise. These events both effectively lower the threshold of response, and lead to small-distance sedimentation, allowing amplification, and integration of the signal. (ii) The role of the cytoskeleton in signal-to-noise modulation and (iii) in signal transduction. In closing, we discuss directions that seem to either not have been explored, or that are still poorly understood.

Light and gravity signals synergize in modulating plant development

Tropisms are growth-mediated plant movements that help plants to respond to changes in environmental stimuli. The availability of water and light, as well as the presence of a constant gravity vector, are all environmental stimuli that plants sense and respond to via directed growth movements (tropisms). The plant response to gravity (gravitropism) and the response to unidirectional light (phototropism) have long been shown to be interconnected growth phenomena. Here, we discuss the similarities in these two processes, as well as the known molecular mechanisms behind the tropistic responses. We also highlight research done in a microgravity environment in order to decouple two tropisms through experiments carried out in the absence of a significant unilateral gravity vector. In addition, alteration of gravity, especially the microgravity environment, and light irradiation produce important effects on meristematic cells, the undifferentiated, highly proliferating, totipotent cells which sustain plant development. Microgravity produces the disruption of meristematic competence, i.e., the decoupling of cell proliferation and cell growth, affecting the regulation of the cell cycle and ribosome biogenesis. Light irradiation, especially red light, mediated by phytochromes, has an activating effect on these processes. Phytohormones, particularly auxin, also are key mediators in these alterations. Upcoming experiments on the International Space Station will clarify some of the mechanisms and molecular players of the plant responses to these environmental signals involved in tropisms and the cell cycle.

Homotypic vacuole fusion requires VTI11 and is regulated by phosphoinositides

Most plant cells contain a large central vacuole that is essential to maintain cellular turgor. We report a new mutant allele of VTI11 that implicates the SNARE protein VTI11 in homotypic fusion of protein storage and lytic vacuoles. Fusion of the multiple vacuoles present in vti11 mutants could be induced by treatment with Wortmannin and LY294002, which are inhibitors of Phosphatidylinositol 3-Kinase (PI3K). We provide evidence that Phosphatidylinositol 3-Phosphate (PtdIns(3)P) regulates vacuole fusion in vti11 mutants, and that fusion of these vacuoles requires intact microtubules and actin filaments. Finally, we show that Wortmannin also induced the fusion of guard cell vacuoles in fava beans, where vacuoles are naturally fragmented after ABA-induced stomata closure. These results suggest a ubiquitous role of phosphoinositides in vacuole fusion, both during the development of the large central vacuole and during the dynamic vacuole remodeling that occurs as part of stomata movements.

A unique HEAT repeat-containing protein SHOOT GRAVITROPISM6 is involved in vacuolar membrane dynamics in gravity-sensing cells of Arabidopsis inflorescence stem

Plant vacuoles play critical roles in development, growth and stress responses. In mature cells, vacuolar membranes (VMs) display several types of structures, which are formed by invagination and folding of VMs into the lumenal side and can gradually move and change shape. Although such VM structures are observed in a broad range of tissue types and plant species, the molecular mechanism underlying their formation and maintenance remains unclear. Here, we report that a novel HEAT-repeat protein, SHOOT GRAVITROPISM6 (SGR6), of Arabidopsis is involved in the control of morphological changes and dynamics of VM structures in endodermal cells, which are the gravity-sensing cells in shoots. SGR6 is a membrane-associated protein that is mainly localized to the VM in stem endodermal cells. The sgr6 mutant stem exhibits a reduced gravitropic response. Higher plants utilize amyloplast sedimentation as a means to sense gravity direction. Amyloplasts are surrounded by VMs in Arabidopsis endodermal cells, and the flexible and dynamic structure of VMs is important for amyloplast sedimentation. We demonstrated that such dynamic features of VMs are gradually lost in sgr6 endodermal cells during a 30 min observation period. Histological analysis revealed that amyloplast sedimentation was impaired in sgr6. Detailed live-cell imaging analyses revealed that the VM structures in sgr6 had severe defects in morphological changes and dynamics. Our results suggest that SGR6 is a novel protein involved in the formation and/or maintenance of invaginated VM structures in gravity-sensing cells.

Mechanosensitive channels are activated by stress in the actin stress fibres, and could be involved in gravity sensing in plants

Mechanosensitive (MS) channels are expressed in a variety of cells. The molecular and biophysical mechanism involved in the regulation of MS channel activities is a central interest in basic biology. MS channels are thought to play crucial roles in gravity sensing in plant cells. To date, two mechanisms have been proposed for MS channel activation. One is that tension development in the lipid bilayer directly activates MS channels. The second mechanism proposes that the cytoskeleton is involved in the channel activation, because MS channel activities are modulated by pharmacological treatments that affect the cytoskeleton. We tested whether tension in the cytoskeleton activates MS channels. Mammalian endothelial cells were microinjected with phalloidin-conjugated beads, which bound to stress fibres, and a traction force to the actin cytoskeleton was applied by dragging the beads with optical tweezers. MS channels were activated when the force was applied, demonstrating that a sub-pN force to the actin filaments activates a single MS channel. Plants may use a similar molecular mechanism in gravity sensing, since the cytoplasmic Ca(2+) concentration increase induced by changes in the gravity vector was attenuated by potential MS channel inhibitors, and by actin-disrupting drugs. These results support the idea that the tension increase in actin filaments by gravity-dependent sedimentation of amyloplasts activates MS Ca(2+) -permeable channels, which can be the molecular mechanism of a Ca(2+) concentration increase through gravistimulation. We review recent progress in the study of tension sensing by actin filaments and MS channels using advanced biophysical methods, and discuss their possible roles in gravisensing.

Calcium mobilizations in response to changes in the gravity vector in Arabidopsis seedlings: possible cellular mechanisms

Gravity influences the growth direction of higher plants. Changes in the gravity vector (gravistimulation) immediately promote the increase in the cytoplasmic free calcium ion concentration ([Ca(2+)]c) in Arabidopsis (Arabidopsis thaliana) seedlings. When the seedlings are gravistimulated by reorientation at 180°, a transient two peaked (biphasic) [Ca(2+)]c-increase arises in their hypocotyl and petioles. Parabolic flights (PFs) can generate a variety of gravity-stimuli, and enables us to measure gravity-induced [Ca(2+)]c-increases without specimen rotation, which demonstrate that Arabidopsis seedlings possess a rapid gravity-sensing mechanism linearly transducing a wide range of gravitational changes into Ca(2+) signals on a sub-second timescale. Hypergravity by centrifugation (20 g or 300 g) also induces similar transient [Ca(2+)]c-increases. In this review, we propose models for possible cellular processes of the garavi-stimulus-induced [Ca(2+)]c-increase, and evaluate those by examining whether the model fits well with the kinetic parameters derived from the [Ca(2+)]c-increases obtained by applying gravistimulus with different amplitudes and time sequences.

Amyloplast displacement is necessary for gravisensing in Arabidopsis shoots as revealed by a centrifuge microscope

The starch-statolith hypothesis proposes that starch-filled amyloplasts act as statoliths in plant gravisensing, moving in response to the gravity vector and signaling its direction. However, recent studies suggest that amyloplasts show continuous, complex movements in Arabidopsis shoots, contradicting the idea of a so-called 'static' or 'settled' statolith. Here, we show that amyloplast movement underlies shoot gravisensing by using a custom-designed centrifuge microscope in combination with analysis of gravitropic mutants. The centrifuge microscope revealed that sedimentary movements of amyloplasts under hypergravity conditions are linearly correlated with gravitropic curvature in wild-type stems. We next analyzed the hypergravity response in the shoot gravitropism 2 (sgr2) mutant, which exhibits neither a shoot gravitropic response nor amyloplast sedimentation at 1 g. sgr2 mutants were able to sense and respond to gravity under 30 g conditions, during which the amyloplasts sedimented. These findings are consistent with amyloplast redistribution resulting from gravity-driven movements triggering shoot gravisensing. To further support this idea, we examined two additional gravitropic mutants, phosphoglucomutase (pgm) and sgr9, which show abnormal amyloplast distribution and reduced gravitropism at 1 g. We found that the correlation between hypergravity-induced amyloplast sedimentation and gravitropic curvature of these mutants was identical to that of wild-type plants. These observations suggest that Arabidopsis shoots have a gravisensing mechanism that linearly converts the number of amyloplasts that settle to the 'bottom' of the cell into gravitropic signals. Further, the restoration of the gravitropic response by hypergravity in the gravitropic mutants that we tested indicates that these lines probably have a functional gravisensing mechanism that is not triggered at 1 g.