Mechanotransduction in gravisensing cells

Plant organs can re-orientate themselves with respect to gravity. Gravisensing cells (statocytes) contain movable amyloplasts whose potential energy is apparently used to activate calcium channels by exerting tension on the actin network and/or pressure on the cytoskeleton elements lining the plasma membrane. The chain of events that follows remains to be further analysed but includes transient pH changes in the cytosol and sustained pH changes in the cell wall. Transduction ends with relocation of the auxin efflux carriers responsible for the lateral transport of auxin, which reorients the root tip in the direction of gravity. Many questions remain to be solved but recent studies now herald a better understanding of the molecular events involved in gravisensing.     

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 .     

Gravisensitivity of plant cells: status and prospects

A discovery of gravisensitivity of plant cells specialized and not specialized to gravity perception stimulated the intensive research of cell biology in altered gravity. In order to better understanding of the possible mechanisms of this phenomenon, it is proposed to distinguish between cell gravisensing and graviperception. It is assumed that proliferative and actively metabolizing cells are the most sensitive to the influence of altered gravity. Grounded on the hypothesis of gravitational decompensation, the consequences of events occurring in plant cells under the microgravity action are discussed. Prospects of future research in this field are proposed.     

Actin Turnover-Mediated Gravity Response in Maize Root Apices: Gravitropism of Decapped Roots Implicates Gravisensing Outside of the Root Cap

The dynamic actin cytoskeleton has been proposed to be linked to gravity sensing in plants but the mechanistic understanding of these processes remains unknown. We have performed detailed pharmacological analyses of the role of the dynamic actin cytoskeleton in gravibending of maize (Zea mays) root apices. Depolymerization of actin filaments with two drugs having different mode of their actions, cytochalasin D and latrunculin B, stimulated root gravibending. By contrast, drug-induced stimulation of actin polymerization and inhibition of actin turnover, using two different agents phalloidin and jasplakinolide, compromised the root gravibending. Importantly, all these actin drugs inhibited root growth to similar extents suggesting that high actin turnover is essential for the gravity-related growth responses rather than for the general growth process. Both latrunculin B and cytochalasin D treatments inhibited root growth but restored gravibending of the decapped root apices, indicating that there is a strong potential for effective actin-mediated gravity sensing outside the cap. This elusive gravity sensing outside the root cap is dependent not only on the high rate of actin turnover but also on weakening of myosin activities, as general inhibition of myosin ATPases induced stimulation of gravibending of the decapped root apices. Collectively, these data provide evidence for the actin turnover-mediated gravity sensing outside the root cap.Key Words: actin cytoskeleton, gravisensing, graviresponding, root cap     

Actomyosin-mediated statolith positioning and sedimentation in gravisensing plant cells studied in microgravity

Graviresponding and tip-growing characean rhizoids and protonemata possess a highly efficient actin-based system to control and correct the position of their statoliths, a prerequisite for gravisensing. Acropetally and basipetally acting actomyosin forces and gravity are the components of the statolith positioning system that also directs sedimenting statoliths to cell-type specific ,oraviperception sites at the plasma membrane where the graviresponse is initiated. These results encourage to propose that similar cytoskeleton-mediated mechanisms for gravity sensing may exist in higher plant statocytes.     

Effects of the myosin ATPase inhibitor 2,3-butanedione monoxime on amyloplast kinetics and gravitropism of Arabidopsis hypocotyls

The actin cytoskeleton is a crucial component in plant gravitropism, and studies confirm that alterations to actin filaments (F-actin) can have dramatic effects on gravitropic curvature in roots and shoots. Many models for gravisensing in higher plants suggest that the key to gravity perception and signal transduction lies in intimate interactions between F-actin and amyloplasts. In this study, we investigated gravitropism in hypocotyls by analyzing the effect of myosin inhibition on gravitropic curvature in order to clarify the role of the actomyosin system in shoot gravitropism. To study amyloplast movement in endodermal cells (i.e., gravity-perceiving statocytes) of living seedlings, we repositioned a confocal laser scanning microscope (CLSM) so that its rotatable stage was oriented vertically. Seedlings containing green fluorescent protein-labeled endodermal amyloplasts were incubated with the ATPase inhibitor 2,3-butanedione monoxime (BDM) and then mounted on the stage so that the hypocotyls were vertical. Using CLSM, we imaged the endodermal amyloplasts, while the hypocotyls were oriented vertically and also after they were reoriented by 90°. Our results show that BDM reduces gravitropic curvature in a concentration-dependent manner. In addition, BDM increases amyloplast movement in hypocotyls of vertical seedlings, but reduces amyloplast movement in hypocotyls of reoriented seedlings, suggesting that myosin may participate in the intracellular transport of amyloplasts in statocytes. These results can be explained in the context of amyloplasts as both noise indicators and gravity susceptors, with BDM producing less coherent amyloplast movement that results in an increased signal-to-noise ratio, which may account for at least part of the observed reduction in gravitopic curvature.     

微管骨架在植物适应低温胁迫中的功能研究进展

植物细胞骨架对低温胁迫的响应是近年来研究的一个活跃的前沿领域。本文综述了该领域研究的进展情况和发展趋势:植物微管骨架的结构和功能的简介,低温诱导植物细胞微管骨架稳定性的变化;并对微管骨架在冷信号传导中的作用进行了探讨。     

The onset of gravisensitivity in the embryonic root of flax

Vertical orientation of emerging roots typically is the first response of plants to gravity. Although root gravitropism has been studied extensively, no conclusive data on the onset of gravisensing exist. We determined the inception of gravisensitivity in flax (Linum usitatissimum) roots by clinorotating germinating seeds after various periods of static orientation (gravistimulation) of imbibed seeds. Gravitropic competency was established about 8 h after imbibition, 11 h prior to germination. The time was determined based on 50% of the newly emerged roots curving in the direction of the gravity vector during static imbibition, despite subsequent clinorotation. The threshold value was affected by the orientation of the seeds. Upward orientation of the micropyle/radicle reduced the number of graviresponding roots to about one-half. Prolonged clinorotation weakened the graviresponse. Gravisensing was accompanied by the development of amyloplasts, but the actin cytoskeleton was not involved because imbibition in Latrunculin B did not affect the onset of gravisensitivity or germination, and the development of F-actin in untreated controls was observed only after the onset of gravisensitivity.     

Dynamics of microtubular cytoskeleton in higher plant meiosis. X. General scheme of cytoskeletal cycle

An overview of a set of our previous papers titled "Dynamics of microtubular cytoskeleton in higher plant meiosis" is presented, in addition to some data on subcellular mechanisms underlying cytoskeleton reorganization during meiotic division in pollen mother cells. An illustrated scheme of cytoskeleton rearrangements during plant meiosis, both with successive and simultaneous cytokinesis, is given.     

Gravity perception in plants: a multiplicity of systems derived by evolution?

The origin and subsequent evolution of life on Earth have taken place within an environment where a 1g gravitational field is omnipresent. Living organisms, at whatever stage in their evolution, have accommodated this variable in both their structure and their function. Systems have also evolved whereby gravitational accelerations are perceived by gravisensors and these, in turn, have led to responses that give particular spatial orientations to living processes. It is proposed that, the higher the evolutionary status of an organism, the more likely it is that it will possess multiple systems for gravisensing because evolution discards little that assists fitness and hence supplements with new gravisensing systems those which already existed within evolutionarily older, less complex organisms. Moreover, in comparison with a single gravisensing system, a multiplicity of systems permits gravity to participate in a wider range of developmental programmes, such as taxes, morphisms and tropisms, through the action of different sensory mechanisms coupled to distinct signalling and response pathways. Whatever the precise mechanism of graviperception in any given set of conditions, all may transduce the g-force by means of a membrane system. Transduction may involve the endoplasmic reticulum and thence the plasma membrane.     

Involvement of plant cytoskeleton in cellular mechanisms of metal toxicity

Literature data and results of the studies carried out us concerning the involvement of plant cell cytoskeleton in cellular mechanisms of metal toxicity are summarized. Characteristics of cytotoxic effect of metals on plant cytoskeleton and, in particular, on microtubules and actin filaments are reviewed. Particular attention is paid to cellular and molecular mechanisms of metal impact on cytoskeleton. The most probable binding sites of heavy metals, as well as alternative mechanisms of their impact on cytoskeleton, are discussed.     

Formation of division spindles in higher plant meiosis

Depolymerisation of the MT cytoskeleton during late prophase makes it impossible to follow the cytoskeleton cycle in centrosomeless plant meiocytes. This paper describes rearrangements of the MT cytoskeleton during plant meiotic spindle formation in normally dividing pollen mother cells in various higher plant species and forms in which the cytoskeleton does not depolymerise at prophase. In such variants of the wild-type, cytoskeleton rearrangements can be observed at late prophase/early prometaphase. Radial MT bundles coalesce in the meridian plane, reorientate tangentially, curve and give rise to a developed ring-shaped perinuclear cytoskeleton system at the meridian. During nuclear envelope breakdown this ring disintegrates and splits into a set of free MT bundles. Three sub-stages of prometaphase are indicated: early prometaphase (disintegration of perinuclear ring and invasion of MTs into the former nuclear area), middle prometaphase or chaotic stage (formation of bipolar spindle fibres), and late prometaphase (formation of bipolar spindle). Analysis of a range of abnormal phenotypes (disintegrated, multiple, polyarchal, chaotic spindles) reveals two previously unknown processes during late prometaphase: axial orientation and consolidation of the spindle fibres.     

Cytoskeleton and plant salt stress tolerance

The plant cytoskeleton is a highly dynamic component of plant cells and mainly based on microtubules (MTs) and actin filaments (AFs). The important functions of dynamic cytoskeletal networks have been indicated for almost every intracellular activity, from cell division to cell movement, cell morphogenesis and cell signal transduction. Recent studies have also indicated a close relationship between the plant cytoskeleton and plant salt stress tolerance. Salt stress is a significant factor that adversely affects crop productivity and quality of agricultural fields worldwide. The complicated regulatory mechanisms of plant salt tolerance have been the subject of intense research for decades. It is well accepted that cellular changes are very important in plant responses to salt stress. Because the organization and dynamics of cytoskeleton may play an important role in enhancing plant tolerance through various cell activities, study on salt stress-induced cytoskeletal network has been a vital topic in the subject of plant salt stress tolerance mechanisms. In this article, we introduce our recent work and review some current information on the dynamic changes and functions of cytoskeletal organization in response to salt stress. The accumulated data point to the existence of highly dynamic cytoskeletal arrays and the activation of complex cytoskeletal regulatory networks in response to salt stresses. The important role played by cytoskeleton in mediating the plant cell''s response to salt stresses is particularly emphasized.Key words: cytoskeleton, microtubules (MTs), microfilaments (MFs), salt stress, response mechanisms, plant tolerance     

A new catch in the SNARE

Vesicle traffic underpins cell homeostasis, growth and development in plants. Traffic is facilitated by a superfamily of proteins known as SNAREs ( soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors) that interact to draw vesicle and target membrane surfaces together for fusion of the bilayers. Several recent findings now indicate that plant SNAREs might not be limited to the conventional 'housekeeping' activities commonly attributed to vesicle trafficking. In the past five years, six different SNAREs have been implicated in stomatal movements, gravisensing and pathogen resistance. These proteins almost certainly do contribute to specific membrane fusion events but they are also essential for signal transduction and response. Some SNAREs can modulate the activity of non-SNARE proteins, notably ion channels. Other examples might reflect SNARE interactions with different scaffolding and structural components of the cell.     

Microtubule dynamics in root hairs of Medicago truncatula

The microtubular cytoskeleton plays an important role in the development of tip-growing plant cells, but knowledge about its dynamics is incomplete. In this study, root hairs of the legume Medicago truncatula have been chosen for a detailed analysis of microtubular cytoskeleton dynamics using GFP-MBD and EB1-YFP as markers and 4D imaging. The microtubular cytoskeleton appears mainly to be composed of bundles which form tracks along which new microtubules polymerise. Polymerisation rates of microtubules are highest in the tip of growing root hairs. Treatment of root hairs with Nod factor and latrunculin B result in a twofold decrease in polymerisation rate. Nonetheless, no direct, physical interaction between the actin filament cytoskeleton and microtubules could be observed. A new picture of how the plant cytoskeleton is organised in apically growing root hairs emerges from these observations, revealing similarities with the organisation in other, non-plant, tip-growing cells.     

Low gravity on earth by magnetic levitation of biological material

The use of a magnetic field gradient levitation apparatus as a tool for investigating gravisensing mechanisms in biological systems and as a low gravity simulator for biological systems is described. The basic principles are described. Differences between its application to pure materials and the heterogeneous materials of biological materials are emphasized.     

Regulation of cytoskeleton‐associated protein activities: Linking cellular signals to plant cytoskeletal function

The plant cytoskeleton undergoes dynamic remodeling in response to diverse developmental and environmental cues. Remodeling of the cytoskeleton coordinates growth in plant cells, including trafficking and exocytosis of membrane and wall components during cell expansion, and regulation of hypocotyl elongation in response to light. Cytoskeletal remodeling also has key functions in disease resistance and abiotic stress responses. Many stimuli result in altered activity of cytoskeleton-associatedproteins,microtubuleassociated proteins(MAPs) and actin-binding proteins(ABPs). MAPs and ABPs are the main players determining the spatiotemporally dynamic nature of the cytoskeleton, functioning in a sensory hub that decodes signals to modulate plant cytoskeletal behavior. Moreover, MAP and ABP activities and levels are precisely regulated during development and environmental responses, but our understanding of this process remains limited. In this review, we summarize the evidence linking multiple signaling pathways, MAP and ABP activities and levels, and cytoskeletal rearrangements in plant cells. We highlight advances in elucidating the multiple mechanisms that regulate MAP and ABP activities and levels, including calcium and calmodulin signaling, ROP GTPase activity, phospholipid signaling, and post-translational modifications.     

Regulation of Actin Dynamics in Pollen Tubes: Control of Actin Polymer Level

Actin cytoskeleton undergoes rapid reorganization in response to internal and external cues. How the dynamics of actin cytoskeleton are regulated, and how its dynamics relate to its function are fundamental questions in plant cell biology. The pollen tube is a well characterized actin-based cell morphogenesis in plants. One of the striking features of actin cytoskeleton characterized in the pollen tube is its surprisingly low level of actin polymer. This special phenomenon might relate to the function of actin cytoskeleton in pollen tubes. Understanding the molecular mechanism underlying this special phenomenon requires careful analysis of actin-binding proteins that modulate actin dynamics directly. Recent biochemical and biophysical analyses of several highly conserved plant actin-binding proteins reveal unusual and unexpected properties, which emphasizes the importance of carefully analyzing their action mechanism and cellular activity. In this review, we highlight an actin monomer sequestering protein, a barbed end capping protein and an F-actin severing and dynamizing protein in plant. We propose that these proteins function in harmony to regulate actin dynamics and maintain the low level of actin polymer in pollen tubes.     

Setting SNAREs in a different wood

Vesicle traffic is essential for cell homeostasis, growth and development in plants, as it is in other eukaryotes, and is facilitated by a superfamily of proteins known as soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors (SNAREs). Although SNAREs are well-conserved across phylla, genomic analysis for two model angiosperm species available to date, rice and Arabidopsis, highlights common patterns of divergence from other eukaryotes. These patterns are associated with the expansion of some gene subfamilies of SNAREs, the absence of others and the appearance of new proteins that show no significant homologies to SNAREs of mammals, yeast or Drosophila. Recent findings indicate that the functions of these plant SNAREs also extend beyond the conventional 'housekeeping' activities associated with vesicle trafficking. A number of SNAREs have been implicated in environmental responses as diverse as stomata movements and gravisensing as well as sensitivity to salt and drought. These proteins are essential for signal transduction and response and, in most cases, appear also to maintain additional roles in membrane trafficking. One common theme to this added functionality lies in control of non-SNARE proteins, notably ion channels. Other examples include interactions between the SNAREs and scaffolding or other structural components within the plant cell.     

A green fluorescent protein fusion to actin-binding domain 2 of Arabidopsis fimbrin highlights new features of a dynamic actin cytoskeleton in live plant cells

The actin cytoskeleton coordinates numerous cellular processes required for plant development. The functions of this network are intricately linked to its dynamic arrangement, and thus progress in understanding how actin orchestrates cellular processes relies on critical evaluation of actin organization and turnover. To investigate the dynamic nature of the actin cytoskeleton, we used a fusion protein between green fluorescent protein (GFP) and the second actin-binding domain (fABD2) of Arabidopsis (Arabidopsis thaliana) fimbrin, AtFIM1. The GFP-fABD2 fusion protein labeled highly dynamic and dense actin networks in diverse species and cell types, revealing structural detail not seen with alternative labeling methods, such as the commonly used mouse talin GFP fusion (GFP-mTalin). Further, we show that expression of the GFP-fABD2 fusion protein in Arabidopsis, unlike GFP-mTalin, has no detectable adverse effects on plant morphology or development. Time-lapse confocal microscopy and fluorescence recovery after photobleaching analyses of the actin cytoskeleton labeled with GFP-fABD2 revealed that lateral-filament migration and sliding of individual actin filaments or bundles are processes that contribute to the dynamic and continually reorganizing nature of the actin scaffold. These new observations of the dynamic actin cytoskeleton in plant cells using GFP-fABD2 reveal the value of this probe for future investigations of how actin filaments coordinate cellular processes required for plant development.