Plant Gravitropism Research Articles

A role for lipid‐mediated signaling in plant gravitropism

Gravitropism is a universal plant response. It is initiated by the sensing of the primary signal (mass or pressure), which is then converted into chemical signals that are transduced and propagated in a precise spatial and temporal fashion, resulting in a differential growth response. Our thesis is that membrane lipids and lipid-mediated signaling pathways play critical roles in the initial signaling and in the establishment of polarity. In this review, we highlight results from recent literature and discuss the major questions that remain unanswered.

GRAVITY PERSISTENT SIGNAL 1 (GPS1) Reveals Novel Cytochrome P450s Involved in Gravitropism

Gravity is an important environmental factor that affects growth and development of plants. In response to changes in gravity, directional growth occurs along the major axes and lateral branches of both shoots and roots. The gravity persistent signal (gps) mutants of Arabidopsis thaliana were previously identified as having an altered response to gravity when reoriented relative to the gravity vector in the cold, with the gps1 mutant exhibiting a complete loss of tropic response under these conditions. Thermal asymmetric interlaced (TAIL) PCR was used to identify the gene defective in gps1. Gene expression data, molecular modeling and computational substrate dockings, quantitative RT-PCR analyses, reporter gene fusions, and physiological analyses of knockout mutants were used to characterize the genes identified. Cloning of the gene defective in gps1 and genetic complementation revealed that GPS1 encodes CYP705A22, a cytochrome P450 monooxygenase (P450). CYP705A5, a closely related family member, was identified as expressed specifically in roots in response to gravistimulation, and a mutation affecting its expression resulted in a delayed gravity response, increased flavonol levels, and decreased basipetal auxin transport. Molecular modeling coupled with in silico substrate docking and diphenylboric acid 2-aminoethyl ester (DBPA) staining indicated that these P450s are involved in biosynthesis of flavonoids potentially involved in auxin transport. The characterization of two novel P450s (CYP705A22 and CYP705A5) and their role in the gravity response has offered new insights into the regulation of the genetic and physiological controls of plant gravitropism.

Beyond the sine law of plant gravitropism

The great German plant physiologist Wilhelm Pfeffer rightly claimed that “no plant is entirely without the power of movement” (1). If this fact remains underappreciated, it is perhaps because plant movements typically unfold over minutes, hours, or days, and thus exceed the attention span of all but the most dedicated observer. Among the large array of plant movements, the tropisms—that is, those movements that are directed toward or away from an external stimulus such as gravity or light—are the most fascinating because they highlight beautifully the sentient nature of plants and the goal-directedness of their growth habit. The pervasiveness of plant tropisms is revealed when one stops to consider the unlikeliness that seeds lodged haphazardly within the crevices of a rugged terrain should sprout stems that reliably find their way up. Were it not for the ability of the young plants to sense light and gravity, forests would be impenetrable tangles of stems and branches growing in all directions. Their prevalence in the plant kingdom explains why tropisms have been an active area of research since the beginning of the 19th century. By the end of the 19th century, the field had reached such a level of development and popularity that eminent biologists, including Charles Darwin and Pfeffer, could devote entire books to the topic (1, 2). Given this long and illustrious tradition of research, one might expect today’s biologists to have extracted all useful information from standard observational research of plant tropisms. A paper published in PNAS should convince the readers that much can still be learned from careful, quantitative observation of biological processes. In a systematic study of shoot gravitropism in 11 taxa, Bastien et al. (3) at once establish the universal response to gravity as a process of initial stem curving followed by apical straightening and debunk the idea that current models of gravitropism offer a plausible explanation for the process.

Gibberellin is required for the formation of tension wood and stem gravitropism in Acacia mangium seedlings.

Angiosperm trees generally form tension wood on the upper sides of leaning stems. The formation of tension wood is an important response to gravitational stimulus. Gibberellin appears to be involved in the differentiation of secondary xylem, but it remains unclear whether gibberellin plays a key role in the formation of tension wood and plant gravitropism. Therefore, a study was designed to investigate the effects of gibberellin and of inhibitors of the synthesis of gibberellin, namely paclobutrazole and uniconazole-P, on the formation of tension wood and negative stem gravitropism in Acacia mangium seedlings. Gibberellic acid (GA(3)), paclobutrazole and uniconazole-P were applied to seedlings via the soil in which they were growing. Distilled water was applied similarly as a control. Three days after such treatment, seedlings were tilted at an angle of 45° from the vertical, and samples of stems were collected for analysis 2 weeks, 2 months and 6 months after tilting. The effects of treatments on the stem recovery degree (Rº) were analysed as an index of the negative gravitropism of seedlings, together the width of the region of tension wood in the upper part of inclined stems. It was found that GA(3) stimulated the negative gravitropism of tilted seedling stems of A. mangium, while paclobutrazole and uniconazole-P inhibited recovery to vertical growth. Moreover, GA(3) stimulated the formation of tension wood in tilted A. mangium seedlings, while paclobutrazole and uniconazole-P strongly suppressed the formation of tension wood, as assessed 2 weeks after tilting. The results suggest that gibberellin plays an important role at the initial stages of formation of tension wood and in stem gravitropism in A. mangium seedlings in response to a gravitational stimulus.

Integral Control of Plant Gravitropism through the Interplay of Hormone Signaling and Gene Regulation

The interplay between hormone signaling and gene regulatory networks is instrumental in promoting the development of living organisms. In particular, plants have evolved mechanisms to sense gravity and orient themselves accordingly. Here, we present a mathematical model that reproduces plant gravitropic responses based on known molecular genetic interactions for auxin signaling coupled with a physical description of plant reorientation. The model allows one to analyze the spatiotemporal dynamics of the system, triggered by an auxin gradient that induces differential growth of the plant with respect to the gravity vector. Our model predicts two important features with strong biological implications: 1), robustness of the regulatory circuit as a consequence of integral control; and 2), a higher degree of plasticity generated by the molecular interplay between two classes of hormones. Our model also predicts the ability of gibberellins to modulate the tropic response and supports the integration of the hormonal role at the level of gene regulation.

Long-Term Care Planning Is More Than an Insurance Topic: Developing a Written Plan for Long-Term Care

<h3>ABSTRACT</h3> 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.

Phylogeny and evolution of charophytic algae and land plants

Charophytic algae and land plants together make up a monophyletic group, streptophytes, which represents one of the main lineages of multicellular eukaryotes and has contributed greatly to the change of the environment on earth in the Phanerozoic Eon. Significant progress has been made to understand phylogenetic relationships among members of this group by phylogenetic studies of morphological and molecular data over the last twenty-five years. Mesostigma viride is now regarded as among the earliest diverging unicellular organisms in streptophytes. Characeae are the sister group to land plants. Liverworts represent the first diverging lineage of land plants. Hornworts and lycophytes are extant representatives of bryophytes and vascular plants, respectively, when early land plants changed from gametophyte to sporophyte as the dominant generation in the life cycle. Equisetum, Psilotaceae, and ferns constitute the monophyletic group of monilophytes, which are sister to seed plants. Gnetales are related to conifers, not to angiosperms as previously thought. Amborella, Nymphaeales, Hydatellaceae, Illiciales, Trimeniaceae, and Austrobaileya represent the earliest diverging lineages of extant angiosperms. These phylogenetic results, together with recent progress on elucidating genetic and developmental aspects of the plant life cycle, multicellularity, and gravitropism, will facilitate evolutionary developmental studies of these key traits, which will help us to gain mechanistic understanding on how plants adapted to environmental challenges when they colonized the land during one of the major transitions in evolution of life.

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.

Cyclotron-based effects on plant gravitropism

Primary roots exhibit positive gravitropism and grow in the direction of the gravitational vector, while shoots respond negatively and grow opposite to the gravitational vector. We first demonstrated that the use of a weak combined magnetic field (CMF), which is comprised of a permanent magnetic field and an alternating magnetic field with the frequency resonance of the cyclotron frequency of calcium ions, can change root gravitropism from a positive direction to negative direction. Two-day-old cress seedlings were gravistimulated in a chamber that was placed into a μ-metal shield where this CMF was created. Using this “new model” of a root gravitropic response, we have studied some of its components including the movement of amyloplasts-statoliths in root cap statocytes and the distribution of Ca 2+ ions in the distal elongation zone during gravistimulation. Unlike results from the control, amyloplasts did not sediment in the distal part of a statocyte, and more Ca 2+ accumulation was observed in the upper side of a gravistimulated root for seedlings treated with the CMF. For plants treated with the CMF, it appears that a root gravitropic reaction occurs by a normal physiological process resulting in root bending although in the opposite direction. These results support the hypothesis that both the amyloplasts in the root cap statocytes and calcium are important signaling components in plant gravitropism.

A universal role for inositol 1,4,5-trisphosphate-mediated signaling in plant gravitropism.

Inositol 1,4,5-trisphosphate (InsP3) has been implicated in the early signaling events of plants linking gravity sensing to the initiation of the gravitropic response. However, at present, the contribution of the phosphoinositide signaling pathway in plant gravitropism is not well understood. To delineate the role of InsP3 in plant gravitropism, we generated Arabidopsis (Arabidopsis thaliana) plants constitutively expressing the human type I inositol polyphosphate 5-phosphatase (InsP 5-ptase), an enzyme that specifically hydrolyzes InsP3. The transgenic plants show no significant differences in growth and life cycle compared to wild-type plants, although basal InsP3 levels are reduced by greater than 90% compared to wild-type plants. With gravistimulation, InsP3 levels in inflorescence stems of transgenic plants show no detectable change, whereas in wild-type plant inflorescences, InsP3 levels increase approximately 3-fold within the first 5 to 15 min of gravistimulation, preceding visible bending. Furthermore, gravitropic bending of the roots, hypocotyls, and inflorescence stems of the InsP 5-ptase transgenic plants is reduced by approximately 30% compared with the wild type. Additionally, the cold memory response of the transgenic plants is attenuated, indicating that InsP3 contributes to gravisignaling in the cold. The transgenic roots were shown to have altered calcium sensitivity in controlling gravitropic response, a reduction in basipetal indole-3-acetic acid transport, and a delay in the asymmetric auxin-induced beta-glucuronidase expression with gravistimulation as compared to the controls. The compromised gravitropic response in all the major axes of growth in the transgenic Arabidopsis plants reveals a universal role for InsP3 in the gravity signal transduction cascade of plants.

Gibberellin-regulated XET is differentially induced by auxin in rice leaf sheath bases during gravitropic bending

The asymmetric distribution of auxin plays a fundamental role in plant gravitropism, yet little is understood about how its lateral distribution stimulates growth. In the present work, the asymmetric distribution not only of auxin, but also that of gibberellins (GAs), was observed in rice leaf sheath bases following gravistimulation. Gravistimulation induced the transient accumulation of greater amounts of both IAA and GA in the lower halves of the leaf sheath bases of rice seedlings. OsGA3ox1, a gene of active GA synthesis, was differentially induced by gravistimulation. Furthermore, 2,3,5-tri-iodobenzoic acid (TIBA), an inhibitor of auxin transport, substantially decreased the asymmetric distribution of IAA and the gradient of OsGA3ox1 expression. Externally applied GA(3) restored the gravitropic curvature of rice leaf sheaths inhibited by either TIBA or by ancymidol, a GA synthesis inhibitor. The expression of XET (encoding xyloglucan endotransglycosylase) was differentially induced in the lower halves of gravistimulated leaf sheath bases and was also up-regulated by exogenous IAA and GA(3). Both ancymidol and TIBA decreased the gradient of XET expression. These data suggest that the asymmetric distribution of auxin effected by gravistimulation induced a gradient of GAs via asymmetric expression of OsGA3ox1 in rice leaf sheath bases, and hence caused the asymmetric expression of XET. Cell wall loosening in the curvature site of the leaf sheath triggered by the expression of XET would contribute to gravitropic growth.

A weak combined magnetic field changes root gravitropism

Although gravitropism has been studied for many decades, many questions on plant gravitropism, including the participation of Ca 2+ ions in graviperception and signal transduction, remain open and require new experiments. We have studied gravistimulation and root gravitropism in the presence of the weak, alternating magnetic field that consisted of a sinusoidal frequency of 32 Hz inside a μ-metal shield. We discovered that this field changes normally positively gravitropic cress root to exhibit negative gravitropism. Because the combined magnetic field was adjusted to the cyclotron frequency of Ca 2+ ions, the obtained data suggest that calcium ion participate in root gravitropism. Simultaneous application of the oscillating magnetic field of the same frequency ion induce oscillation of Ca 2+ ions and can change the rate and/or the direction of Ca 2+ ion flux in roots. Control and magnetic field-exposed roots were examined for change in the distribution of amyloplasts and cellular organelles by light, electron, and confocal laser microscopy.

Molecular Mechanism of Plant Gravitropism

Molecular Mechanism of Plant Gravitropism

The promotion of gravitropism in Arabidopsis roots upon actin disruption is coupled with the extended alkalinization of the columella cytoplasm and a persistent lateral auxin gradient.

The actin cytoskeleton has been implicated in regulating plant gravitropism. However, its precise role in this process remains uncertain. We have shown previously that disruption of the actin cytoskeleton with Latrunculin B (Lat B) strongly promoted gravitropism in maize roots. These effects were most evident on a clinostat as curvature that would exceed 90 degrees despite short periods of horizontal stimulation. To probe further the cellular mechanisms underlying these enhanced gravity responses, we extended our studies to roots of Arabidopsis. Similar to our observations in other plant species, Lat B enhanced the response of Arabidopsis roots to gravity. Lat B (100 nm) and a stimulation time of 5-10 min were sufficient to induce enhanced bending responses during clinorotation. Lat B (100 nm) disrupted the fine actin filament network in different regions of the root and altered the dynamics of amyloplasts in the columella but did not inhibit the gravity-induced alkalinization of the columella cytoplasm. However, the duration of the alkalinization response during continuous gravistimulation was extended in Lat B-treated roots. Indirect visualization of auxin redistribution using the DR5:beta-glucuronidase (DR5:GUS) auxin-responsive reporter showed that the enhanced curvature of Lat B-treated roots during clinorotation was accompanied by a persistent lateral auxin gradient. Blocking the gravity-induced alkalinization of the columella cytoplasm with caged protons reduced Lat B-induced curvature and the development of the lateral auxin gradient. Our data indicate that the actin cytoskeleton is unnecessary for the initial perception of gravity but likely acts to downregulate gravitropism by continuously resetting the gravitropic-signaling system.

Plant gravitropism. Unraveling the ups and downs of a complex process.

Wind and heavy rain can have a devastating effect on crop production if they occur late during the life cycle of the plants. They often flatten the crop on the ground, leaving seed and other harvestable products at the mercy of soil moisture and pathogens and inaccessible to mechanical harvesting

Microinjection - a tool to study gravitropism

Despite extensive studies on plant gravitropism this phenomenon is still poorly understood. The separation of gravity sensing, signal transduction and response is a common concept but especially the mechanism of gravisensing remains unclear. This paper focuses on microinjection as powerful tool to investigate gravisensing in plants. We describe the microinjection of magnetic beads in rhizoids of the green alga Chara and related subsequent manipulation of the gravisensing system. After injection, an external magnet can control the movement of the magnetic beads. We demonstrate successful injection of magnetic beads into rhizoids and describe a multitude of experiments that can be carried out to investigate gravitropism in Chara rhizoids. In addition to examining mechanical properties, bead microinjection is also useful for probing the function of the cytoskeleton by coating beads with drugs that interfere with the cytoskeleton. The injection of fluorescently labeled beads or probes may reveal the involvement of the cytoskeleton during gravistimulation and response in living cells.

Enhanced gravitropism of roots with a disrupted cap actin cytoskeleton.

The actin cytoskeleton has been proposed to be a major player in plant gravitropism. However, understanding the role of actin in this process is far from complete. To address this problem, we conducted an analysis of the effect of Latrunculin B (Lat B), a potent actin-disrupting drug, on root gravitropism using various parameters that included detailed curvature kinetics, estimation of gravitropic sensitivity, and monitoring of curvature development after extended clinorotation. Lat B treatment resulted in a promotion of root curvature after a 90 degrees reorientation in three plant species tested. More significantly, the sensitivity of maize (Zea mays) roots to gravity was enhanced after actin disruption, as determined from a comparison of presentation time of Lat B-treated versus untreated roots. A short 10-min gravistimulus followed by extended rotation on a 1-rpm clinostat resulted in extensive gravitropic responses, manifested as curvature that often exceeded 90 degrees. Application of Lat B to the cap or elongation zone of maize roots resulted in the disruption of the actin cytoskeleton, which was confined to the area of localized Lat B application. Only roots with Lat B applied to the cap displayed the strong curvature responses after extended clinorotation. Our study demonstrates that disrupting the actin cytoskeleton in the cap leads to the persistence of a signal established by a previous gravistimulus. Therefore, actin could function in root gravitropism by providing a mechanism to regulate the proliferation of a gravitropic signal originating from the cap to allow the root to attain its correct orientation or set point angle.

The cytoskeleton and gravitropism in higher plants.

The cellular and molecular mechanisms underlying the gravitropic response of plants have continued to elude plant biologists despite more than a century of research. Lately there has been increased attention on the role of the cytoskeleton in plant gravitropism, but several controversies and major gaps in our understanding of cytoskeletal involvement in gravitropism remain. A major question in the study of plant gravitropism is how the cytoskeleton mediates early sensing and signal transduction events in plants. Much has been made of the actin cytoskeleton as the cellular structure that sedimenting amyloplasts impinge upon to trigger the downstream signaling events leading to the bending response. There is also strong molecular and biochemical evidence that the transport of auxin, an important player in gravitropism, is regulated by actin. Organizational changes in microtubules during the growth response phase of gravitropism have also been well documented, but the significance of such reorientations in controlling differential cellular growth is unclear. Studies employing pharmacological approaches to dissect cytoskeletal involvement in gravitropism have led to conflicting results and therefore need to be interpreted with caution. Despite the current controversies, the revolutionary advances in molecular, biochemical, and cell biological techniques have opened up several possibilities for further research into this difficult area. The myriad proteins associated with the plant cytoskeleton that are being rapidly characterized provide a rich assortment of candidate regulators that could be targets of the gravity signal transduction chain. Cytoskeletal and ion imaging in real time combined with mutant analysis promises to provide a fresh start into this controversial area of research.

Restoration of gravitropic sensitivity in starch‐deficient mutants of Arabidopsis by hypergravity

Despite the extensive study of plant gravitropism, there have been few experiments which have utilized hypergravity as a tool to investigate gravisensitivity in flowering plants. Previous studies have shown that starch-deficient mutants of Arabidopsis are less sensitive to gravity compared to the wild-type (WT). In this report, the question addressed was whether hypergravity could restore the sensitivity of starch-deficient mutants of Arabidopsis. The strains examined include a WT, a starchless mutant and a reduced-starch mutant. Vertical orientation studies with dark-grown seedlings indicate that increased centrifugal acceleration improves orientation relative to the acceleration vector for all strains, even the WT. For starchless roots, growth of seedlings under constant 5 g acceleration was required to restore orientation to the level of the WT at 1 g. In contrast, approximately 10 g was required to restore the orientation of the starchless mutant hypocotyls to a WT level at 1 g. Examination of plastid position in root cap columella cells of the starchless mutant revealed that the restoration of gravitropic sensitivity was correlated with the sedimentation of plastids toward the distal cell wall. Even in WT plants, hypergravity caused greater sedimentation of plastids and improved gravitropic capability. Collectively, these experiments support the hypothesis of a statolith-based system of gravity perception in plants. As far as is known, this is the first report to use hypergravity to study the mechanisms of gravitropism in Arabidopsis.

Restoration of gravitropic sensitivity in starch‐deficient mutants of Arabidopsis by hypergravity

Despite the extensive study of plant gravitropism, there have been few experiments which have utilized hypergravity as a tool to investigate gravisensitivity in flowering plants. Previous studies have shown that starch‐deficient mutants of Arabidopsis are less sensitive to gravity compared to the wild‐type (WT). In this report, the question addressed was whether hypergravity could restore the sensitivity of starch‐deficient mutants of Arabidopsis . The strains examined include a WT, a starchless mutant and a reduced‐starch mutant. Vertical orientation studies with dark‐grown seedlings indicate that increased centrifugal acceleration improves orientation relative to the acceleration vector for all strains, even the WT. For starchless roots, growth of seedlings under constant 5 g acceleration was required to restore orientation to the level of the WT at 1 g . In contrast, approximately 10 g was required to restore the orientation of the starchless mutant hypocotyls to a WT level at 1 g . Examination of plastid position in root cap columella cells of the starchless mutant revealed that the restoration of gravitropic sensitivity was correlated with the sedimentation of plastids toward the distal cell wall. Even in WT plants, hypergravity caused greater sedimentation of plastids and improved gravitropic capability. Collectively, these experiments support the hypothesis of a statolith‐based system of gravity perception in plants. As far as is known, this is the first report to use hypergravity to study the mechanisms of gravitropism in Arabidopsis .