Gravitropism of hypocotyls of wild-type and starch-deficient Arabidopsis seedlings in spaceflight studies

Conducting Plant Experiments in Space and on the Moon

The growth and development of plants during spaceflight have important implications for both basic and applied research supported by NASA and other international space agencies. While there have been many reviews of plant space biology, this chapter attempts to fill a gap in the literature on the actual process and methods of performing plant research in the spaceflight environment. One of the authors (JZK) has been a principal investigator on eight spaceflight projects. These experiences include using the U.S. Space Shuttle, the former Russian Space Station Mir, and the International Space Station, utilizing the Space Shuttle and Space X as launch vehicles. While there are several ways to fly an experiment into space and to obtain a spaceflight opportunity, this review focuses on using the NASA peer-reviewed sciences approach to get an experiment manifested for flight. Three narratives for the implementation of plant space biology experiments are considered from rapid turn around of a few months to a project with new hardware development that lasted 6 years. The many challenges of spaceflight research include logistical and resource constraints such as crew time, power, cold stowage, data downlinks, among others. Additional issues considered are working at NASA centers, hardware development, safety concerns, and the engineering versus science culture in space agencies. The difficulties of publishing the results from spaceflight research based on such factors as the lack of controls, limited sample size, and the indirect effects of the spaceflight environment also are summarized. Lessons learned from these spaceflight experiences are discussed in the context of improvements for future space-based research projects with plants. We also will consider new opportunities for Moon-based research via NASA's Artemis lunar exploration program.

Advantages and Limitations of Current Microgravity Platforms for Space Biology Research

Human Space exploration has created new challenges and new opportunities for science. Reaching beyond the Earth’s surface has raised the issue of the importance of gravity for the development and the physiology of biological systems, while giving scientists the tools to study the mechanisms of response and adaptation to the microgravity environment. As life has evolved under the constant influence of gravity, gravity affects biological systems at a very fundamental level. Owing to limited access to spaceflight platforms, scientists rely heavily on on-ground facilities that reproduce, to a different extent, microgravity or its effects. However, the technical constraints of counterbalancing the gravitational force on Earth add complexity to data interpretation. In-flight experiments are also not without their challenges, including additional stressors, such as cosmic radiation and lack of convection. It is thus extremely important in Space biology to design experiments in a way that maximizes the scientific return and takes into consideration all the variables of the chosen setup, both on-ground or on orbit. This review provides a critical analysis of current ground-based and spaceflight facilities. In particular, the focus was given to experimental design to offer the reader the tools to select the appropriate setup and to appropriately interpret the results.

Gravity Signaling in Flowering Plant Roots

Roots typically grow downward into the soil where they anchor the plant and take up water and nutrients necessary for plant growth and development. While the primary roots usually grow vertically downward, laterals often follow a gravity set point angle that allows them to explore the surrounding environment. These responses can be modified by developmental and environmental cues. This review discusses the molecular mechanisms that govern root gravitropism in flowering plant roots. In this system, the primary site of gravity sensing within the root cap is physically separated from the site of curvature response at the elongation zone. Gravity sensing involves the sedimentation of starch-filled plastids (statoliths) within the columella cells of the root cap (the statocytes), which triggers a relocalization of plasma membrane-associated PIN auxin efflux facilitators to the lower side of the cell. This process is associated with the recruitment of RLD regulators of vesicular trafficking to the lower membrane by LAZY proteins. PIN relocalization leads to the formation of a lateral gradient of auxin across the root cap. Upon transmission to the elongation zone, this auxin gradient triggers a downward curvature. We review the molecular mechanisms that control this process in primary roots and discuss recent insights into the regulation of oblique growth in lateral roots and its impact on root-system architecture, soil exploration and plant adaptation to stressful environments.

Vegetative and reproductive growth of Arabidopsis under microgravity conditions in space

We have performed a seed-to-seed experiment in the cell biology experiment facility (CBEF) installed in the Kibo (Japanese Experiment Module) in the International Space Station. The CBEF has a 1 × g compartment on a centrifuge and a microgravity compartment, to investigate the effects of microgravity on the vegetative and reproductive growth of Arabidopsis thaliana (L.) Heynh. Seeds germinated irrespective of gravitational conditions after water supply on board. Thereafter, seedlings developed rosette leaves. The time of bolting was slightly earlier under microgravity than under space 1 × g. Microgravity enhanced the growth rate of peduncles as compared with space 1 × g or ground control. Plants developed flowers, siliques and seeds, completing their entire life cycle during 62-days cultivation. Although the flowering time was not significantly affected under microgravity, the number of flowers in a bolted plant significantly increased under microgravity as compared with space 1 × g or ground control. Microscopic analysis of reproductive organs revealed that the longitudinal length of anthers was significantly shorter under microgravity when compared with space 1 × g, while the length of pistils and filaments was not influenced by the gravitational conditions. Seed mass significantly increased under microgravity when compared with space 1 × g. In addition, seeds produced in space were found not to germinate on the ground. These results indicate that microgravity significantly influenced the reproductive development of Arabidopsis plants even though Earth's gravitational environment is not absolutely necessary for them to complete their life cycle.

Variation in the transcriptome of different ecotypes of Arabidopsis thaliana reveals signatures of oxidative stress in plant responses to spaceflight

PREMISE OF THE STUDY

Spaceflight provides a unique environment in which to dissect plant stress response behaviors and to reveal potentially novel pathways triggered in space. We therefore analyzed the transcriptomes of Arabidopsis thaliana plants grown on board the International Space Station to find the molecular fingerprints of these space-related response networks.

METHODS

Four ecotypes (Col-0, Ws-2, Ler-0 and Cvi-0) were grown on orbit and then their patterns of transcript abundance compared to ground-based controls using RNA sequencing.

KEY RESULTS

Transcripts from heat-shock proteins were upregulated in all ecotypes in spaceflight, whereas peroxidase transcripts were downregulated. Among the shared and ecotype-specific changes, gene classes related to oxidative stress and hypoxia were detected. These spaceflight transcriptional response signatures could be partly mimicked on Earth by a low oxygen environment and more fully by oxidative stress (H₂ O₂ ) treatments.

CONCLUSIONS

These results suggest that the spaceflight environment is associated with oxidative stress potentially triggered, in part, by hypoxic response. Further, a shared spaceflight response may be through the induction of molecular chaperones (such as heat shock proteins) that help protect cellular machinery from the effects of oxidative damage. In addition, this research emphasizes the importance of considering the effects of natural variation when designing and interpreting changes associated with spaceflight experiments.

Space, the final frontier: A critical review of recent experiments performed in microgravity

Space biology provides an opportunity to study plant physiology and development in a unique microgravity environment. Recent space studies with plants have provided interesting insights into plant biology, including discovering that plants can grow seed-to-seed in microgravity, as well as identifying novel responses to light. However, spaceflight experiments are not without their challenges, including limited space, limited access, and stressors such as lack of convection and cosmic radiation. Therefore, it is important to design experiments in a way to maximize the scientific return from research conducted on orbiting platforms such as the International Space Station. Here, we provide a critical review of recent spaceflight experiments and suggest ways in which future experiments can be designed to improve the value and applicability of the results generated. These potential improvements include: utilizing in-flight controls to delineate microgravity versus other spaceflight effects, increasing scientific return via next-generation sequencing technologies, and utilizing multiple genotypes to ensure results are not unique to one genetic background. Space experiments have given us new insights into plant biology. However, to move forward, special care should be given to maximize science return in understanding both microgravity itself as well as the combinatorial effects of living in space.

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.

Transcriptional response of Arabidopsis seedlings during spaceflight reveals peroxidase and cell wall remodeling genes associated with root hair development

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PREMISE OF THE STUDY

Plants will be an important component of advanced life support systems during space exploration missions. Therefore, understanding their biology in the spacecraft environment will be essential before they can be used for such systems.•

METHODS

Seedlings of Arabidopsis thaliana were grown for 2 wk in the Biological Research in Canisters (BRIC) hardware on board the second to the last mission of the space shuttle Discovery (STS-131). Transcript profiles between ground controls and space-grown seedlings were compared using stringent selection criteria.•

KEY RESULTS

Expression of transcripts associated with oxidative stress and cell wall remodeling was repressed in microgravity. These downregulated genes were previously shown to be enriched in root hairs consistent with seedling phenotypes observed in space. Mutations in genes that were downregulated in microgravity, including two uncharacterized root hair-expressed class III peroxidase genes (PRX44 and PRX57), led to defective polar root hair growth on Earth. PRX44 and PRX57 mutants had ruptured root hairs, which is a typical phenotype of tip-growing cells with defective cell walls and those subjected to stress.•

CONCLUSIONS

Long-term exposure to microgravity negatively impacts tip growth by repressing expression of genes essential for normal root hair development. Whereas changes in peroxidase gene expression leading to reduced root hair growth in space are actin-independent, root hair development modulated by phosphoinositides could be dependent on the actin cytoskeleton. These results have profound implications for plant adaptation to microgravity given the importance of tip growing cells such as root hairs for efficient nutrient capture.

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.

Seed-to-seed-to-seed growth and development of Arabidopsis in microgravity

Arabidopsis thaliana was grown from seed to seed wholly in microgravity on the International Space Station. Arabidopsis plants were germinated, grown, and maintained inside a growth chamber prior to returning to Earth. Some of these seeds were used in a subsequent experiment to successfully produce a second (back-to-back) generation of microgravity-grown Arabidopsis. In general, plant growth and development in microgravity proceeded similarly to those of the ground controls, which were grown in an identical chamber. Morphologically, the most striking feature of space-grown Arabidopsis was that the secondary inflorescence branches and siliques formed nearly perpendicular angles to the inflorescence stems. The branches grew out perpendicularly to the main inflorescence stem, indicating that gravity was the key determinant of branch and silique angle and that light had either no role or a secondary role in Arabidopsis branch and silique orientation. Seed protein bodies were 55% smaller in space seed than in controls, but protein assays showed only a 9% reduction in seed protein content. Germination rates for space-produced seed were 92%, indicating that the seeds developed in microgravity were healthy and viable. Gravity is not necessary for seed-to-seed growth of plants, though it plays a direct role in plant form and may influence seed reserves.

Plant Growth and Morphogenesis under Different Gravity Conditions: Relevance to Plant Life in Space

The growth and morphogenesis of plants are entirely dependent on the gravitational acceleration of earth. Under microgravity conditions in space, these processes are greatly modified. Recent space experiments, in combination with ground-based studies, have shown that elongation growth is stimulated and lateral expansion suppressed in various shoot organs and roots under microgravity conditions. Plant organs also show automorphogenesis in space, which consists of altered growth direction and spontaneous curvature in the dorsiventral (back and front) directions. Changes in cell wall properties are responsible for these modifications of growth and morphogenesis under microgravity conditions. Plants live in space with interesting new sizes and forms.

Effects of the Extraterrestrial Environment on Plants: Recommendations for Future Space Experiments for the MELiSSA Higher Plant Compartment

Due to logistical challenges, long-term human space exploration missions require a life support system capable of regenerating all the essentials for survival. Higher plants can be utilized to provide a continuous supply of fresh food, atmosphere revitalization, and clean water for humans. Plants can adapt to extreme environments on Earth, and model plants have been shown to grow and develop through a full life cycle in microgravity. However, more knowledge about the long term effects of the extraterrestrial environment on plant growth and development is necessary. The European Space Agency (ESA) has developed the Micro-Ecological Life Support System Alternative (MELiSSA) program to develop a closed regenerative life support system, based on micro-organisms and higher plant processes, with continuous recycling of resources. In this context, a literature review to analyze the impact of the space environments on higher plants, with focus on gravity levels, magnetic fields and radiation, has been performed. This communication presents a roadmap giving directions for future scientific activities within space plant cultivation. The roadmap aims to identify the research activities required before higher plants can be included in regenerative life support systems in space.

Microsome-associated proteome modifications of Arabidopsis seedlings grown on board the International Space Station reveal the possible effect on plants of space stresses other than microgravity

Growing plants in space for using them in bioregenerative life support systems during long-term human spaceflights needs improvement of our knowledge in how plants can adapt to space growth conditions. In a previous study performed on board the International Space Station (GENARA A experiment STS-132) we evaluate the global changes that microgravity can exert on the membrane proteome of Arabidopsis seedlings. Here we report additional data from this space experiment, taking advantage of the availability in the EMCS of a centrifuge to evaluate the effects of cues other than microgravity on the relative distribution of membrane proteins. Among the 1484 membrane proteins quantified, 227 proteins displayed no abundance differences between µ g and 1 g in space, while their abundances significantly differed between 1 g in space and 1 g on ground. A majority of these proteins (176) were over-represented in space samples and mainly belong to families corresponding to protein synthesis, degradation, transport, lipid metabolism, or ribosomal proteins. In the remaining set of 51 proteins that were under-represented in membranes, aquaporins and chloroplastic proteins are majority. These sets of proteins clearly appear as indicators of plant physiological processes affected in space by stressful factors others than microgravity.

Analyses of a gravistimulation-specific Ca2+ signature in Arabidopsis using parabolic flights

Gravity is a critical environmental factor affecting the morphology and functions of organisms on the Earth. Plants sense changes in the gravity vector (gravistimulation) and regulate their growth direction accordingly. In Arabidopsis (Arabidopsis thaliana) seedlings, gravistimulation, achieved by rotating the specimens under the ambient 1g of the Earth, is known to induce a biphasic (transient and sustained) increase in cytoplasmic calcium concentration ([Ca(2+)]c). However, the [Ca(2+)]c increase genuinely caused by gravistimulation has not been identified because gravistimulation is generally accompanied by rotation of specimens on the ground (1g), adding an additional mechanical signal to the treatment. Here, we demonstrate a gravistimulation-specific Ca(2+) response in Arabidopsis seedlings by separating rotation from gravistimulation by using the microgravity (less than 10(-4)g) conditions provided by parabolic flights. Gravistimulation without rotating the specimen caused a sustained [Ca(2+)]c increase, which corresponds closely to the second sustained [Ca(2+)]c increase observed in ground experiments. The [Ca(2+)]c increases were analyzed under a variety of gravity intensities (e.g. 0.5g, 1.5g, or 2g) combined with rapid switching between hypergravity and microgravity, demonstrating that Arabidopsis seedlings possess a very rapid gravity-sensing mechanism linearly transducing a wide range of gravitational changes (0.5g-2g) into Ca(2+) signals on a subsecond time scale.

Transcriptome analyses of Arabidopsis thaliana seedlings grown in space: implications for gravity-responsive genes

The transcriptome of seedlings was analyzed from experiments performed on the International Space Station to study the interacting effects of light and gravity on plant tropisms (project named TROPI-2; Kiss et al. 2012). Seeds of Arabidopsis were germinated in space, and seedlings were then grown in the European Modular Cultivation System for 4 days at ~1g followed by exposure to a range of gravitational accelerations (from microgravity to 1g) and two light treatments (blue light with or without a 1 h pretreatment with red). At the end of the experiments, the cassettes containing the seedlings were frozen in the minus eighty laboratory freezer and returned to Earth on space shuttle mission STS-131. The RNA was extracted from whole seedlings and used for the transcriptome analyses. A comparison of 1g spaceflight samples with 1g ground controls identified 230 genes that were differentially regulated at least twofold, emphasizing the need for "in situ" tissue fixation on a 1g centrifuge as an important control for spaceflight experiments. A further comparison of all spaceflight samples with ground controls identified approximately 280 genes that were differentially regulated at least twofold. Of these genes, several were involved in regulating cell polarity (i.e., auxin, calcium, lipid metabolism), cell-wall development, oxygen status, and cell defense or stress. However, when the transcriptome of the all g-treated spaceflight samples was compared with microgravity samples, only ~130 genes were identified as being differently regulated (P ≤ 0.01). Of this subset, only 27 genes were at least twofold differently regulated between microgravity and 1g space samples and included putative/pseudo/undefined genes (14), transposable elements (5), an expansin (ATEXP24; At1g21240), a cell-wall kinase (WAK3; At1g21240), a laccase-like flavonoid oxidase (TT10; At5g48100), among others.

Hypocotyl directional growth in Arabidopsis: a complex trait

The growth direction of the Arabidopsis (Arabidopsis thaliana) etiolated-seedling hypocotyl is a complex trait that is controlled by extrinsic signals such as gravity and touch as well as intrinsic signals such as hormones (brassinosteroid [BR], auxin, cytokinin, ethylene) and nutrient status (glucose [Glc], sucrose). We used a genetic approach to identify the signaling elements and their relationship underlying hypocotyl growth direction. BR randomizes etiolated-seedling growth by inhibiting negative gravitropism of the hypocotyls via modulating auxin homeostasis for which we designate as reset, not to be confused with the gravity set point angle. Cytokinin signaling antagonizes this BR reset of gravity sensing and/or tropism by affecting ethylene biosynthesis/signaling. Glc also antagonizes BR reset but acts independently of cytokinin and ethylene signaling pathways via inhibiting BR-regulated gene expression quantitatively and spatially, by altering protein degradation, and by antagonizing BR-induced changes in microtubule organization and cell patterning associated with hypocotyl agravitropism. This BR reset is reduced in the presence of the microtubule organization inhibitor oryzalin, suggesting a central role for cytoskeleton reorganization. A unifying and hierarchical model of Glc and hormone signaling interplay is proposed. The biological significance of BR-mediated changes in hypocotyl graviresponse lies in the fact that BR signaling sensitizes the dark-grown seedling hypocotyl to the presence of obstacles, overriding gravitropism, to enable efficient circumnavigation through soil.

An endogenous growth pattern of roots is revealed in seedlings grown in microgravity

In plants, sensitive and selective mechanisms have evolved to perceive and respond to light and gravity. We investigated the effects of microgravity on the growth and development of Arabidopsis thaliana (ecotype Landsberg) in a spaceflight experiment. These studies were performed with the Biological Research in Canisters (BRIC) hardware system in the middeck region of the space shuttle during mission STS-131 in April 2010. Seedlings were grown on nutrient agar in Petri dishes in BRIC hardware under dark conditions and then fixed in flight with paraformaldehyde, glutaraldehyde, or RNAlater. Although the long-term objective was to study the role of the actin cytoskeleton in gravity perception, in this article we focus on the analysis of morphology of seedlings that developed in microgravity. While previous spaceflight studies noted deleterious morphological effects due to the accumulation of ethylene gas, no such effects were observed in seedlings grown with the BRIC system. Seed germination was 89% in the spaceflight experiment and 91% in the ground control, and seedlings grew equally well in both conditions. However, roots of space-grown seedlings exhibited a significant difference (compared to the ground controls) in overall growth patterns in that they skewed to one direction. In addition, a greater number of adventitious roots formed from the axis of the hypocotyls in the flight-grown plants. Our hypothesis is that an endogenous response in plants causes the roots to skew and that this default growth response is largely masked by the normal 1 g conditions on Earth.

A novel phototropic response to red light is revealed in microgravity

The aim of this study was to investigate phototropism in plants grown in microgravity conditions without the complications of a 1-g environment. Experiments performed on the International Space Station (ISS) were used to explore the mechanisms of both blue-light- and red-light-induced phototropism in plants. This project utilized the European Modular Cultivation System (EMCS), which has environmental controls for plant growth as well as centrifuges for gravity treatments used as a 1-g control. Images captured from video tapes were used to analyze the growth, development, and curvature of Arabidopsis thaliana plants that developed from seed in space. A novel positive phototropic response to red light was observed in hypocotyls of seedlings that developed in microgravity. This response was not apparent in seedlings grown on Earth or in the 1-g control during the space flight. In addition, blue-light-based phototropism had a greater response in microgravity compared with the 1-g control. Although flowering plants are generally thought to lack red light phototropism, our data suggest that at least some flowering plants may have retained a red light sensory system for phototropism. Thus, this discovery may have important implications for understanding the evolution of light sensory systems in plants.

Plant cell proliferation and growth are altered by microgravity conditions in spaceflight

Seeds of Arabidopsis thaliana were sent to space and germinated in orbit. Seedlings grew for 4d and were then fixed in-flight with paraformaldehyde. The experiment was replicated on the ground in a Random Positioning Machine, an effective simulator of microgravity. In addition, samples from a different space experiment, processed in a similar way but fixed in glutaraldehyde, including a control flight experiment in a 1g centrifuge, were also used. In all cases, comparisons were performed with ground controls at 1g. Seedlings grown in microgravity were significantly longer than the ground 1g controls. The cortical root meristematic cells were analyzed to investigate the alterations in cell proliferation and cell growth. Proliferation rate was quantified by counting the number of cells per millimeter in the specific cell files, and was found to be higher in microgravity-grown samples than in the control 1g. Cell growth was appraised through the rate of ribosome biogenesis, assessed by morphological and morphometrical parameters of the nucleolus and by the levels of the nucleolar protein nucleolin. All these parameters showed a depletion of the rate of ribosome production in microgravity-grown samples versus samples grown at 1g. The results show that growth in microgravity induces alterations in essential cellular functions. Cell growth and proliferation, which are strictly associated functions under normal ground conditions, appeared divergent after gravity modification; proliferation was enhanced, whereas growth was depleted. We suggest that the cause of these changes could be an alteration in the cell cycle regulation, at the levels of checkpoints regulating cell cycle progression, leading to a shortened G2 period.

Microgravity environment uncouples cell growth and cell proliferation in root meristematic cells: the mediator role of auxin

Experiments performed in space have evidenced that, in root meristematic cells, the absence of gravity results in the uncoupling of cell growth and cell proliferation, two essential cellular functions that support plant growth and development, which are strictly coordinated under normal ground gravity conditions. In space, cell proliferation appears enhanced whereas cell growth is depleted. Since coordination of cell growth and proliferation is a major feature of meristematic cells, the observed uncoupling is a serious stress condition for these cells producing important alterations in the developmental pattern of the plant. Auxin plays a major role in these processes both by assuring the coupling of cell growth and proliferation under normal conditions and by exerting a decisive influence in the uncoupling under altered gravity conditions. Auxin is a mediator of the transduction of the gravitropic signal and its distribution in the root is altered subsequent to a change in the gravity conditions. This altered distribution may produce changes in the expression of specific growth coordinators leading to the alteration of cell cycle and protein synthesis. Therefore, available data indicate that the effects of altered gravity on cell growth and proliferation are the consequence of the transduction of the gravitropic signal perceived by columella cells, in the root tip.

EGY1 plays a role in regulation of endodermal plastid size and number that are involved in ethylene-dependent gravitropism of light-grown Arabidopsis hypocotyls

Egy1 was isolated as an ethylene-dependent gravitropism-deficient Arabidopsis mutant. Molecular studies reveal that EGY1 gene encodes a 59-kDa plastid-targeted metalloprotease. It is actively expressed in hypocotyl tissue and targets to endodermal and cortex plastid. Its protein level is up-regulated by both ethylene and light. CAB protein accumulation and chlorophyll level is severely reduced in hypocotyls and endodermal cells, respectively. Sucrose is able to restore the severely reduced starch and lipid contents as well as the deficient endodermal plastid size found in light-grown egy1 hypocotyls yet it fails to rescue the reduced plastid number and chlorophyll level in egy1 endodermal cells. The loss-of-function egy1 mutation results in a smaller size (1.9 +/- 0.3 microm in diameter) and less number (5 +/- 1) of plastids in endodermal cells, which are nearly 50% of the wild-type. EGY1 is specially required for the development of full-size endodermal plastid in seedlings that are grown on sucrose-free media under light. It plays a direct role in controlling the light-induced chlorophyll production, grana formation and plastid replication in endodermal cell. However, it plays an indirect role in regulation of endodermal plastid size. It is likely that the ethylene-dependent gravitropism-deficient phenotype of egy1 hypocotyls may result from the smaller size and less number of endodermal plastids. Gravicurvature assays performed on ethylene-insensitive mutants, etr1-1, etr2-1, ers2-1, ein4-1 and ein2-5, have clearly demonstrated the necessary role for ethylene in vigorous gravitropism of light-grown hypocotyls. The degree of ethylene-dependent gravicurvature is positively correlated with the combined state of endodermal plastid mass and number. Neither ethylene nor EGY1-regulated full-size endodermal plastid is sufficient for promotion of vigorous hypocotyl gravitropism. Presence of 4 full-size plastids per endodermal cell together with ethylene pretreatment of hypocotyls becomes sufficient to trigger vigorous gravicurvature in light-grown seedlings. A model is therefore proposed to address the role of EGY1 in regulation of endodermal plastid size and number as well as the stimulatory effect of ethylene on hypocotyl gravitropism.

RCN1-regulated phosphatase activity and EIN2 modulate hypocotyl gravitropism by a mechanism that does not require ethylene signaling

The roots curl in naphthylphthalamic acid1 (rcn1) mutant of Arabidopsis (Arabidopsis thaliana) has altered auxin transport, gravitropism, and ethylene response, providing an opportunity to analyze the interplay between ethylene and auxin in control of seedling growth. Roots of rcn1 seedlings were previously shown to have altered auxin transport, growth, and gravitropism, while rcn1 hypocotyl elongation exhibited enhanced ethylene response. We have characterized auxin transport and gravitropism phenotypes of rcn1 hypocotyls and have explored the roles of auxin and ethylene in controlling these phenotypes. As in roots, auxin transport is increased in etiolated rcn1 hypocotyls. Hypocotyl gravity response is accelerated, although overall elongation is reduced, in etiolated rcn1 hypocotyls. Etiolated, but not light grown, rcn1 seedlings also overproduce ethylene, and mutations conferring ethylene insensitivity restore normal hypocotyl elongation to rcn1. Auxin transport is unaffected by treatment with the ethylene precursor 1-aminocyclopropane carboxylic acid in etiolated hypocotyls of wild-type and rcn1 seedlings. Surprisingly, the ethylene insensitive2-1 (ein2-1) and ein2-5 mutations dramatically reduce gravitropic bending in hypocotyls. However, the ethylene resistant1-3 (etr1-3) mutation does not significantly affect hypocotyl gravity response. Furthermore, neither the etr1 nor the ein2 mutation abrogates the accelerated gravitropism observed in rcn1 hypocotyls, indicating that both wild-type gravity response and enhanced gravity response in rcn1 do not require an intact ethylene-signaling pathway. We therefore conclude that the RCN1 protein affects overall hypocotyl elongation via negative regulation of ethylene synthesis in etiolated seedlings, and that RCN1 and EIN2 modulate hypocotyl gravitropism and ethylene responses through independent pathways.

Ethylene modulates flavonoid accumulation and gravitropic responses in roots of Arabidopsis

Plant organs change their growth direction in response to reorientation relative to the gravity vector. We explored the role of ethylene in Arabidopsis (Arabidopsis thaliana) root gravitropism. Treatment of wild-type Columbia seedlings with the ethylene precursor 1-aminocyclopropane carboxylic acid (ACC) reduced root elongation and gravitropic curvature. The ethylene-insensitive mutants ein2-5 and etr1-3 had wild-type root gravity responses, but lacked the growth and gravity inhibition by ACC found in the wild type. We examined the effect of ACC on tt4(2YY6) seedlings, which have a null mutation in the gene encoding chalcone synthase, the first enzyme in flavonoid synthesis. The tt4(2YY6) mutant makes no flavonoids, has elevated indole-3-acetic acid transport, and exhibits a delayed gravity response. Roots of tt4(2YY6), the backcrossed line tt4-2, and two other tt4 alleles had wild-type sensitivity to growth inhibition by ACC, whereas the root gravitropic curvature of these tt4 alleles was much less inhibited by ACC than wild-type roots, suggesting that ACC may reduce gravitropic curvature by altering flavonoid synthesis. ACC treatment induced flavonoid accumulation in root tips, as judged by a dye that becomes fluorescent upon binding flavonoids in wild type, but not in ein2-5 and etr1-3. ACC also prevented a transient peak in flavonoid synthesis in response to gravity. Together, these experiments suggest that elevated ethylene levels negatively regulate root gravitropism, using EIN2- and ETR1-dependent pathways, and that ACC inhibition of gravity response occurs through altering flavonoid synthesis.

Gravitropic bending and plant hormones

Gravitropism is a complex multistep process that redirects the growth of roots and various above-ground organs in response to changes in the direction of the gravity vector. The anatomy and morphology of these graviresponding organs indicates a certain spatial separation between the sensing region and the responding one, a situation that strongly suggests the requirement of phytohormones as mediators to coordinate the process. The Cholodny-Went hypothesis suggested auxin as the main mediator of gravitropism. So far, ample evidence has been gathered with regard to auxin asymmetrical detection, polar and lateral transport involving influx and efflux carriers, response signaling pathway, and possible modes of action in differential cell elongation, supports its major role in gravitropism at least in roots. However, it is becoming clear that the participation of other hormones, acting in concert with auxin, is necessary as well. Of particular importance is the role of ethylene in shoot gravitropism, possibly associated with the modulation of auxin transport or sensitivity, and the key role implicated for cytokinin as the putative root cap inhibitor that controls early root gravitropism. Therefore, the major advances in the understanding of transport and signaling of auxin, ethylene, and cytokinin may shed light on the possibly tight and complicated interactions between them in gravitropism. Not much convincing evidence has been accumulated regarding the participation of other phytohormones, such as gibberellins, abscisic acid, brassinosteroids, jasmonates, and salicylic acid, in gravitropism. However, the emerging concept of cooperative hormone action opens new possibilities for a better understanding of the complex interactions of all phytohormones and their possible synergistic effects and involvement in the gravitropic bending process.

Calcium signaling in plant cells in altered gravity

Changes in the intracellular Ca2+ concentration in altered gravity (microgravity and clinostating) evidence that Ca2+ signaling can play a fundamental role in biological effects of microgravity. Calcium as a second messenger is known to play a crucial role in stimulus-response coupling for many plant cellular signaling pathways. Its messenger functions are realized by transient changes in the cytosolic ion concentration induced by a variety of internal and external stimuli such as light, hormones, temperature, anoxia, salinity, and gravity. Although the first data on the changes in the calcium balance in plant cells under the influence of altered gravity have appeared in 80th, a review highlighting the performed research and the possible significance of such Ca2+ changes in the structural and metabolic rearrangements of plant cells in altered gravity is still lacking. In this paper, an attempt was made to summarize the available experimental results and to consider some hypotheses in this field of research. It is proposed to distinguish between cell gravisensing and cell graviperception; the former is related to cell structure and metabolism stability in the gravitational field and their changes in microgravity (cells not specialized to gravity perception), the latter is related to active use of a gravitational stimulus by cells presumebly specialized to gravity perception for realization of normal space orientation, growth, and vital activity (gravitropism, gravitaxis) in plants. The main experimental data concerning both redistribution of free Ca2+ ions in plant cell organelles and the cell wall, and an increase in the intracellular Ca2+ concentration under the influence of altered gravity are presented. Based on the gravitational decompensation hypothesis, the consequence of events occurring in gravisensing cells not specialized to gravity perception under altered gravity are considered in the following order: changes in the cytoplasmic membrane surface tension --> alterations in the physicochemical properties of the membrane --> changes in membrane permeability, ion transport, membrane-bound enzyme activity, etc. --> metabolism rearrangements --> physiological responses. An analysis of data available on biological effects of altered gravity at the cellular level allows one to conclude that microgravity environment appears to affect cytoskeleton, carbohydrate and lipid metabolism, cell wall biogenesis via changes in enzyme activity and protein expression, with involvement of regulatory Ca2+ messenger system. Changes in Ca2+ influx/efflux and possible pathways of Ca2+ signaling in plant cell biochemical regulation in altered gravity are discussed.

New aspects of gravity responses in plant cells

Plants show two distinct responses to gravity: gravity-dependent morphogenesis (gravimorphogenesis) and gravity resistance. In gravitropism, a typical mechanism of gravimorphogenesis, gravity is utilized as a signal to establish an appropriate form. The response has been studied in a gravity-free environment, where plant seedlings were found to perform spontaneous morphogenesis, termed automorphogenesis. Automorphogenesis consists of a change in growth direction and spontaneous curvature in dorsiventral directions. The spontaneous curvature is caused by a difference in the capacity of the cell wall to expand between the dorsal and the ventral sides of organs, which originates from the inherent structural anisotropy. Gravity resistance is a response that enables the plant to develop against the gravitational force. To resist the force, the plant constructs a tough body by increasing the cell wall rigidity that suppresses growth. The mechanical properties of the cell wall are changed by modification of the cell wall metabolism and cell wall environment, especially pH. In gravitropism, gravity is perceived by amyloplasts in statocytes, whereas gravity resistance may be mediated by mechanoreceptors on the plasma membrane.

High phosphorylase activity is correlated with increased potato minituber formation and starch content during extended clinorotation

The major purpose of these experiments were to investigate growth of potato storage organs and starch synthesis in minitubers at slow horizontal clinorotation (2 rpm), which partly mimics microgravity, and a secondary goal was to study the activity and localization of phosphorylase (EC 2.4.1.1) in storage parenchyma under these conditions. Miniplants of Solanum tuberosum L. (cv Adreta) were grown in culture for 30 days for both the vertical control and the horizontal clinorotation. During long-term clinorotation, an acceleration of minituber formation, and an increase of amyloplast number and size in storage parenchyma cells, as well as increased starch content, was observed in the minitubers. The differences among cytochemical reaction intensity, activity of phosphorylase, and carbohydrate content in storage parenchyma cells of minitubers grown in a horizontal clinostat were established by electron-cytochemical and biochemical methods. It is shown that high phosphorylase activity is correlated with increased starch content during extended clinorotation. The results demonstrate the increase in carbohydrate metabolism and possible accelerated growth of storage organs under the influence of microgravity, as mimicked by clinorotation; therefore, clinorotation can be used as a basis for future studies on mechanisms of starch synthesis under microgravity.

Arabidopsis thaliana: A Model for the Study of Root and Shoot Gravitropism

For most plants, shoots grow upward and roots grow downward. These growth patterns illustrate the ability for plant organs to guide their growth at a specified angle from the gravity vector (gravitropism). They allow shoots to grow upward toward light, where they can photosynthesize, and roots to grow downward into the soil, where they can anchor the plant as well as take up water and mineral ions.Gravitropism involves several steps organized in a specific response pathway. These include the perception of a gravistimulus (reorientation within the gravity field), the transduction of this mechanical stimulus into a physiological signal, the transmission of this signal from the site of sensing to the site of response, and a curvature-response which allows the organ tip to resume growth at a predefined set angle from the gravity vector.The primary sites for gravity sensing are located in the cap for roots, and in the endodermis for shoots. The curvature response occurs in the elongation zones for each organ. Upon gravistimulation, a gradient of auxin appears to be generated across the stimulated organ, and be transmitted to the site of response where it promotes a differential growth response. Therefore, while the gravity-induced auxin gradient has to be transmitted from the cap to the elongation zones in roots, there is no need for a longitudinal transport in shoots, as sites for gravity sensing and response overlap in this organ.A combination of molecular genetics, physiology, biochemistry and cell biology, coupled with the utilization of Arabidopsis thaliana as a model system, have recently allowed the identification of a number of molecules involved in the regulation of each phase of gravitropism in shoots and roots of higher plants. In this review, we attempt to summarize the results of these experiments, and we conclude by comparing the molecular and physiological mechanisms that underlie gravitropism in these organs.

ABBREVIATIONS

GSPA: gravitational set point angle; IAA: indole-3-acetic acid; NAA: 1-naphthalene acetic acid; NPA: 1-N-naphthylphthalamic acid; 2,4-D: 2,4-dichlorphenoxy acetic acid; TIBA: 2,3,5-triiodobenzoic acid.

MAPK and SRC-kinases control EGR-1 and NF-kappa B inductions by changes in mechanical environment in osteoblasts

Bone loss occurs in microgravity whereas an increase in bone mass is observed after skeletal loading. This tissue adaptation involves changes in osteoblastic proliferation and differentiation whose mechanisms remain largely unknown. In this context, we investigated the expression and the nuclear translocation of Egr-1 and NF-kappa B, in a simulated microgravity model (clinostat) and in a model of mechanical strain (Flexcell). We performed RT-PCR and immunocytochemistry analyses at baseline and up to 2 h after stimulation (a mitogenic regimen, 1% stretch, 0.05 Hz, 10 min, or clinorotation 50 rpm, 10 min) in osteoblastic ROS17/2.8 cells. Egr-1 induction as well as NF-kappa B nuclear translocation were activated by mechanical changes. PKC downregulation and COX1/2 inhibition did not alter these inductions. In contrast, ERK1/2, p38(MAPK) and src-kinases pathways were differentially involved in both models. Thus, we demonstrated that changes in the mechanical environment induced an activation of Egr-1 and NF-kappa B with specific kinetics and involved various transduction pathways including MAPKs and src-kinases. These could partially explain the later alterations of proliferation observed.

Transgene expression patterns indicate that spaceflight affects stress signal perception and transduction in arabidopsis

The use of plants as integral components of life support systems remains a cornerstone of strategies for long-term human habitation of space and extraterrestrial colonization. Spaceflight experiments over the past few decades have refined the hardware required to grow plants in low-earth orbit and have illuminated fundamental issues regarding spaceflight effects on plant growth and development. Potential incipient hypoxia, resulting from the lack of convection-driven gas movement, has emerged as a possible major impact of microgravity. We developed transgenic Arabidopsis containing the alcohol dehydrogenase (Adh) gene promoter linked to the beta-glucuronidase (GUS) reporter gene to address specifically the possibility that spaceflight induces the plant hypoxia response and to assess whether any spaceflight response was similar to control terrestrial hypoxia-induced gene expression patterns. The staining patterns resulting from a 5-d mission on the orbiter Columbia during mission STS-93 indicate that the Adh/GUS reporter gene was activated in roots during the flight. However, the patterns of expression were not identical to terrestrial control inductions. Moreover, although terrestrial hypoxia induces Adh/GUS expression in the shoot apex, no apex staining was observed in the spaceflight plants. This indicates that either the normal hypoxia response signaling is impaired in spaceflight or that spaceflight inappropriately induces Adh/GUS activity for reasons other than hypoxia.

The molecular characterization and in situ expression pattern of pea SCARECROW gene

Certain mutants of shoot gravitropism were reported to be ascribed to the SCR and SHR loci in Arabidopsis thaliana. The SCR gene was known to regulate the development of endodermis cells that are responsible for sensing gravity in a shoot. With the aim of elucidating the molecular mechanism for gravitropic responses in pea seedlings, we have isolated a putative pea SCR ortholog from a shoot cDNA library. Analyses of the cDNA clones revealed the structure of a full-length ORF coding for 819 amino acid residues. A remarkable feature of pea SCR protein was the presence of asparagine stretches at the N-terminal transcriptional activation domain, which was distinct from the occurrence of glutamine or alanine stretches in the Arabidopsis or maize SCR. A Northern blot analysis revealed a single 3.2-kb pea SCR transcript in addition to a closely related 2.5-kb transcript. Our in situ hybridization data indicated that pea SCR mRNA accumulated in the shoot apical meristem, leaf primordia and a root single cell layer corresponding to the endodermis. The expression patterns were similar to those reported for A. thaliana and Zea mays, suggesting that SCR may be functionally conserved among plants and involved in the differentiation of the endodermis.

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.

Automorphogenesis and gravitropism of plant seedlings grown under microgravity conditions

Plant seedlings exhibit automorphogenesis on clinostats. The occurrence of automorphogenesis was confirmed under microgravity in Space Shuttle STS-95 flight. Rice coleoptiles showed an inclination toward the caryopsis in the basal region and a spontaneous curvature in the same adaxial direction in the elongating region both on a three-dimensional (3-D) clinostat and in space. Both rice roots and Arabidopsis hypocotyls also showed a similar morphology in space and on the 3-D clinostat. In rice coleoptiles, the mechanisms inducing such an automorphic curvature were studied. The faster-expanding convex side of rice coleoptiles showed a higher extensibility of the cell wall than the opposite side. Also, in the convex side, the cell wall thickness was smaller, the turnover of the matrix polysaccharides was more active, and the microtubules oriented more transversely than the concave side, and these differences appear to be causes of the curvature. When rice coleoptiles grown on the 3-D clinostat were placed horizontally, the gravitropic curvature was delayed as compared with control coleoptiles. In clinostatted coleoptiles, the corresponding suppression of the amyloplast development was also observed. Similar results were obtained in Arabidopsis hypocotyls. Thus, the induction of automorphogenesis and a concomitant decrease in graviresponsiveness occurred in plant shoots grown under microgravity conditions.

Development and growth of several strains of Arabidopsis seedlings in microgravity

Growth and development of dark-grown Arabidopsis thaliana seedlings were studied in microgravity during space shuttle mission STS-84. The major purpose of this project was to determine if there were developmental differences among the four ecotypes studied--Wassilewskija (Ws), Columbia (Col), Landsberg erecta (Ler), and C24--and to evaluate whether particular ecotypes are better suited for spaceflight experimentation compared with others. A secondary goal was to study the growth of three starch-deficient strains of Arabidopsis by extending the observations made in a previously published report. For all strains, seed germination was not affected by microgravity, but seedlings were smaller in the spaceflight samples compared with the ground controls. The starch-deficient strains continued to exhibit vigorous growth until the termination of the experiment at 121 h after imbibition of seeds. However, ethylene effects, i.e., reduced growth and exaggerated hypocotyl hooks, were observed in all strains studied. Nevertheless, the Ler and C24 ecotypes seem to be more suitable for spaceflight research, compared with the other two ecotypes, based on measurements of their relative and absolute growth. This type of information should aid in the design of plant experiments for the International Space Station.