The gravity-induced re-localization of auxin efflux carrier CsPIN1 in cucumber seedlings: spaceflight experiments for immunohistochemical microscopy

Comprehensive analyses of plant hormones in etiolated pea and maize seedlings grown under microgravity conditions in space: Relevance to the International Space Station experiment "Auxin Transport"

Functional relationships between endogenous levels of plant hormones in the growth and development of shoots in etiolated Alaska pea and etiolated Golden Cross Bantam maize seedlings under different gravities were investigated in the "Auxin Transport" experiment aboard the International Space Station (ISS). Comprehensive analyses of 31 species of plant hormones of pea and maize seedlings grown under microgravity (μg) in space and 1 g conditions were conducted. Principal component analysis (PCA) and a multiple regression analysis with the dataset from the plant hormone analysis of the etiolated pea seedlings grown under μg and 1 g conditions in the presence and absence of 2,3,5-triiodobenzoic acid (TIBA) revealed endogenous levels of auxin correlated positively with bending and length of epicotyls. Endogenous cytokinins correlated negatively with them. These results suggest an interaction of auxin and cytokinins in automorphogenesis and growth inhibition of etiolated Alaska pea epicotyls grown under μg conditions in space. Less polar auxin transport with reduced endogenous levels of auxin increased endogenous levels of cytokinins, resulting in changing the growth direction of epicotyls and inhibiting growth. On the other hand, almost no close relationship between endogenous plant hormone levels and growth and development in etiolated maize seedlings grown was observed under μg conditions in space, as per Schulze et al. (1992). However, endogenous levels of IAA in the seedlings grown under μg conditions in space were significantly higher than those grown on Earth, similar to the cases of polar auxin transport already reported.

Genome-Wide Comparison of Structural Variations and Transposon Alterations in Soybean Cultivars Induced by Spaceflight

Space mutation causes genetic and phenotypic changes in biological materials. Transposon activation is an adaptive mechanism for organisms to cope with changes in the external environment, such as space mutation. Although transposon alterations have been widely reported in diverse plant species, few studies have assessed the global transposon alterations in plants exposed to the space environment. In this study, for the first time, the effects of transposon alterations in soybean caused by space mutation were considered. A new vegetable soybean variety, 'Zhexian 9' (Z9), derived from space mutation treatment of 'Taiwan 75' (T75), was genetically analyzed. Comparative analyses of these two soybean genomes uncovered surprising structural differences, especially with respect to translocation breakends, deletions, and inversions. In total, 12,028 structural variations (SVs) and 29,063 transposable elements (TEs) between T75 and Z9 were detected. In addition, 1336 potential genes were variable between T75 and Z9 in terms of SVs and TEs. These differential genes were enriched in functions such as defense response, cell wall-related processes, epigenetics, auxin metabolism and transport, signal transduction, and especially methylation, which implied that regulation of epigenetic mechanisms and TE activity are important in the space environment. These results are helpful for understanding the role of TEs in response to the space environment and provide a theoretical basis for the selection of wild plant materials suitable for space breeding.

Plants in Microgravity: Molecular and Technological Perspectives

Plants are vital components of our ecosystem for a balanced life here on Earth, as a source of both food and oxygen for survival. Recent space exploration has extended the field of plant biology, allowing for future studies on life support farming on distant planets. This exploration will utilize life support technologies for long-term human space flights and settlements. Such longer space missions will depend on the supply of clean air, food, and proper waste management. The ubiquitous force of gravity is known to impact plant growth and development. Despite this, we still have limited knowledge about how plants can sense and adapt to microgravity in space. Thus, the ability of plants to survive in microgravity in space settings becomes an intriguing topic to be investigated in detail. The new knowledge could be applied to provide food for astronaut missions to space and could also teach us more about how plants can adapt to unique environments. Here, we briefly review and discuss the current knowledge about plant gravity-sensing mechanisms and the experimental possibilities to research microgravity-effects on plants either on the Earth or in orbit.

Microgravity modelling by two-axial clinorotation leads to scattered organisation of cytoskeleton in Arabidopsis seedlings

Proper plant development in a closed ecosystem under weightlessness will be crucial for the success of future space missions. To supplement spaceflight experiments, such conditions of microgravity are modelled on Earth using a two-axial (2A) clinorotation, and in several fundamental studies resulted in the data on proteome and metabolome adjustments, embryo development, cell cycle regulation, etc. Nevertheless, our understanding of the cytoskeleton responses to the microgravity is still limited. In the present work, we study the adjustment of actin microfilaments (MFs) and microtubules (MTs) in Arabidopsis thaliana (L.) Heynh. seedlings under 2A clinorotation. Modelled microgravity resulted in not only the alteration of seedlings phenotype, but also a transient increase of the hydrogen peroxide level and in the cytoskeleton adjustment. Using GFP-fABD2 and Lifeact-Venus transgenic lines, we demonstrate that MFs became 'scattered' in elongating root and hypocotyl cells under 2A clinorotation. In addition, in GFP-MAP4 and GFP-TUA6 lines the tubulin cytoskeleton had higher fractions of transverse MTs under 2A clinorotation. Remarkably, the first static gravistimulation of continuously clinorotated seedlings reverted MF organisation to a longitudinal one in roots within 30 min. Our data suggest that the 'scattered' organisation of MFs in microgravity can serve as a good basis for the rapid cytoskeleton conversion to a 'longitudinal' structure under the gravity force.

Gravity sensing in plant and animal cells

Gravity determines shape of body tissue and affects the functions of life, both in plants and animals. The cellular response to gravity is an active process of mechanotransduction. Although plants and animals share some common mechanisms of gravity sensing in spite of their distant phylogenetic origin, each species has its own mechanism to sense and respond to gravity. In this review, we discuss current understanding regarding the mechanisms of cellular gravity sensing in plants and animals. Understanding gravisensing also contributes to life on Earth, e.g., understanding osteoporosis and muscle atrophy. Furthermore, in the current age of Mars exploration, understanding cellular responses to gravity will form the foundation of living in space.

Plant responses to real and simulated microgravity

Plant biology experiments in real and simulated microgravity have significantly contributed to our understanding of physiology and behavior of plants. How do plants perceive microgravity? How that perception translates into stimulus? And in turn plant's response and adaptation to microgravity through physiological, cellular, and molecular changes have been reasonably well documented in the literature. Knowledge gained through these plant biology experiments in microgravity helped to successfully cultivate crops in space. For instance, salad crop such as red romaine lettuce grown on the International Space Station (ISS) is allowed to incorporate into the crew's supplementary diet. However, the use of plants as a sustainable bio-regenerative life support system (BLSS) to produce fresh food and O2, reduce CO2 level, recycle metabolic waste, and efficient water management for long-duration space exploration missions requires critical gap filling research. Hence, it is inevitable to reflect and review plant biology microgravity research findings time and again with a new set of data available in the literature. With that in focus, the current article discusses phenotypic, physiological, biochemical, cell cycle, cell wall changes and molecular responses of plants to microgravity both in real and simulated conditions with the latest literature.

From Spaceflight to Mars g-Levels: Adaptive Response of A. Thaliana Seedlings in a Reduced Gravity Environment Is Enhanced by Red-Light Photostimulation

The response of plants to the spaceflight environment and microgravity is still not well understood, although research has increased in this area. Even less is known about plants' response to partial or reduced gravity levels. In the absence of the directional cues provided by the gravity vector, the plant is especially perceptive to other cues such as light. Here, we investigate the response of Arabidopsis thaliana 6-day-old seedlings to microgravity and the Mars partial gravity level during spaceflight, as well as the effects of red-light photostimulation by determining meristematic cell growth and proliferation. These experiments involve microscopic techniques together with transcriptomic studies. We demonstrate that microgravity and partial gravity trigger differential responses. The microgravity environment activates hormonal routes responsible for proliferation/growth and upregulates plastid/mitochondrial-encoded transcripts, even in the dark. In contrast, the Mars gravity level inhibits these routes and activates responses to stress factors to restore cell growth parameters only when red photostimulation is provided. This response is accompanied by upregulation of numerous transcription factors such as the environmental acclimation-related WRKY-domain family. In the long term, these discoveries can be applied in the design of bioregenerative life support systems and space farming.

Altered localisation of ZmPIN1a proteins in plasma membranes responsible for enhanced-polar auxin transport in etiolated maize seedlings under microgravity conditions in space

In the International Space Station experiment 'Auxin Transport', polar auxin transport (PAT) in shoots of etiolated maize (Zea mays L. cv. Golden Cross Bantam) grown under microgravity in space was substantially enhanced compared with those grown on Earth. To clarify the mechanism, the effects of microgravity on expression of ZmPIN1a encoding essential auxin efflux carrier and cellular localisation of its products were investigated. The amounts of ZmPIN1a mRNA in the coleoptiles and the mesocotyls in space-grown seedlings were almost the same as those in 1 g-grown seedlings, but its products were not. Immunohistochemical analysis with anti-ZmPIN1a antibody revealed a majority of ZmPIN1a localised in the basal side of plasma membranes of endodermal cells in the coleoptiles and the mesocotyls, and in the basal and lateral sides of plasma membranes in coleoptile parenchymatous cells, in which it directed towards the radial direction, but not towards the vascular bundle direction. Microgravity dramatically altered ZmPIN1a localisation in plasma membranes in coleoptile parenchymatous cells, shifting mainly towards the vascular bundle direction. These results suggest that mechanism of microgravity-enhanced PAT in maize shoots is more likely to be due to the enhanced ZmPIN1a accumulation and the altered ZmPIN1a localisation in parenchymatous cells of the coleoptiles.

Gravity-regulated localization of PsPIN1 is important for polar auxin transport in etiolated pea seedlings: Relevance to the International Space Station experiment

To clarify the mechanism of gravity-controlled polar auxin transport, we conducted the International Space Station (ISS) experiment "Auxin Transport" (identified by NASA's operation nomenclature) in 2016 and 2017, focusing on the expression of genes related to auxin efflux carrier protein PsPIN1 and its localization in the hook and epicotyl cells of etiolated Alaska pea seedlings grown for three days in the dark under microgravity (μg) and artificial 1 g conditions on a centrifuge in the Cell Biology Experiment Facility (CBEF) in the ISS, and under 1 g conditions on Earth. Regardless of gravity conditions, the accumulation of PsPIN1 mRNA in the proximal side of epicotyls of the seedlings was not different, but tended to be slightly higher as compared with that in the distal side. 2,3,5-Triiodobenzoic acid (TIBA) also did not affect the accumulation of PsPIN1 mRNA in the proximal and distal sides of epicotyls. However, in the apical hook region, TIBA increased the accumulation of PsPIN1 mRNA under μg conditions as compared with that under artificial 1 g conditions in the ISS. The accumulation of PsPIN1 proteins in epicotyls determined by western blotting was almost parallel to that of mRNA of PsPIN1. Immunohistochemical analysis with a specific polyclonal antibody of PsPIN1 revealed that a majority of PsPIN1 in the apical hook and subapical regions of the seedlings grown under artificial 1 g conditions in the ISS localized in the basal side (rootward) of the plasma membrane of the endodermal tissues. Conversely, in the seedlings grown under μg conditions, localization of PsPIN1 was greatly disarrayed. TIBA substantially altered the cellular localization pattern of PsPIN1, especially under μg conditions. These results strongly suggest that the mechanisms by which gravity controls polar auxin transport are more likely to be due to the membrane localization of PsPIN1. This physiologically valuable report describes a close relationship between gravity-controlled polar auxin transport and the localization of auxin efflux carrier PsPIN1 in etiolated pea seedlings based on the μg experiment conducted in space.

Circumnutational movement in rice coleoptiles involves the gravitropic response: analysis of an agravitropic mutant and space-grown seedlings

Plants exhibit helical growth movements known as circumnutation in growing organs. Some studies indicate that circumnutation involves the gravitropic response, but this notion is a matter of debate. Here, using the agravitropic rice mutant lazy1 and space-grown rice seedlings, we found that circumnutation was reduced or lost during agravitropic growth in coleoptiles. Coleoptiles of wild-type rice exhibited circumnutation in the dark, with vigorous oscillatory movements during their growth. The gravitropic responses in lazy1 coleoptiles differed depending on the growth stage, with gravitropic responses detected during early growth and agravitropism during later growth. The nutation-like movements observed in lazy1 coleoptiles at the early stage of growth were no longer detected with the disappearance of the gravitropic response. To verify the relationship between circumnutation and gravitropic responses in rice coleoptiles, we conducted spaceflight experiments in plants under microgravity conditions on the International Space Station. Wild-type rice seeds were germinated, and the resulting seedlings were grown under microgravity or a centrifuge-generated 1 g environment in space. We began filming the seedlings 2 days after seed imbibition and obtained images of seedling growth every 15 min. The seed germination rate in space was 92-100% under both microgravity and 1 g conditions. LED-synchronized flashlight photography induced an attenuation of coleoptile growth and circumnutational movement due to cumulative light exposure. Nevertheless, wild-type rice coleoptiles still showed circumnutational oscillations under 1 g but not microgravity conditions. These results support the idea that the gravitropic response is involved in plant circumnutation.

Polar auxin transport is essential to maintain growth and development of etiolated pea and maize seedlings grown under 1 g conditions: Relevance to the international space station experiment

We conducted "Auxin Transport" space experiments in 2016 and 2017 in the Japanese Experiment Module (JEM) on the International Space Station (ISS), with the principal objective being integrated analyses of the growth and development of etiolated pea (Pisum sativum L. cv Alaska) and maize (Zea mays L. cv Golden Cross Bantam) seedlings under true microgravity conditions in space relative to auxin dynamics. Etiolated pea seedlings grown under microgravity conditions in space for 3 days showed automorphogenesis. Epicotyls and roots bent ca. 45° and 20° toward the direction away from the cotyledons, respectively, whereas those grown under artificial 1 g conditions produced by a centrifuge in the Cell Biology Experimental Facility (CBEF) in space showed negative and positive gravitropic response in epicotyls and in roots, respectively. On the other hand, the coleoptiles of 4-day-old etiolated maize seedlings grew almost straight, but the mesocotyls curved and grew toward a random direction under microgravity conditions in space. In contrast, the coleoptiles and mesocotyls of etiolated maize seedlings grown under 1 g conditions on Earth were almost straight and grew upward or toward the direction against the gravity vector. The polar auxin transport activity in etiolated pea epicotyls and in maize shoots was significantly inhibited and enhanced, respectively, under microgravity conditions in space as compared with artificial 1 g conditions in space or 1 g conditions on Earth. An inhibitor of polar auxin transport, 2,3,5-triiodobenzoic acid (TIBA) substantially affected the growth direction and polar auxin transport activity in etiolated pea seedlings grown under both artificial 1 g and microgravity conditions in space. These results strongly suggest that adequate polar auxin transport is essential for gravitropic response in plants. Possible mechanisms enhancing polar auxin transport in etiolated maize seedlings grown under microgravity conditions in space are also proposed.

Simulated microgravity and the antagonistic influence of strigolactone on plant nutrient uptake in low nutrient conditions

Human-assisted space exploration will require efficient methods of food production. Large-scale farming in presence of an Earth-like atmosphere in space faces two main challenges: plant yield in microgravity and plant nutrition in extraterrestrial soils, which are likely low in nutrients compared to terrestrial farm lands. We propose a plant-fungal symbiosis (i.e. mycorrhiza) as an efficient tool to increase plant biomass production in extraterrestrial environments. We tested the mycorrhization of Solanaceae on the model plant Petunia hybrida using the arbuscular mycorrhizal fungus Rhizophagus irregularis under simulated microgravity (s0-g) conditions obtained through a 3-D random positioning machine. Our results show that s0-g negatively affects mycorrhization and plant phosphate uptake by inhibiting hyphal elongation and secondary branching. However, in low nutrient conditions, the mycorrhiza can still support plant biomass production in s0-g when colonized plants have increased SL root exudation. Alternatively, s0-g in high nutrient conditions boosts tissue-specific cell division and cell expansion and overall plant size in Petunia, which has been reported for other plants species. Finally, we show that the SL mimic molecule rac-GR24 can still induce hyphal branching in vitro under simulated microgravity. Based on these results, we propose that in nutrient limited conditions strigolactone root exudation can challenge the negative microgravity effects on mycorrhization and therefore might play an important role in increasing the efficiency of future space farming.

The Role of the Primary Cell Wall in Plant Morphogenesis

Morphogenesis remains a riddle, wrapped in a mystery, inside an enigma. It remains a formidable problem viewed from many different perspectives of morphology, genetics, and computational modelling. We propose a biochemical reductionist approach that shows how both internal and external physical forces contribute to plant morphogenesis via mechanical stress⁻strain transduction from the primary cell wall tethered to the plasma membrane by a specific arabinogalactan protein (AGP). The resulting stress vector, with direction defined by Hechtian adhesion sites, has a magnitude of a few piconewtons amplified by a hypothetical Hechtian growth oscillator. This paradigm shift involves stress-activated plasma membrane Ca2+ channels and auxin-activated H⁺-ATPase. The proton pump dissociates periplasmic AGP-glycomodules that bind Ca2+. Thus, as the immediate source of cytosolic Ca2+, an AGP-Ca2+ capacitor directs the vectorial exocytosis of cell wall precursors and auxin efflux (PIN) proteins. In toto, these components comprise the Hechtian oscillator and also the gravisensor. Thus, interdependent auxin and Ca2+ morphogen gradients account for the predominance of AGPs. The size and location of a cell surface AGP-Ca2+ capacitor is essential to differentiation and explains AGP correlation with all stages of morphogenetic patterning from embryogenesis to root and shoot. Finally, the evolutionary origins of the Hechtian oscillator in the unicellular Chlorophycean algae reflect the ubiquitous role of chemiosmotic proton pumps that preceded DNA at the dawn of life.

Procedures for chemical fixation in immunohistochemical analyses of PIN proteins regulating polar auxin transport: Relevance to spaceflight experiments

The mechanism by which gravity controls the polar transport of auxin, a plant hormone regulating multiple physiological processes in higher plants, remains unclear, although an important role of PIN proteins as efflux carriers/facilitators in polar auxin transport is suggested. We are going to study the effect of microgravity on the polar transport of auxin, focusing on the cellular localization of its efflux carrier, PsPIN1 in etiolated pea seedlings and ZmPIN1a in etiolated maize seedlings grown under microgravity conditions on the International Space Station (ISS) using immunohistochemical analyses according to space experimental plans (Ueda, 2016). To obtain adequate results regarding the cellular localization of functional proteins, prolonged chemical fixation processes as well as chemical fixatives should be well-matched to the properties of functional proteins as antigens since experimental analyses will be performed on the ground after keeping samples for a long duration on the ISS. As a result of ground verification, clear detection of the cellular localization of PsPIN1 and ZmPIN1a immunohistochemically was successful based on the results of several kinds of chemical fixation tested, even when etiolated pea and maize seedlings were fixed by immersion in chemical fixative for a long duration. The addition of 0.1% (w/v) Nonidet P-40 to chemical fixative composed of 50% (v/v) ethanol and 5% (v/v) acetic acid or that of 50% (v/v) methanol and 5% (v/v) acetic acid has led to a significant improvement in the immunohistochemical detection of PsPIN1 or ZmPIN1a. These chemical fixatives were also shown to be storage-stable for a long time before use. In this study, adequate chemical fixatives and fixation protocols were developed, which can be used to detect localization of PsPIN1 and ZmPIN1a proteins in young etiolated pea and maize seedlings, respectively, using anti PsPIN1 and ZmPIN1a antibodies. These protocols can be used in spaceflight experiments to investigate the effects of the microgravity environment on the ISS on PIN protein localization in pea and maize seedlings.

Single-base resolution methylome analysis shows epigenetic changes in Arabidopsis seedlings exposed to microgravity spaceflight conditions on board the SJ-10 recoverable satellite

DNA methylation is a very important epigenetic modification that participates in many biological functions. Although many studies of DNA methylation have been reported in various plant species, few studies have assessed the global DNA methylation pattern in plants challenged by exposure to microgravity conditions. In this report, we mapped the Arabidopsis genome methylation pattern changes associated with microgravity conditions on board the Chinese recoverable scientific satellite SJ-10 at single-base resolution. Interestingly, we found epigenetic differences in Arabidopsis seedlings exposed to microgravity in that the Arabidopsis genome exhibits lower methylation levels in the CHG, CHH, and CpG contexts under microgravity conditions. Microgravity stimulation was related to altered methylation of a number of genes, including DNA methylation-associated genes, hormone signaling related genes, cell-wall modification genes and transposable elements (TEs). Relatively unstable DNA methylation of TEs was responsible for the induction of active transposons. These observations suggest that DNA demethylation within TEs may affect the transcription of transposons in response to microgravity conditions. In summary, the results of this investigation are beneficial for understanding the mechanism of plant adaptation to microgravity and improve strategies to allow plants to adapt to space.

Spaceflight alters the pattern of chemical tags that adorn DNA in plant seedlings. Weiming Cai and colleagues from the Shanghai Institute of Plant Physiology and Ecology, China, profiled the genome-wide epigenetic patterns of Arabidopsis thaliana seedlings that spent 60 h in the microgravity of low Earth orbit aboard the Shijian-10 recoverable satellite. They analyzed the distribution of methyl tags across the genome — an epigenetic mark that affects gene expression levels — and found that seedlings exposed to microgravity had lower methylation on average than control plants grown on the ground, although certain genes related to methylation, transcription factors and hormones tended to be more methylated. Epigenetic differences were also observed among genes involved in cell-wall modification and in transposable elements. The findings could help inform the design of plants optimized for growth in space.

Regulation of asymmetric polar auxin transport by PsPIN1 in endodermal tissues of etiolated Pisum sativum epicotyls: focus on immunohistochemical analyses

This manuscript reports the production of specific polyclonal antibodies for PsPIN1, a putative auxin efflux carrier in Alaska pea (Pisum sativum L.) plants, and the cellular immunolocalization of PsPIN1. When pea seeds were set with the seed axis horizontal to the upper surface of a rockwool block, and allowed to germinate and grow for 3 days in the dark, the epicotyl grew upward. On the other hand, the application of 2,3,5-triiodobenzoic acid (TIBA) inhibited graviresponse. In the subapical epicotyl regions, PsPIN1 has been found to localize in the basal side of the plasma membrane of cells in endodermal tissues. Asymmetric PsPIN1 localization between the proximal and distal sides of the epicotyl was observed, the total amounts of PsPIN1 being more abundant in the proximal side. The asymmetric PsPIN1 distribution between the proximal and distal sides of the epicotyl was well correlated with unequal polar auxin transport as well as asymmetric accumulation of mRNA of PsPIN1 (Ueda et al. in Biol Sci Space 26:32-41, 2012; Ueda et al. in Plant Biol 16(suppl 1):43-49, 2014). In the proximal side of an apical hook, PsPIN1 localized in the basal side of the plasma membrane of cells in endodermal tissues, whereas in the distal side, the abundant distribution of PsPIN1 localized in the basal-lower (endodermal) side of the basal plasma membrane, suggesting possible lateral auxin movement from the distal side to the proximal side in this region. The application of TIBA significantly reduced the amount of PsPIN1 in the proximal side of epicotyls, but little in the distal side. These results suggest that unequal auxin transport in epicotyls during the early growth stage of etiolated pea seedlings is derived from asymmetric PsPIN1 localization in the apical hook and subapical region of epicotyls, and that asymmetric transport between the proximal and distal sides of epicotyls is required for the graviresponse of epicotyls.

Persistence of plant hormone levels in rice shoots grown under microgravity conditions in space: its relationship to maintenance of shoot growth

We investigated the effects of microgravity environment on growth and plant hormone levels in dark-grown rice shoots cultivated in artificial 1 g and microgravity conditions on the International Space Station (ISS). Growth of microgravity-grown shoots was comparable to that of 1 g-grown shoots. Endogenous levels of indole-3-acetic acid (IAA) in shoots remained constant, while those of abscisic acid (ABA), jasmonic acid (JA), cytokinins (CKs) and gibberellins (GAs) decreased during the cultivation period under both conditions. The levels of auxin, ABA, JA, CKs and GAs in rice shoots grown under microgravity conditions were comparable to those under 1 g conditions. These results suggest microgravity environment in space had minimal impact on levels of these plant hormones in rice shoots, which may be the cause of the persistence of normal growth of shoots under microgravity conditions. Concerning ethylene, the expression level of a gene for 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, the key enzyme in ethylene biosynthesis, was reduced under microgravity conditions, suggesting that microgravity may affect the ethylene production. Therefore, ethylene production may be responsive to alterations of the gravitational force.