Plants in Space: Microgravity, Biochemistry, and Development

Introduction

With the current rise in space travel research, there arises the question of bioregenerative life support systems (BLSS). BLSS are closed environments that utilize a complex system of biological and technological components to sustain human life during long-term space missions. Such systems mainly rely on plants to recycle nutrients and air, as well as to filter out compounds harmful to human health. Thus, to maintain a BLSS, it is imperative to cultivate healthy and successful plants. Spacecraft conditions complicate this task, as microgravity, ionizing radiation, hypomagnetic fields, and other factors all influence and often stress plant development [1]. The objectives of this review are to: 1) evaluate the effects of microgravity on plant development through the metrics of bioactive compound content, germination rate, and morphology, and 2) investigate means of offsetting said effects in plants.

Background

Gravity-Sensing Mechanisms

Gravity sensing in plants occurs through multiple possible pathways, most notably starch statoliths and protoplast sensing. The starch statolith hypothesis postulates that columella cells in the root cap possess amyloplasts (starch statoliths) that settle and sediment towards the direction of the gravity vector. Amyloplasts possess LAZY proteins, which participate in gravity-induced signaling to regulate integral membrane-localized PIN-FORMED (PIN)  proteins. PIN proteins are involved in modulating auxin efflux and generating differential growth [2]. In contrast, the gravitational pressure hypothesis alternatively proposes that protoplasts act as gravity-influenced bodies; protoplast deformation exerts pressure to induce cellular signaling, initiating PIN protein movement. Downstream PIN protein functionality and auxin efflux are largely the same as in the starch statolith hypothesis [3]. While these mechanisms detect gravitational vectors, various environmental conditions—in this case microgravity—may alter or inhibit their functionality.

Microgravity & Other Stressors

Microgravity environments cause objects to display extremely small apparent accelerations in relation to a reference frame, typically because both the objects and the reference frame are in a constant state of freefall [4]. It is primarily experienced on orbiting spacecraft—for example the ISS—and secondarily on parabolic flights and in drop towers. Due to the difficulty of establishing true microgravity environments for testing, such conditions are often simulated with clinostats and random positioning machines (RPMs). Both machines utilize rotation to disturb the influence of a constant downward gravity vector [5]. Aside from microgravity, the cultivation of plants in space involves various other stressors. Astronauts and plants alike experience elevated ionizing radiation levels, as well as hypomagnetic fields [6, 7]. Such stressors trigger various defense mechanisms that often combine and compete with microgravity in terms of phenotypic effect.

One notable phenotype involves the accumulation of reactive oxygen species (ROS). ROS—produced in a wide variety of harmful abiotic conditions—serve as a broad indicator of plant stress and play critical roles in cellular proliferation and stress response pathways. The exact mechanism through which ROS occurs is still a subject of debate, though experimental data has suggested that it may be associated with mitochondrial, chloroplastic, or signaling dysfunction [6, 8]. To reduce ROS levels, plants synthesize phenolic compounds, including phenolic acids, flavonoids, and lignans, many of which serve as antioxidants. These biocompounds—a wide variety of which are synthesized through the phenylpropanoid pathway—mitigate the effects of oxidative stress in plant tissues, including cellular damage and various degenerative conditions.

Effects of Microgravity

Oxidative Stress, Phenolic Compounds, & Lignin

Current studies indicate that simulated microgravity increases concentrations of both reactive oxygen species (ROS) and antioxidants. For example, Trifolium repens (white clover) grown from seed in an RPM has been shown to exhibit elevated isoflavone levels [9]. Other studies report similar results; clinorotated Ocimum basilicum (basil) seedlings possessed increased phenol and carotenoid contents, while clinorotated Zea mays (corn) seedlings exhibited increased antioxidant activity through higher flavonoid and total phenol contents [10, 11]. Brassica rapa (field mustard) under RPM rotation and ionizing radiation reportedly accumulated both ROS and phenols [12]. ROS were distributed uniformly across tissues. In contrast, polyphenol distribution was localized at the roots’ steles when treated with simulated microgravity but was homogenous when treated with a combination of simulated microgravity and ionizing radiation. This phenomenon may inform future crop choices when farming under microgravity conditions, as ionizing radiation may influence antioxidant localization within crops.

Contrary to the previous studies, RPM-rotated Lactuca sativa (lettuce) possessed lower phenol contents than control samples [7]. Carotenoid levels remained relatively consistent between both treatments, with no significant differences. ROS levels saw significant reductions in root samples. These observations may be attributed to the reported downregulation of the Respiratory Burst Oxidase Homolog D (RBOHD) gene [7], which encodes for ROS-producing NADPH oxidase enzymes [13]. This downregulation may partially explain the decrease in phenol content, as ROS trigger signaling cascades that activate both enzymatic antioxidant-producing genes and secondary metabolite pathways [14, 15].

These experiments reveal magnified antioxidant production in Trifolium repens, Ocimum basilicum, Zea mays, and Brassica rapa, likely due to stress-induced elevated ROS levels. This lines up with mechanistic explanations for ROS production, as microgravity-induced rearrangement of cellular architecture has been linked to free radical generation [6]. However, in Lactuca sativa, ROS and antioxidant levels both declined. It can thus be generalized that while microgravity stress in plants tends to trigger the formation of oxidative species that then mediate antioxidant production, regulation of related pathways is species-dependent.

In comparison to controls, microgravity-treated mung bean seedlings have been found to possess significant reductions in lignin content [16]. Lignin acts as a structural polymer critical to the development of plant secondary cell walls. As such, lignin is intrinsically connected to plant morphology. Lignin—being composed of phenylpropanoid units—secondarily possesses radical-scavenging properties as an important destination in the phenylpropanoid pathway [17]. This pathway also branches off to produce anthocyanins, flavonols, hydroxycinnamic acids, and other secondary metabolites. The broad influence of microgravity on these compounds suggests that the phenylpropanoid pathway is modulated at one or more of its steps during spaceflight.

Germination, Cell Proliferation, & Development

Microgravity appears to influence both germination and biomass accumulation, though high variability in current studies complicates the determination of broad directional trends. Clinorotated Zea mays, Arachis hypogaea (peanut), and Solanum lycopersicum (tomato) displayed increased alpha-amylase, lipase, and beta-glucanase activity [11]. Alpha-amylase and lipase are germination-associated enzymes that fuel embryonic growth through the catabolism of starches and lipids, respectively, while beta-glucanase is associated with cell elongation through the remodeling of polysaccharides in the cell wall [18]. This indicates that microgravity may improve seedling germination and growth rate. Such results are consistent with prior research, which reports increased seedling weight, root length, and shoot length in clinorotated Ocimum basilicum that intensified with lower gravity treatments [10]. Moreover, microgravity-treated samples possessed greater total carbohydrate and total protein levels. This phenomenon may be attributed to the observed upregulation of Rubisco genes, which promote photosynthetic activity. However, separate research has indicated that clinorotated Zea mays displays lowered germination rates and seedling weights [19]. Increasing clinorotation from 0 rpm (control) to 2.0 rpm was correlated with a gradual decrease in seedling dry weight. In addition, clinorotation at and above 1.0 rpm increased seed germination time. The inconsistencies regarding the germination activity of clinorotated Zea mays may be caused by differences in clinorotation speed. Observations of increased alpha-amylase and lipase activity were based on clinorotation speeds of 85 rpm, while observations of decreased germination rate were based on speeds of 0.5, 1.0, and 2.0 rpm. Higher rotation speeds (among other factors such as developmental stage, exposure duration, and rotational axis) may confound data by increasing shear stress and altering fluid dynamics, as neither occurs in real microgravity [5]. Additionally, increases in alpha-amylase and lipase activity only suggest a positive relationship between microgravity and germination through the mechanistic lens of enzyme activity. Due to the influence of other biochemical systems, this may not translate into specific germination patterns similar to the phenotype-based data in the opposing study. Thus, differences in observations stem from separate regulatory layers that lack direct causality.

Attempts to germinate seeds of 14 different species within the Fabaceae family saw little success; only Trifolium repens (white clover) sprouted [9]. This was coupled with observed increases in phenylalanine ammonia lyase (PAL)—key to the first step in the phenylpropanoid pathway—activity. Such results contradict other research within the field, as microgravity-treated pine seedlings have been reported to possess reduced PAL activity [16]. The upregulation of PAL has been previously associated with increased germination rate and viability under high temperature and humidity stress [20]. While temperature and humidity stress differ from microgravity stress, the data altogether suggests that other factors may have influenced both PAL activity and germination success among the Fabaceae family. Moreover, decreases in Fabaceae germination cannot be concretely extrapolated to broader contexts, as the previous literature on corn, peanut, and tomato plants suggests that microgravity’s impact on germination success is species-dependent. However, the findings indicate that similar to antioxidant levels (which then mechanistically extend to ROS levels), germination rates may also be tied to modulation of the phenylpropanoid pathway.

Post-germination, microgravity conditions have been correlated with disrupted meristematic activity. Meristematic activity involves the capacity of cells to engage in developmental organization through the balance and coordination of cellular proliferation and growth; thus, it is fundamental to morphology. Both RPM and true microgravity treatment of Arabidopsis thaliana have been reported to increase cellular proliferation in root meristematic cells and decrease cellular growth [21]. This—along with the previously discussed lignin reductions observed in microgravity—may explain the morphological differences between plants grown in microgravity and 1g. Moreover, karyological analysis of Avena sativa (oat), Vigna radiata (mung bean), and Helianthus annuus (sunflower) has revealed chromosomal aberration among Avena sativa and Helianthus annuus samples following exposure of all three plants to microgravity [22]. Avena sativa exhibited chromosomal fracture, while Helianthus annuus displayed aneuploidy and improper chromosomal segregation during cellular division. Taken together, these results suggest that microgravity influences cellular proliferation and growth, possibly interacting with chromosomal segregation during mitosis, though a mechanistic explanation for these observations has yet to be discovered.

Offsetting the Effects of Microgravity

The effects of microgravity can be offset through phototropism, hormone supplementation, and other directional stimulation. For example, experimentation on transparent testa 3 (tt3) mutant Arabidopsis thaliana (thale cress) plants highlights how phototropic response in roots can guide both root and shoot growth [23]. TT3 encodes dihydroflavonol 4-reductase (DFR), a critical junction point in the phenylpropanoid pathway, as it competes with flavonol synthase (FLS) to delineate the starting points of the anthocyanin/proanthocyanidin and flavonol pathways, respectively. Because flavonols modulate PIN and auxin efflux carrier ATP-binding cassette B (ABCB) as inhibitory regulators of auxin transport, DFR’s ability to compete against FLS is implicated as a factor in gravitropic curvature and growth. tt3 mutants, through their encoding of defective DFR, have been reported to accumulate flavonols kaempferol and quercetin and possess inhibited polar auxin transport, consistent with the phenylpropanoid pathway [24]. However, tt3 samples with only shoot light exposure expressed growth similar to wild-type samples under the same treatment. This contrasts with treatment of both shoots and roots to light, where tt3 mutants expressed significantly more disordered growth patterns than their wild-type counterparts. Experimentation on RPM-treated wild-type Arabidopsis thaliana provides similar results; between photoperiod and darkness treatments under both 1g conditions and simulation microgravity, photoperiod-treated plants under RPM conditions displayed growth more similar to their 1g counterparts than darkness-treated plants under RPM conditions with their respective counterparts [25]. Thus, phototropic responses may compensate for gravitropic impairment in terms of growth direction. More specifically, these growth pattern “corrections” may be connected to hypocotyl positive phototropism and root negative phototropism, which may localize auxin in microgravity-grown plants that cannot stably generate such asymmetries through gravitropic systems.

In addition, increased strigolactone (SL) exudation may aid arbuscular mycorrhizal fungi (AMF) symbiosis and promote biomass production. Upon treating Petunia hybrida (garden petunia) and AMF symbiotic systems with Earth gravity and simulated microgravity via RPM, microgravity samples displayed fewer arbuscules than Earth gravity samples [26]. Arbuscules—formed from the interfacing of fungal hyphae and plant root cortical cells—serve as markers of plant-fungal nutrient exchange. Moreover, microgravity samples exhibited lower levels of phosphate sequestration, which suggests that plants and AMF struggle to aid each other under microgravity conditions due to lack of root-hyphae connectivity. This complicates agriculture in low-nutrient environments where symbiotic nutrient exchange could prove crucial to supplement standard uptake. Symbiotic growth tests with PDR1 overexpressed (OE) petunia samples—known for increased SL exudation—under microgravity conditions led to increased biomass production and arbuscule count compared to wild-type counterparts. This suggests that localized SL exudation through PDR1 OE disinhibits AMF hyphal extension and consequently promotes nutrient uptake.

These studies highlight the importance of directional signaling in maintaining growth along an axis. Microgravity lacks a consistent gravitational vector, which interferes with plant systems that rely on gravitropic sensation. However, localized applications of light and SLs aid in mitigating undesirable phenotypes by compensating for a lack of stable gravity-sensing through alternative directional systems. Further research is necessary to determine how the polarization of other directional plant systems would impact phenotypic response.

Discussion

Studying plant responses in microgravity yields inconsistent experimental results partially due to a lack of standardization in terms of species choice. Different species, even within the same families, vary in terms of their genomic, proteomic, and metabolomic landscapes. As such, one can expect them to also differ in their responses to microgravity. However, species diversity is necessary in space agriculture applications; unless merely supplementing other dietary plans, it is critical to cultivate a variety of crops to meet the astronauts’ nutritional requirements [1]. Inconsistency in results may also be attributed to technological and methodological differences. Clinostats and RPMs, while conceptually similar, differ in mechanical functionality. Clinostats rotate samples over a single axis. RPMs, while also able to rotate samples on a single axis, mainly use two independent axes to randomize orientation in a three-dimensional field. While studies on macrophages show no statistically significant difference between two- and three-dimensional rotation in terms of ROS production, there is an absence of similar testing done with plants and with metrics aside from ROS production [27]. Rotational speeds affect microgravity simulation efficacy by modulating the speed at which the plant axis is reoriented in relation to the gravity vector. High rotational speeds may increase centrifugal forces and fluid shear stress, confounding results. Though previous research has noted phenotypic similarities between plants grown in simulated microgravity and true microgravity conditions, the two cannot be equated due to the fundamental differences in how they operate [25]. Phenotypes observed during spaceflight additionally may be caused by factors other than microgravity, including radiation and magnetic fields. Thus, true spaceflight conditions act as a complex interplay of unique environmental factors—of which microgravity is only one.

Inconsistencies in germination and biomass accumulation can also possibly be attributed, in part, to the dual nature of reactive oxygen species. At lower levels, ROS are essential for cellular replication as signaling molecules within the cell cycle. However, high levels of ROS can cause oxidative damage that disrupts regular cellular processes. Therefore, metrics that involve cellular replication and plant development may differ by ROS concentration and their associated experimental conditions.

Nearly every phenotype described in this paper has been associated with the phenylpropanoid pathway. Modulations of the phenylpropanoid pathway have been correlated with changes in antioxidant levels, ROS levels, lignin biosynthesis, germination success, and plant morphology. Conflicts in results between studies may suggest that microgravity influences this pathway at multiple levels, creating a complex regulatory system that requires further research to break down.

Conclusion

The study of astrobotany is a continuously expanding field, built upon a foundation of microgravity simulation to make up for the difficulty of establishing natural microgravitational conditions. Current evidence generally indicates that microgravity generally induces oxidative stress in plants, as shown through increases in ROS and phenol content within a variety of species, though some studies provide evidence towards the contrary. There are also pronounced effects on germination rate and biomass accumulation, though it is difficult to identify any exact trends due to conflicts in data. The effects of microgravity may be offset through alternative directional signaling, notably with phototropic manipulation and localized strigolactone application. Many microgravity-induced changes to plant biochemistry have been associated with components of the phenylpropanoid pathway, though a direct mechanistic connection between microgravity environments and the pathway has yet to be elucidated. Additional research is needed to investigate the intricacies of gravitropic sensing and response, as well as to establish more effective bioregenerative life support systems in the future.

About the Author: Tyler Murai

Tyler is a 3rd year Plant Biology major and Computational Biology minor. He serves as a Senior Editor and as the Director of Outreach for The Aggie Transcript. Outside of the journal, he works at the Shabek Lab to investigate the roles of various E3 ubiquitin ligases in plant systems. True to the expectations of those who see him working with plants all day, he abides by the hippie-ish maxim of “peace and love.”

Author’s Note

I wrote this paper in UWP 104E under Dr. Pamela Chisum. Plants have always been treated by the scientific community as inferior to animals (and I can’t even imagine how mycologists feel in all of this), but I say that the age of animal dominance must come to an end. Plants make up a major portion of our diets, play a key role in the bioremediation of our environment, and even had a brief stint in the world of vaccine production with Covifenz. My paper centers on the role of plants in space, inspired by one of my favorite childhood books: Andy Weir’s The Martian. Could we create successful potato farms on Mars? It remains science fiction for now. However, with our current research into shipping humans to faraway planets, plants have found a little nook in the space scene. Nutrients—water—air—they recycle nearly everything a person needs. This, I hope, is plant biology’s chance to escape large-scale agriculture for just a moment and find some new relevance amongst the stars.

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