Changes in Arabidopsis leaf ultrastructure, chlorophyll and carbohydrate content during spaceflight depend on ventilation

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.

RNA-seq analyses of Arabidopsis thaliana seedlings after exposure to blue-light phototropic stimuli in microgravity

PREMISE

Plants synthesize information from multiple environmental stimuli when determining their direction of growth. Gravity, being ubiquitous on Earth, plays a major role in determining the direction of growth and overall architecture of the plant. Here, we utilized the microgravity environment on board the International Space Station (ISS) to identify genes involved influencing growth and development of phototropically stimulated seedlings of Arabidopsis thaliana.

METHODS

Seedlings were grown on the ISS, and RNA was extracted from 7 samples (pools of 10-15 plants) grown in microgravity (μg) or Earth gravity conditions (1-g). Transcriptomic analyses via RNA sequencing (RNA-seq) of differential gene expression was performed using the HISAT2-Stringtie-DESeq2 RNASeq pipeline. Differentially expressed genes were further characterized by using Pathway Analysis and enrichment for Gene Ontology classifications.

RESULTS

For 296 genes that were found significantly differentially expressed between plants in microgravity compared to 1-g controls, Pathway Analysis identified eight molecular pathways that were significantly affected by reduced gravity conditions. Specifically, light-associated pathways (e.g., photosynthesis-antenna proteins, photosynthesis, porphyrin, and chlorophyll metabolism) were significantly downregulated in microgravity.

CONCLUSIONS

Gene expression in A. thaliana seedlings grown in microgravity was significantly altered compared to that of the 1-g control. Understanding how plants grow in conditions of microgravity not only aids in our understanding of how plants grow and respond to the environment but will also help to efficiently grow plants during long-range space missions.

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.

Long-term exposure of rapeseed (Brassica napus L.) to ZnO nanoparticles: anatomical and ultrastructural responses

Rapid development of nanotechnology in recent years has raised concerns about nanoparticle (NPs) release into the environment and its adverse effects on living organisms. The present study is the first comprehensive report on the anatomical and ultrastructural changes of a variety of cells after long-term exposure of plant to NPs or bulk material particles (BPs). Light and electron microscopy revealed some anatomical and ultrastructural modifications of the different types of cell in the root and leaf, induced by both types of treatment. Zinc oxide (ZnO) BPs-induced modifications were surprisingly more than those induced by ZnO NPs. The modifications induced by ZnO BPs or ZnO NPs were almost similar to those induced by excess Zn. Zn content of the root and leaf of both ZnO NPs- and ZnO BPs-treated plants was severely increased, where the increase was greater in the plants treated with ZnO BPs. Overall, these results indicate that the modifications induced by ZnO particles can be attributed, at least partly, to the Zn(2+) dissolution by ZnO particles rather than their absorption by root and their subsequent effects.

Clinorotation impacts root apex respiration and the ultrostructure of mitochondria

Mitochondrial respiration in plants provides energy for biosynthesis, and its balance with photosynthesis determines the rate of plant biomass accumulation. However, there are very limited data on the influence of altered gravity on the functional status of plant mitochondria. In the given paper, we presented the results of our investigations of root respiration, the mitochondrion ultrastructure, and AOX expression of pea 1-, 3- and 5-day old seedlings grown under slow horizontal clinorotation by using an inhibitor analysis, electron microscopy, and quantitative real-time RT-PCR. It was in the first time shown that enhancement of the respiration rate in root apices of pea etiolated seedlings at the 5th day of clinorotation does not connected with increasing of both alternative oxidize capacity and AOX expression. We assumed this phenomenon is provided by more intensive oxidation of respiratory substrates. At the structural level, mitochondria in cells of the distal elongation zone were the most sensitive to clinorotation that confirms the special physiological status of this zone. The performed investigation revealed an enough resistance of plant mitochondria to the influence of altered gravity that, on our opinion, is one of components providing plant adaptation to microgravity in space flight.

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.

3D Plant cell architecture of Arabidopsis thaliana (Brassicaceae) using focused ion beam-scanning electron microscopy

PREMISE OF THE STUDY

Focused ion beam-scanning electron microscopy (FIB-SEM) combines the ability to sequentially mill the sample surface and obtain SEM images that can be used to create 3D renderings with micron-level resolution. We have applied FIB-SEM to study Arabidopsis cell architecture. The goal was to determine the efficacy of this technique in plant tissue and cellular studies and to demonstrate its usefulness in studying cell and organelle architecture and distribution. •

METHODS

Seed aleurone, leaf mesophyll, stem cortex, root cortex, and petal lamina from Arabidopsis were fixed and embedded for electron microscopy using protocols developed for animal tissues and modified for use with plant cells. Each sample was sectioned using the FIB and imaged with SEM. These serial images were assembled to produce 3D renderings of each cell type. •

RESULTS

Organelles such as nuclei and chloroplasts were easily identifiable, and other structures such as endoplasmic reticula, lipid bodies, and starch grains were distinguishable in each tissue. •

DISCUSSION

The application of FIB-SEM produced 3D renderings of five plant cell types and offered unique views of their shapes and internal content. These results demonstrate the usefulness of FIB-SEM for organelle distribution and cell architecture studies.

Characterization of photosystem I in rice (Oryza sativa L.) seedlings upon exposure to random positioning machine

To gain a better understanding of how photosynthesis is adapted under altered gravity forces, photosynthetic apparatus and its functioning were investigated in rice (Oryza sativa L.) seedlings grown in a random positioning machine (RPM). A decrease in fresh weight and dry weight was observed in rice seedlings grown under RPM condition. No significant changes were found in the chloroplast ultrastructure and total chlorophyll content between the RPM and control samples. Analyses of chlorophyll fluorescence and thermoluminescence demonstrate that PSII activity was unchanged under RPM condition. However, PSI activity decreased significantly under RPM condition. 77 K fluorescence emission spectra show a blue-shift and reduction of PSI fluorescence emission peak in the RPM seedlings. In addition, RPM caused a significant decrease in the amplitude of absorbance changes of P700 at 820 nm (A 820) induced by saturated far-red light. Moreover, the PSI efficiency (Φ I) decreased significantly under RPM condition. Immunoblot and blue native gel analyses further illustrate that accumulation of PSI proteins was greatly decreased in the RPM seedlings. Our results suggest that PSI, but not PSII, is down-regulated under RPM condition.

Analyses of tropistic responses using metabolomics

PREMISE OF THE STUDY

Characterization of phototropism and gravitropism has been through gene expression studies, assessment of curvature response, and protein expression experiments. To our knowledge, the current study is the first to determine how the metabolome, the complete set of small-molecule metabolites within a plant, is impacted during these tropisms.

METHODS

We have determined the metabolic profile of plants during gravitropism and phototropism. Seedlings of Arabidopsis thaliana wild type (WT) and phyB mutant were exposed to unidirectional light (red or blue) or reoriented to induce a tropistic response, and small-molecule metabolites were assayed and quantified. A subset of the WT was analyzed using microarray experiments to obtain gene profiling data.

KEY RESULTS

Analyses of the metabolomic data using principal component analysis showed a common profile in the WT during the different tropistic curvatures, but phyB mutants produced a distinctive profile for each tropism. Interestingly, the gravity treatment elicited the greatest changes in gene expression of the WT, followed by blue light, then by red light treatments. For all tropisms, we identified genes that were downregulated by a large magnitude in carbohydrate metabolism and secondary metabolism. These included ATCSLA15, CELLULOSE SYNTHASE-LIKE, and ATCHS/SHS/TT4, CHALCONE SYNTHASE. In addition, genes involved in amino acid biosynthesis were strongly upregulated, and these included THA1 (THREONINE ALDOLASE 1) and ASN1 (DARK INDUCIBLE asparagine synthase).

CONCLUSIONS

We have established the first metabolic profile of tropisms in conjunction with transcriptomic analyses. This approach has been useful in characterizing the similarities and differences in the molecular mechanisms involved with phototropism and gravitropism.

Fundamental plant biology enabled by the space shuttle

The relationship between fundamental plant biology and space biology was especially synergistic in the era of the Space Shuttle. While all terrestrial organisms are influenced by gravity, the impact of gravity as a tropic stimulus in plants has been a topic of formal study for more than a century. And while plants were parts of early space biology payloads, it was not until the advent of the Space Shuttle that the science of plant space biology enjoyed expansion that truly enabled controlled, fundamental experiments that removed gravity from the equation. The Space Shuttle presented a science platform that provided regular science flights with dedicated plant growth hardware and crew trained in inflight plant manipulations. Part of the impetus for plant biology experiments in space was the realization that plants could be important parts of bioregenerative life support on long missions, recycling water, air, and nutrients for the human crew. However, a large part of the impetus was that the Space Shuttle enabled fundamental plant science essentially in a microgravity environment. Experiments during the Space Shuttle era produced key science insights on biological adaptation to spaceflight and especially plant growth and tropisms. In this review, we present an overview of plant science in the Space Shuttle era with an emphasis on experiments dealing with fundamental plant growth in microgravity. This review discusses general conclusions from the study of plant spaceflight biology enabled by the Space Shuttle by providing historical context and reviews of select experiments that exemplify plant space biology science.

Plant Vacuolar ATP-binding Cassette Transporters That Translocate Folates and Antifolates in Vitro and Contribute to Antifolate Tolerance in Vivo

The vacuoles of pea (Pisum sativum) leaves and red beet (Beta vulgaris) storage root are major sites for the intracellular compartmentation of folates. In the light of these findings and preliminary experiments indicating that some plant multidrug resistance-associated protein (MRP) subfamily ATP-binding cassette transporters are able to transport compounds of this type, the Arabidopsis thaliana vacuolar MRP, AtMRP1 (AtABCC1), and its functional equivalent(s) in vacuolar membrane vesicles purified from red beet storage root were studied. In so doing, it has been determined that heterologously expressed AtMRP1 and its equivalents in red beet vacuolar membranes are not only competent in the transport of glutathione conjugates but also folate monoglutamates and antifolates as exemplified by pteroyl-l-glutamic acid and methotrexate (MTX), respectively. In agreement with the results of these in vitro transport measurements, analyses of atmrp1 T-DNA insertion mutants of Arabidopsis ecotypes Wassilewskia and Columbia disclose an MTX-hypersensitive phenotype. atmrp1 knock-out mutants are more sensitive than wild-type plants to growth retardation by nanomolar concentrations of MTX, and this is associated with impaired vacuolar antifolate sequestration. The vacuoles of protoplasts isolated from the leaves of Wassilewskia atmrp1 mutants accumulate 50% less [(3)H]MTX than the vacuoles of protoplasts from wild-type plants when incubated in media containing nanomolar concentrations of this antifolate, and vacuolar membrane-enriched vesicles purified from the mutant catalyze MgATP-dependent [(3)H]MTX uptake at only 40% of the capacity of the equivalent membrane fraction from wild-type plants. AtMRP1 and its counterparts in other plant species therefore have the potential for participating in the vacuolar accumulation of folates and related compounds.

Microgravity effects on leaf morphology, cell structure, carbon metabolism and mRNA expression of dwarf wheat

The use of higher plants as the basis for a biological life support system that regenerates the atmosphere, purifies water, and produces food has been proposed for long duration space missions. The objective of these experiments was to determine what effects microgravity (microg) had on chloroplast development, carbohydrate metabolism and gene expression in developing leaves of Triticum aestivum L. cv. USU Apogee. Gravity naive wheat plants were sampled from a series of seven 21-day experiments conducted during Increment IV of the International Space Station. These samples were fixed in either 3% glutaraldehyde or RNAlater or frozen at -25 degrees C for subsequent analysis. In addition, leaf samples were collected from 24- and 14-day-old plants during the mission that were returned to Earth for analysis. Plants grown under identical light, temperature, relative humidity, photoperiod, CO(2), and planting density were used as ground controls. At the morphological level, there was little difference in the development of cells of wheat under microg conditions. Leaves developed in mug have thinner cross-sectional area than the 1g grown plants. Ultrastructurally, the chloroplasts of microg grown plants were more ovoid than those developed at 1g, and the thylakoid membranes had a trend to greater packing density. No differences were observed in the starch, soluble sugar, or lignin content of the leaves grown in microg or 1g conditions. Furthermore, no differences in gene expression were detected leaf samples collected at microg from 24-day-old leaves, suggesting that the spaceflight environment had minimal impact on wheat metabolism.

Dynamics of storage reserve deposition during Brassica rapa L. pollen and seed development in microgravity

Pollen and seeds share a developmental sequence characterized by intense metabolic activity during reserve deposition before drying to a cryptobiotic form. Neither pollen nor seed development has been well studied in the absence of gravity, despite the importance of these structures in supporting future long-duration manned habitation away from Earth. Using immature seeds (3-15 d postpollination) of Brassica rapa L. cv. Astroplants produced on the STS-87 flight of the space shuttle Columbia, we compared the progress of storage reserve deposition in cotyledon cells during early stages of seed development. Brassica pollen development was studied in flowers produced on plants grown entirely in microgravity on the Mir space station and fixed while on orbit. Cytochemical localization of storage reserves showed differences in starch accumulation between spaceflight and ground control plants in interior layers of the developing seed coat as early as 9 d after pollination. At this age, the embryo is in the cotyledon elongation stage, and there are numerous starch grains in the cotyledon cells in both flight and ground control seeds. In the spaceflight seeds, starch was retained after this stage, while starch grains decreased in size in the ground control seeds. Large and well-developed protein bodies were observed in cotyledon cells of ground control seeds at 15 d postpollination, but their development was delayed in the seeds produced during spaceflight. Like the developing cotyledonary tissues, cells of the anther wall and filaments from the spaceflight plants contained numerous large starch grains, while these were rarely seen in the ground controls. The tapetum remained swollen and persisted to a later developmental stage in the spaceflight plants than in the ground controls, even though most pollen grains appeared normal. These developmental markers indicate that Brassica seeds and pollen produced in microgravity were physiologically younger than those produced in 1 g. We hypothesize that microgravity limits mixing of the gaseous microenvironments inside the closed tissues and that the resulting gas composition surrounding the seeds and pollen retards their development.

Plant Reproductive Development during Spaceflight

Reproductive development in microgravity has now been studied in a variety of plants; Arabidopsis, Brassica, and Triticum have been especially well studied. Earlier indications that gravity might be required for some stage of reproductive development have now been refuted. Nevertheless, the spaceflight environment presents many unique challenges that have often compromised the ability of plants to reproduce. These include limitations in hardware design to compensate for the unique environmental characteristics of microgravity, especially absence of convective air movement. Pollen development has been shown to be sensitive to high concentrations of ethylene prevailing on various orbital platforms. Barring these gross environmental problems, androecium and gynoecium development occur normally in microgravity, in that functional propagules are produced. Nonetheless, qualitative changes in anther and pistil development have been shown, and significant qualitative changes occur in storage reserve deposition during seed development. Apart from the intrinsic biological importance of these results, consequences of diminished seed quality when plants are grown in the absence of gravity will detract from the utility of plant-based life support systems. By understanding gravity's role in determining the microenvironments that prevail during reproductive development, counter-measures to these obstacles can be found, while at the same time providing basic knowledge that will have broader agricultural significance.

Pigment composition and concentrations within the plant (Ceratophyllum demersum L.) component of the STS-89 C.E.B.A.S. Mini-Module spaceflight experiment

The Closed Equilibrated Biological Aquatic System (C.E.B.A.S.) Mini-Module, a Space Shuttle middeck locker payload which supports a variety of aquatic inhabitants (fish, snails, plants and bacteria) in an enclosed 8.6 L chamber, was tested for its biological stability in microgravity. The aquatic plant, Ceratophyllum demersum L., was critical for the vitality and functioning of this artificial mini-ecosystem. Its photosynthetic pigment concentrations were of interest due to their light harvesting and protective functions. "Post-flight" chlorophyll and carotenoid concentrations within Ceratophyllum apical segments were directly related to the quantities of light received in the experiments, with microgravity exposure (STS-89) failing to account for any significant deviation from ground control studies.

The effect of a microgravity (space) environment on the expression of expansins from the peg and root tissues of Cucumis sativus

In young cucumber seedlings, the peg is a polar outgrowth of tissue that functions by snagging the seed coat, thereby freeing the cotyledons. The development of the peg is thought to be gravity-dependent and has become a model system for plant-gravity response. Peg development requires rapid cell expansion, a process thought to be catalyzed by alpha-expansins, and thus was a good system to identify expansins that were regulated by gravity. This study identified 7 new alpha-expansin cDNAs from cucumber seedlings (Cucumis sativus L. cv Burpee Hybrid II) and examined their expression patterns. Two alpha-expansins (CsExp3 and CsExp4) were more highly expressed in the peg and the root. Earlier reports stated that pegs tend not to form in the absence of gravity, so the expression levels were compared in the pegs of seedlings grown in space (STS-95), on a clinostat, and on earth (1 g). Pegs were observed to form at high frequency on clinostat and space-grown seedlings, yet on clinostats there was more than a 4-fold reduction in the expression of CsExp3 in the pegs of seedlings grown on clinostats vs. those grown at 1 g, while the CsExp4 gene appeared to be turned off (below detection limits). There were no detectable differences in expansin gene expression levels for the pegs of seedlings grown in space or in the orbiter environmental simulator (OES) (1 g) at NASA. The microgravity environment did not affect the expression of CsExp3 or CsExp4, and the clinostat did not simulate the microgravity environment well.

Evidence of root zone hypoxia in Brassica rapa L. grown in microgravity

A series of experiments was conducted aboard the U.S. space shuttle and the Mir space station to evaluate microgravity-induced root zone hypoxia in rapid-cycling Brassica (Brassica rapa L.), using both root and foliar indicators of low-oxygen stress to the root zone. Root systems from two groups of plants 15 and 30 d after planting, grown in a phenolic foam nutrient delivery system on the shuttle (STS-87), were harvested and fixed for microscopy or frozen for enzyme assays immediately postflight or following a ground-based control. Activities of fermentative enzymes were measured as indicators of root zone hypoxia and metabolism. Following 16 d of microgravity, ADH (alcohol dehydrogenase) activity was increased in the spaceflight roots 47% and 475% in the 15-d-old and 30-d-old plants, respectively, relative to the ground control. Cytochemical localization showed ADH activity in only the root tips of the space-grown plants. Shoots from plants that were grown from seed in flight in a particulate medium on the Mir station were harvested at 13 d after planting and quick-frozen and stored in flight in a gaseous nitrogen freezer or chemically fixed in flight for subsequent microscopy. When compared to material from a high-fidelity ground control, concentrations of shoot sucrose and total soluble carbohydrate were significantly greater in the spaceflight treatment according to enzymatic carbohydrate analysis. Stereological analysis of micrographs of sections from leaf and cotyledon tissue fixed in flight and compared with ground controls indicated no changes in the volume of protoplast, cell wall, and intercellular space in parenchyma cells. Within the protoplasm, the volume occupied by starch was threefold higher in the spaceflight than in the ground control, with a concomitant decrease in vacuolar volume in the spaceflight treatment. Both induction of fermentative enzyme activity in roots and accumulation of carbohydrates in foliage have been repeatedly shown to occur in response to root zone oxygen deprivation. These results indicate that root zone hypoxia is a persistent challenge in spaceflight plant growth experiments and may be caused by microgravity-induced changes in fluid and gas distribution.

Inhibitory effect of hypergravity on photosynthetic carbon dioxide fixation in Euglena gracilis

Photosynthesis, the conversion of light energy into chemical energy, is a critical biological process, whereby plants synthesize carbohydrates from light, carbon dioxide (CO2) and water. The influence of gravity on this biological process, however, is not well understood. Thus, centrifugation was used to alter the gravity environment of Euglena gracilis grown on nutritive agar plates illuminated with red and blue light emitting diodes. The results showed that hypergravity (up to 10xg) had an inhibitory effect on photosynthetic CO2 fixation. Chlorophyll accumulation per cell was essentially unaffected by treatment; however, Chl a/Chl b ratios decreased in hypergravity when compared to 1xg controls. Photosynthesis in Euglena appears to have limited tolerance for even moderate changes in gravitational acceleration.

Pollination and embryo development in Brassica rapa L. in microgravity

Plant reproduction under spaceflight conditions has been problematic in the past. In order to determine what aspect of reproductive development is affected by microgravity, we studied pollination and embryo development in Brassica rapa L. during 16 d in microgravity on the space shuttle (STS-87). Brassica is self-incompatible and requires mechanical transfer of pollen. Short-duration access to microgravity during parabolic flights on the KC-135A aircraft was used initially to confirm that equal numbers of pollen grains could be collected and transferred in the absence of gravity. Brassica was grown in the Plant Growth Facility flight hardware as follows. Three chambers each contained six plants that were 13 d old at launch. As these plants flowered, thin colored tape was used to indicate the date of hand pollination, resulting in silique populations aged 8-15 d postpollination at the end of the 16-d mission. The remaining three chambers contained dry seeds that germinated on orbit to produce 14-d-old plants just beginning to flower at the time of landing. Pollen produced by these plants had comparable viability (93%) with that produced in the 2-d-delayed ground control. Matched-age siliques yielded embryos of equivalent developmental stage in the spaceflight and ground control treatments. Carbohydrate and protein storage reserves in the embryos, assessed by cytochemical localization, were also comparable. In the spaceflight material, growth and development by embryos rescued from siliques 15 d after pollination lagged behind the ground controls by 12 d; however, in the subsequent generation, no differences between the two treatments were found. The results demonstrate that while no stage of reproductive development in Brassica is absolutely dependent upon gravity, lower embryo quality may result following development in microgravity.