SummarySumoylation is a posttranslational regulatory process in higher eukaryotes modifying substrate proteins through conjugation of small ubiquitin-related modifiers (SUMOs). Sumoylation modulates protein stability, subcellular localization and activity; thus, it regulates most cellular functions including response to environmental stress in plants. To study the feasibility of manipulating SUMO E3 ligase, one of the important components in the sumoylation pathway in transgenic (TG) crop plants for improving overall plant performance under adverse environmental conditions, we have analysed TG creeping bentgrass (Agrostis stolonifera L.) plants constitutively expressing OsSIZ1, a rice SUMO E3 ligase. Overexpression of OsSIZ1 led to increased photosynthesis and overall plant growth. When subjected to water deficiency and heat stress, OsSIZ1 plants exhibited drastically enhanced performance associated with more robust root growth, higher water retention and cell membrane integrity than wild-type (WT) controls. OsSIZ1 plants also displayed significantly better growth than WT controls under phosphatestarvation conditions, which was associated with a higher uptake of phosphate (Pi) and other minerals, such as potassium and zinc. Further analysis revealed that overexpression of OsSIZ1 enhanced stress-induced SUMO conjugation to substrate in TG plants, which was associated with modified expression of stress-related genes. This strongly supports a role sumoylation plays in regulating multiple molecular pathways involved in plant stress response, establishing a direct link between sumoylation and plant response to environmental adversities. Our results demonstrate the great potential of genetic manipulation of sumoylation process in TG crop species for improved resistance to broad abiotic stresses.

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.

a b s t r a c t Solar tracking in the common sunflower, Helianthus annuus, is a dramatic example of a diurnal rhythm in plants. During the day, the shoot apex continuously reorients, following the sun's relative position so that the developing heads track from east to west. At night, the reverse happens, and the heads return and face east in anticipation of dawn. This daily cycle dampens and eventually stops at anthesis, after which the sunflower head maintains an easterly orientation. Although shoot apical heliotropism has long been the subject of physiological studies in sunflower, the underlying developmental, cellular, and molecular mechanisms that drive the directional growth and curvature of the stem in response to extrinsic and perhaps intrinsic cues are not known. Furthermore, the ecological functions of solar tracking and the easterly orientation of mature heads have been the subject of significant but unresolved speculation. In this review, we discuss the current state of knowledge about this complex, dynamic trait. Candidate mechanisms that may contribute to daytime and nighttime movement are highlighted, including light signaling, hormonal action, and circadian regulation of growth pathways. The merits of the diverse hypotheses advanced to explain the adaptive significance of heliotropism in sunflower are also considered.

Introduction: Traveling to nearby extraterrestrial objects having a reduced gravity level (partial gravity) compared to Earth’s gravity is becoming a realistic objective for space agencies. The use of plants as part of life support systems will require a better understanding of the interactions among plant growth responses including tropisms, under partial gravity conditions.Materials and Methods: Here, we present results from our latest space experiments on the ISS, in which seeds of Arabidopsis thaliana were germinated, and seedlings grew for six days under different gravity levels, namely micro-g, several intermediate partial-g levels, and 1g, and were subjected to irradiation with blue light for the last 48 h. RNA was extracted from 20 samples for subsequent RNAseq analysis. Transcriptomic analysis was performed using the HISAT2-Stringtie-DESeq pipeline. Differentially expressed genes were further characterized for global responses using the GEDI tool, gene networks and for Gene Ontology (GO) enrichment.Results: Differential gene expression analysis revealed only one differentially expressed gene (AT4G21560, VPS28-1 a vacuolar protein) across all gravity conditions using FDR correction (q < 0.05). However, the same 14 genes appeared differentially expressed when comparing either micro-g, low-g level (< 0.1g) or the Moon g-level with 1g control conditions. Apart from these 14-shared genes, the number of differentially expressed genes was similar in microgravity and the Moon g-level and increased in the intermediate g-level (< 0.1g), but it was then progressively reduced as the difference with the Earth gravity became smaller. The GO groups were differentially affected at each g-level: light and photosynthesis GO under microgravity, genes belonged to general stress, chemical and hormone responses under low-g, and a response related to cell wall and membrane structure and function under the Moon g-level.Discussion: Transcriptional analyses of plants under blue light stimulation suggests that root blue-light phototropism may be enough to reduce the gravitational stress response caused by the lack of gravitropism in microgravity. Competition among tropisms induces an intense perturbation at the micro-g level, which shows an extensive stress response that is progressively attenuated. Our results show a major effect on cell wall/membrane remodeling (detected at the interval from the Moon to Mars gravity), which can be potentially related to graviresistance mechanisms.

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.

Kiss. 2019. 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. ACKNOWLEDGMENTSFunding was provided by grants NNX12A065G and 80NSSC17K0546 from NASA (PI = J. Z. Kiss). We thank the reviewers for insightful comments and review in preparation of this manuscript. DATA AVAILABILITYAll sequences generated in this study are deposited in NASA GenLab (https ://genel ab.nasa.gov/) as data set GLDS 251. SUPPORTING INFORMATIONAdditional Supporting Information may be found online in the supporting information tab for this article. APPENDIX S1. FASTQC results for pooled sample #111: microgravity (A) forward and (B) reverse paired-end reads. APPENDIX S2. FASTQC results for pooled sample #114: microgravity (A) forward and (B) reverse paired-end reads. APPENDIX S3. FASTQC results for pooled sample #116: microgravity (A) forward and (B) reverse paired-end reads. APPENDIX S4. FASTQC results for pooled sample #120: microgravity (A) forward and (B) reverse paired-end reads. APPENDIX S5. FASTQC results for pooled sample #175: 1-g (A) forward and (B) reverse paired-end reads.APPENDIX S6. FASTQC results for pooled sample # 179: 1-g (A) forward and (B) reverse paired-end reads. APPENDIX S7. FASTQC results for pooled sample # 235-239: 1-g (A) forward and (B) reverse paired-end reads. AP...

Main conclusion Blue-light positive phototropism in roots is masked by gravity and revealed in conditions of microgravity. In addition, the magnitude of red-light positive phototropic curvature is correlated to the magnitude of gravity.Due to their sessile nature, plants utilize environmental cues to grow and respond to their surroundings. Two of these cues, light and gravity, play a substantial role in plant orientation and directed growth movements (tropisms). However, very little is currently known about the interaction between light-(phototropic) and gravity (gravitropic)-mediated growth responses. Utilizing the European Modular Cultivation System on board the International Space Station, we investigated the interaction between phototropic and gravitropic responses in three Arabidopsis thaliana genotypes, Landsberg wild type, as well as mutants of phytochrome A and phytochrome B. Onboard centrifuges were used to create a fractional gravity gradient ranging from reduced gravity up to 1g. A novel positive blue-light phototropic response of roots was observed during conditions of microgravity, and this response was attenuated at 0.1g. In addition, a red-light pretreatment of plants enhanced the magnitude of positive phototropic curvature of roots in response to blue illumination. In addition, a positive phototropic response of roots was observed when exposed to red light, and a decrease in response was gradual and correlated with the increase in gravity. The positive red-light phototropic curvature of hypocotyls when exposed to red light was also confirmed. Both red-light and blue-light phototropic responses were also shown to be affected by directional light intensity. To our knowledge, this is the first characterization of a positive blue-light phototropic response in Arabidopsis roots, as well as the first description of the relationship between these phototropic responses in fractional or reduced gravities.Correspondence to: John Z. Kiss. Electronic supplementary material The online version of this article

No abstract