Plants are able to sense and respond to minute tilt from the vertical direction of the gravity, which is key to maintain their upright posture during development. When a shoot or a root is tilted, a complex signaling pathway involving the redistribution of growth hormones within the tissue is triggered. This leads to differential growth between the two sides of the plant organ and the bending of the organ toward the vertical direction. It has long been assumed that this starts by gravisensing in plants relies on a sensor made of starch-rich grains (statoliths), located in specific cells, called statocytes. How such a sensor can detect inclination is not fully clear, and two hypotheses have been proposed. Experiments including space experiments and results apparently support that the hypothesis of a force sensor, where gravity was detected by sensing statoliths’ weight on the cell edges (1) or on the cytoskeleton network (2). More recent studies using a biophysical approaches on Earth provide support to the hypothesis of an inclination sensor, independently of the force level (3, 4). The contribution of space experiments designed to asses these hypotheses, and their comparison with ground experiments will be presented. We will show that a biophysical model called A-SD-M (5) can unify both hypotheses and explain the gravisensing in the whole range from micro- to hyper-gravity.

Research on gravitropism has been dominated by two main ideas: that gravity is perceived through the sedimentation of starch-rich plastids within specialised gravity-sensing cells (Starch-statolith hypothesis), and that tropic growth is driven by auxin asymmetry across the graviresponding organ (Cholodny-Went hypothesis). Our recent work on gravity-dependent, non-vertical growth in lateral organs in Arabidopsis, has highlighted the importance of a third, even older concept in gravitropism: angle dependence. However, the mechanistic basis of how statolith sedimentation, and eventually Cholodny-Went driven auxin asymmetry, translates into angle dependent gravitropic behaviour remains unexplored. Here, using a combination of cutting edge vertical confocal imaging with time lapse tracking software, we characterize for the first time the dynamics of gra visensing in the columella of the Arabidopsis primary roots. We observed that statolith sedimentation across individual tiers of columella cells occurs according to the angle of displacement from the vertical axis. We also demonstrate how statolith sedimentation leads to angle dependent PIN3/7 polarization in specific columella domains. This detail analysis shows that different PINs/columella tiers play distinct roles in establishing the asymmetric auxin gradient at different angles. Our findings provide a fundamental framework to further explore the mechanisms that regulate angle dependent gravitropic responses in both primary and lateral organs, with major implications for crop improvement.

Prolonged human missions in space, whether in orbit or interplanetary travel, will require access to a sustainable food supply. During multi-year missions, key nutrients like vitamins C and B1 will degrade in packaged food, but the integration of plants into advanced Bioregenerative Life Support Systems could represent a sustainable and economically attractive way to provide a balanced diet during long-duration missions.
Although a variety of small plant species have been grown on the International Space Station (ISS), the cultivation of larger crop plants in μg still needs to be fully developed. One of the practical issues that requires attention is the control of root growth in the absence of gravity. A mature plant develops a complex root system made of tens to hundreds of single root tips, whose growth rate and orientation are regulated by environmental signals such as gravity, light, water potential, chemical gradients and electric charges. In fact, the ability to change growth direction in response to an external cue (tropism) is a key characteristic of plant development. Crucially, the biological response to gravity (gravitropism) is one of the dominant forces shaping complex root systems on Earth.
In microgravity environments, plant roots cannot use gravitropism to establish and maintain their optimal spatial distribution. To directly address this challenge, we are proposing to adopt the natural tendency of roots to reorient towards negative charges (electrotropism) and use artificial electric fields to mimic the effect of gravitropism on roots.

In this talk, I will present a quantitative characterization of the tropic response exhibited by roots of the plant model system Arabidopsis thaliana exposed to external electric fields in laboratory conditions on Earth. Our study elucidates the dose-response kinetics of early electrotropism stages, exhibiting a power law reminiscent of physiological reactions in animals. I will also discuss long-term electrotropism traits, such as overshoot, habituation, and the role of past exposures in the response to electric fields (hysteresis), offering quantitative insights into the intricate nature of root behaviour in these conditions.
Overall, this study represents a first step towards the possible adoption of electrotropism as a replacement for gravitropism when growing plants in microgravity environments.

Long-duration missions will require plants adapted to the spaceflight environment. Limitations in growth chamber area and down mass returned from the International Space Station prevent many experiments from producing enough tissue for robust proteomic analyses. Transcriptional profiling has shown that plants grown during spaceflight exhibit altered molecular responses. However, changes in transcript levels do not necessarily correlate with changes in protein abundance. Instead of directly assessing protein content, Targeted Ribosome Affinity Purification (TRAP) followed by RNAseq allows for quantification of transcripts that are actively recruited to ribosomes. These transcripts make up the translatome. The goal of the APEX-07 spaceflight experiment was to compare the transcriptome, translatome and small RNA landscape of Arabidopsis seedlings grown on the ISS to obtain a deeper understanding of spaceflight regulation. APEX-07 successfully identified core points of post transcriptional regulation in both roots and stems. Less than 50% of the differentially expressed genes had consistent patterns of expression in both the transcriptome and translatome. Genes that did share patterns in both the polysomal and total mRNA show ontological enrichment in regulatory proteins including phosphatases, kinases and small molecular binding proteins. Some genes, including several ion and small molecule transporters, showed opposite patterns of expression across extraction types. The opposing patterns in transcription and translation points towards post transcriptional regulation being especially important for transmembrane transporters. The polysomal fraction also identified pathways that were not differentially regulated in the total mRNA. Transcripts for cell wall remodeling genes were only upregulated in the polysomal fraction with the same genes showing no significant difference in the total mRNA. The miRNA sequencing also identified targets of post transcriptional regulation. Several miRNAs known to target MYB and ARF family transcription factors were increased in spaceflight suggesting that those transcription factors were post transcriptionally downregulated in response to flight. This trend was seen in ARF 6, 8, 10 and 16 where miRNA was increased, and mRNA decreased in response to flight. Processes related to ROS and telomeric function were also significantly regulated at the transcript level suggesting processes important to spaceflight are potentially regulated at levels which are not visible when only using standard RNAseq. Total mRNA sequencing has the power to reveal transcripts that are differentially expressed in space, post transcriptional regulation gives better insights into the functional response of the plant to the spaceflight environment. Partially supported by NASA grant 80NSSC19k1481 to PIs S.E.W. and I.Y.P.

Long-duration space missions require a deep understanding of how plants respond to the altered gravity environment. In microgravity, fluid flow and diffusion-dependent biochemical processes, like the circadian clock, might be disrupted.
The circadian clock is an internal timekeeping system in plants that coordinates their internal processes with the external day-night cycle. This anticipation allows plants to optimize their performance and survival. It relies heavily on precise movement and interactions within cells, making it a sensitive model for studying microgravity's effects. Moreover, understanding the impacts of microgravity on the circadian clock will be essential for astronaut and plant health on long-duration space flights. Therefore, we addressed the question: Does reduced gravity affect the plant's circadian clock?
We used a Random Positioning Machine (RPM) to simulate microgravity and studied the response of the Arabidopsis plant's circadian clock genes. We collected root and shoot samples every 2 hours over 48 hours, allowing us to track changes in gene expression. RNA analysis revealed disruptions in the rhythmic expression patterns of core circadian clock genes like CCA1 and LHY. This disruption cascaded to affect downstream clock-controlled genes as well. Overall, RNA sequencing showed a widespread shift in the plant's circadian rhythmicity under simulated microgravity.
In conclusion, this research highlights the potential disruption of the plant's internal timing system under microgravity conditions. Further study is needed to understand the long-term implications for plant growth and development in space.