The gravistimulation-induced very slow Ca2+ increase in Arabidopsis seedlings requires MCA1, a Ca2+-permeable mechanosensitive channel

Prolonged exposure to hypergravity increases number and size of cells and enhances lignin deposition in the stem of Arabidopsis thaliana

We have performed a lab-based hypergravity cultivation experiment using a centrifuge equipped with a lighting system and examined long-term effects of hypergravity on the development of the main axis of the Arabidopsis (Arabidopsis thaliana (L.) Heynh.) primary inflorescence, which comprises the rachis and peduncle, collectively referred to as the main stem for simplicity. Plants grown under 1 × g (gravitational acceleration on Earth) conditions for 20-23 days and having the first visible flower bud were exposed to hypergravity at 8 × g for 10 days. We analyzed the effect of prolonged hypergravity conditions on growth, lignin deposition, and tissue anatomy of the main stem. As a result, the length of the main stem decreased and cross-sectional area, dry mass per unit length, cell number, and lignin content of the main stem significantly increased under hypergravity. Lignin content in the rosette leaves also increased when they were exposed to hypergravity during their development. Except for interfascicular fibers, cross-sectional areas of the tissues composing the internode significantly increased under hypergravity in most types of the tissues in the basal part than the apical part of the main stem, indicating that the effect of hypergravity is more pronounced in the basal part than the apical part. The number of cells in the fascicular cambium and xylem significantly increased under hypergravity both in the apical and basal internodes of the main stem, indicating a possibility that hypergravity stimulates procambium activity to produce xylem element more than phloem element. The main stem was suggested to be strengthened through changes in its morphological characteristics as well as lignin deposition under prolonged hypergravity conditions.

Deciphering the role of mechanosensitive channels in plant root biology: perception, signaling, and adaptive responses

MAIN CONCLUSION

Mechanosensitive channels are integral membrane proteins that rapidly translate extrinsic or intrinsic mechanical tensions into biological responses. They can serve as potential candidates for developing smart-resilient crops with efficient root systems. Mechanosensitive (MS) calcium channels are molecular switches for mechanoperception and signal transduction in all living organisms. Although tremendous progress has been made in understanding mechanoperception and signal transduction in bacteria and animals, this remains largely unknown in plants. However, identification and validation of MS channels such as Mid1-complementing activity channels (MCAs), mechanosensitive-like channels (MSLs), and Piezo channels (PIEZO) has been the most significant discovery in plant mechanobiology, providing novel insights into plant mechanoperception. This review summarizes recent advances in root mechanobiology, focusing on MS channels and their related signaling players, such as calcium ions (Ca2+), reactive oxygen species (ROS), and phytohormones. Despite significant advances in understanding the role of Ca2+ signaling in root biology, little is known about the involvement of MS channel-driven Ca2+ and ROS signaling. Additionally, the hotspots connecting the upstream and downstream signaling of MS channels remain unclear. In light of this, we discuss the present knowledge of MS channels in root biology and their role in root developmental and adaptive traits. We also provide a model highlighting upstream (cell wall sensors) and downstream signaling players, viz., Ca2+, ROS, and hormones, connected with MS channels. Furthermore, we highlighted the importance of emerging signaling molecules, such as nitric oxide (NO), hydrogen sulfide (H2S), and neurotransmitters (NTs), and their association with root mechanoperception. Finally, we conclude with future directions and knowledge gaps that warrant further research to decipher the complexity of root mechanosensing.

Polarized distribution of extracellular nucleotides promotes gravity-directed polarization of development in spores of Ceratopteris richardii

Gravity directs the polarization of Ceratopteris fern spores. This process begins with the uptake of calcium through channels at the bottom of the spore, a step necessary for the gravity response. Data showing that extracellular ATP (eATP) regulates calcium channels led to the hypothesis that extracellular nucleotides could play a role in the gravity-directed polarization of Ceratopteris spores. In animal and plant cells ATP can be released from mechanosensitive channels. This report tests the hypothesis that the polarized release of ATP from spores could be activated by gravity, preferentially along the bottom of the spore, leading to an asymmetrical accumulation of eATP. In order to carry out this test, an ATP biosensor was used to measure the [eATP] at the bottom and top of germinating spores during gravity-directed polarization. The [eATP] along the bottom of the spore averaged 7-fold higher than the concentration at the top. All treatments that disrupted eATP signaling resulted in a statistically significant decrease in the gravity response. In order to investigate the source of ATP release, spores were treated with Brefeldin A (BFA) and gadolinium trichloride (GdCl3). These treatments resulted in a significant decrease in gravity-directed polarization. An ATP biosensor was also used to measure ATP release after treatment with both BFA and GdCl3. Both of these treatments caused a significant decrease in [ATP] measured around spores. These results support the hypothesis that ATP could be released from mechanosensitive channels and secretory vesicles during the gravity-directed polarization of Ceratopteris spores.

Sound perception in plants: from ecological significance to molecular understanding

In addition to positive effects on plant growth and resilience, sound alerts plants of potential danger and aids in defense. Sound guides plants towards essential resources, like water, through phonotropic root growth. Sound also facilitates mutualistic interactions such as buzz pollination. Molecularly, sound induces Ca2+ signatures, K+ fluxes, and an increase in reactive oxygen species (ROS) levels in a mechanosensitive ion channel-dependent fashion. We review the two major open questions in the field of plant acoustics: (i) what is the ecological relevance of sound in plant life, and (ii) how is sound sensed and transduced to evoke a morphophysiological response? We highlight the clear need to combine the ecological and molecular perspectives for a more holistic approach to better understand plant behavior.

Plant responses to hypergravity: a comprehensive review

Hypergravity is an effective novel stimulus to elucidate plant gravitational and mechanobiological behaviour. Here, we review the current understanding of phenotypic, physio-biochemical, and molecular plant responses to simulated hypergravity. Plants readily respond to altered gravity conditions, such as microgravity or hypergravity. Hypergravity-a gravitational force higher than that on the Earth's surface (> 1g)-can be simulated using centrifuges. Exposing seeds, seedlings, or plant cell cultures to hypergravity elicits characteristic morphological, physio-biochemical, and molecular changes. While several studies have provided insights into plant responses and underlying mechanisms, much is still elusive, including the interplay of hypergravity with gravitropism. Moreover, hypergravity is of great significance for mechano- and space/gravitational biologists to elucidate fundamental plant behaviour. In this review, we provide an overview of the phenotypic, physiological, biochemical, and molecular responses of plants to hypergravity. We then discuss the involvement of hypergravity in plant gravitropism-the directional growth along the gravity vector. Finally, we highlight future research directions to expand our understanding of hypergravity in plant biology.

Entanglement of Arabidopsis Seedlings to a Mesh Substrate under Microgravity Conditions in KIBO on the ISS

The International Space Station (ISS) provides a precious opportunity to study plant growth and development under microgravity (micro-G) conditions. In this study, four lines of Arabidopsis seeds (wild type, wild-type MCA1-GFP, mca1-knockout, and MCA1-overexpressed) were cultured on a nylon lace mesh placed on Gelrite-solidified MS-medium in the Japanese experiment module KIBO on the ISS, and the entanglement of roots with the mesh was examined under micro-G and 1-G conditions. We found that root entanglement with the mesh was enhanced, and root coiling was induced under the micro-G condition. This behavior was less pronounced in mca1-knockout seedlings, although MCA1-GFP distribution at the root tip of the seedlings was nearly the same in micro-G-grown seedlings and the ground control seedlings. Possible involvement of MCA1 in the root entanglement is discussed.

Sensing Mechanisms: Calcium Signaling Mediated Abiotic Stress in Plants

Plants are exposed to various environmental stresses. The sensing of environmental cues and the transduction of stress signals into intracellular signaling are initial events in the cellular signaling network. As a second messenger, Ca²⁺ links environmental stimuli to different biological processes, such as growth, physiology, and sensing of and response to stress. An increase in intracellular calcium concentrations ([Ca²⁺]_i) is a common event in most stress-induced signal transduction pathways. In recent years, significant progress has been made in research related to the early events of stress signaling in plants, particularly in the identification of primary stress sensors. This review highlights current advances that are beginning to elucidate the mechanisms by which abiotic environmental cues are sensed via Ca²⁺ signals. Additionally, this review discusses important questions about the integration of the sensing of multiple stress conditions and subsequent signaling responses that need to be addressed in the future.

Role of the Earth's Motions in Plant Orientation - Planetary Mechanism

According to the proposed theory, the starch-rich particles (statoliths) help the plant to convert the signals from Earth's motions into the signals necessary for the plant to perceive its orientation relative to the gravity vector while moving freely because of inertia in the sensory cells (statocytes) of roots and stems. Motions of the Earth are never constant, which, in particular, refers to the so-called polar motions and oscillations of the planet's rotation axis. Statoliths at any given moment move in the cytoplasmic liquid of statocytes due to inertial motion initiated by the action of the Earth's movements, maintaining the trajectory set by the previous movement of the oscillating planet. Unlike statoliths, the walls of a statocyte move in space along with the entire plant and with the Earth, in strict accordance with the current direction of motion of the planet's axis. This leads to the inevitable collision of statoliths with the statocytic wall/membrane. Cytoplasmic liquid, as a substance that is not able to maintain its shape, does not interfere with the inertial motions of the statoliths and collision with the wall of the statocyte. By striking the membrane, statoliths cause the release of ions and other factors at the impact site, which further participate in the gravitropic process. Pressure of the sediment of statoliths at the bottom of the statocyte, as well as position of this sediment, are not the defining factors of gravitropism.

MCAs in Arabidopsis are Ca2+-permeable mechanosensitive channels inherently sensitive to membrane tension

Mechanosensitive (MS) ion channels respond to mechanical stress and convert it into intracellular electric and ionic signals. Five MS channel families have been identified in plants, including the Mid1-Complementing Activity (MCA) channel; however, its activation mechanisms have not been elucidated in detail. We herein demonstrate that the MCA2 channel is a Ca²⁺-permeable MS channel that is directly activated by membrane tension. The N-terminal 173 residues of MCA1 and MCA2 were synthesized in vitro, purified, and reconstituted into artificial liposomal membranes. Liposomes reconstituted with MCA1(1-173) or MCA2(1-173) mediate Ca²⁺ influx and the application of pressure to the membrane reconstituted with MCA2(1-173) elicits channel currents. This channel is also activated by voltage. Blockers for MS channels inhibit activation by stretch, but not by voltage. Since MCA proteins are found exclusively in plants, these results suggest that MCA represent plant-specific MS channels that open directly with membrane tension.

Mechanosensitive ion channels convert mechanical stimuli into intracellular electric and ionic signals. Here the authors show that Arabidopsis MCA2 is a Ca²⁺-permeable mechanosensitive channel that is directly activated by membrane tension.

Accumulation of Abnormal Amyloplasts in Pulp Cells Induces Bitter Pit in Malus domestica

Apple bitter pit primarily occurs during fruit ripening and storage; however, its formation mechanism remains unclear. Although it is considered that Ca2+ deficiency causes metabolic disorders in apples, there have been few studies on the mechanism of the bitter pit from the perspective of cell structure. At the fruit ripening stage, the fruit with a bitter pit on the tree was taken as the research material. In this study, the microscopic observation revealed numerous amyloplasts in the pulp cells of apples affected with bitter pit, but not in the healthy pulp. Furthermore, the results of fluorescence staining and transmission electron microscopy (TEM) revealed that the bitter pit pulp cells undergo programmed cell death (PCD), their nuclear chromosomes condense, and amyloplast forms autophagy. The cytoplasmic Ca2+ concentration in the healthy fruits was lowest near the peduncle, followed by that in the calyx, whereas it was highest at the equator. In contrast, the cytoplasmic Ca2+ concentration in apple fruits showing bitter pit disorder was lowest near the peduncle and highest in the calyx. Moreover, the cytosolic Ca2+ concentration in the flesh cells of apples with the bitter pit was much lower than that in the healthy apple flesh cells; however, the concentration of Ca2+ in the vacuoles of fruits with the bitter pit was higher than that in the vacuoles of healthy fruits. In summary, bitter pit pulp cells contain a large number of amyloplasts, which disrupts the distribution of Ca2+ in the pulp cells and causes PCD. These two processes lead to an imbalance in cell metabolism and induce the formation of a bitter pit.

With an Ear Up against the Wall: An Update on Mechanoperception in Arabidopsis

Cells interpret mechanical signals and adjust their physiology or development appropriately. In plants, the interface with the outside world is the cell wall, a structure that forms a continuum with the plasma membrane and the cytoskeleton. Mechanical stress from cell wall damage or deformation is interpreted to elicit compensatory responses, hormone signalling, or immune responses. Our understanding of how this is achieved is still evolving; however, we can refer to examples from animals and yeast where more of the details have been worked out. Here, we provide an update on this changing story with a focus on candidate mechanosensitive channels and plasma membrane-localized receptors.