Transgene expression patterns indicate that spaceflight affects stress signal perception and transduction in arabidopsis

Calcium signaling in plant cells in altered gravity

Changes in the intracellular Ca2+ concentration in altered gravity (microgravity and clinostating) evidence that Ca2+ signaling can play a fundamental role in biological effects of microgravity. Calcium as a second messenger is known to play a crucial role in stimulus-response coupling for many plant cellular signaling pathways. Its messenger functions are realized by transient changes in the cytosolic ion concentration induced by a variety of internal and external stimuli such as light, hormones, temperature, anoxia, salinity, and gravity. Although the first data on the changes in the calcium balance in plant cells under the influence of altered gravity have appeared in 80th, a review highlighting the performed research and the possible significance of such Ca2+ changes in the structural and metabolic rearrangements of plant cells in altered gravity is still lacking. In this paper, an attempt was made to summarize the available experimental results and to consider some hypotheses in this field of research. It is proposed to distinguish between cell gravisensing and cell graviperception; the former is related to cell structure and metabolism stability in the gravitational field and their changes in microgravity (cells not specialized to gravity perception), the latter is related to active use of a gravitational stimulus by cells presumebly specialized to gravity perception for realization of normal space orientation, growth, and vital activity (gravitropism, gravitaxis) in plants. The main experimental data concerning both redistribution of free Ca2+ ions in plant cell organelles and the cell wall, and an increase in the intracellular Ca2+ concentration under the influence of altered gravity are presented. Based on the gravitational decompensation hypothesis, the consequence of events occurring in gravisensing cells not specialized to gravity perception under altered gravity are considered in the following order: changes in the cytoplasmic membrane surface tension --> alterations in the physicochemical properties of the membrane --> changes in membrane permeability, ion transport, membrane-bound enzyme activity, etc. --> metabolism rearrangements --> physiological responses. An analysis of data available on biological effects of altered gravity at the cellular level allows one to conclude that microgravity environment appears to affect cytoskeleton, carbohydrate and lipid metabolism, cell wall biogenesis via changes in enzyme activity and protein expression, with involvement of regulatory Ca2+ messenger system. Changes in Ca2+ influx/efflux and possible pathways of Ca2+ signaling in plant cell biochemical regulation in altered gravity are discussed.

Seed-to-seed growth of Arabidopsis thaliana on the International Space Station

The assembly of the International Space Station (ISS) as a permanent experimental outpost has provided the opportunity for quality plant research in space. To take advantage of this orbital laboratory, engineers and scientists at the Wisconsin Center for Space Automation and Robotics (WCSAR), University of Wisconsin-Madison, developed a plant growth facility capable of supporting plant growth in the microgravity environment. Utilizing this Advanced Astroculture (ADVASC) plant growth facility, an experiment was conducted with the objective to grow Arabidopsis thaliana plants from seed-to-seed on the ISS. Dry Arabidopsis seeds were anchored in the root tray of the ADVASC growth chamber. These seeds were successfully germinated from May 10 until the end of June 2001. Arabidopsis plants grew and completed a full life cycle in microgravity. This experiment demonstrated that ADVASC is capable of providing environment conditions suitable for plant growth and development in microgravity. The normal progression through the life cycle, as well as the postflight morphometric analyses, demonstrate that Arabidopsis thaliana does not require the presence of gravity for growth and development.

New aspects of gravity responses in plant cells

Plants show two distinct responses to gravity: gravity-dependent morphogenesis (gravimorphogenesis) and gravity resistance. In gravitropism, a typical mechanism of gravimorphogenesis, gravity is utilized as a signal to establish an appropriate form. The response has been studied in a gravity-free environment, where plant seedlings were found to perform spontaneous morphogenesis, termed automorphogenesis. Automorphogenesis consists of a change in growth direction and spontaneous curvature in dorsiventral directions. The spontaneous curvature is caused by a difference in the capacity of the cell wall to expand between the dorsal and the ventral sides of organs, which originates from the inherent structural anisotropy. Gravity resistance is a response that enables the plant to develop against the gravitational force. To resist the force, the plant constructs a tough body by increasing the cell wall rigidity that suppresses growth. The mechanical properties of the cell wall are changed by modification of the cell wall metabolism and cell wall environment, especially pH. In gravitropism, gravity is perceived by amyloplasts in statocytes, whereas gravity resistance may be mediated by mechanoreceptors on the plasma membrane.

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.

Plants in space

Virtually all scenarios for the long-term habitation of spacecraft and other extraterrestrial structures involve plants as important parts of the contained environment that would support humans. Recent experiments have identified several effects of spaceflight on plants that will need to be more fully understood before plant-based life support can become a reality. The International Space Station (ISS) is the focus for the newest phase of space-based research, which should solve some of the mysteries of how spaceflight affects plant growth. Research carried out on the ISS and in the proposed terrestrial facility for Advanced Life Support testing will bring the requirements for establishing extraterrestrial plant-based life support systems into clearer focus.