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Assessment of Programs in Space Biology and Medicine 1991 6 Plant Biology Plants may be regarded as the most important organisms on our planet as they are key to the entire biological system that has developed on it. It is essential to understand the effects of gravity and its absence in order to employ this knowledge to growing plants in space as well as to enhance their growth and use on Earth. However, our knowledge concerning the growth of plants in the microgravity environment of space is minimal. Of tremendous practical importance is the simple question of whether or not plants can grow and reproduce with normal efficiency through multiple generations in space. Answering this question will require long-term flight experiments together with carefully designed on-orbit and ground controls. Recommendations and objectives contained in the three SSB/CSBM reports being considered in this assessment remain largely unfulfilled. The 1979 report calls for studies on how plant cell polarity first develops and emphasizes the use of horizontal rotation as a substitute for microgravity. The 1987 strategy is a more comprehensive statement of experimental goals and recommendations related to the growth, development, and behavior of plants in space. The 1988 report questions the threshold of gravity-sensing systems in plantsâarguing that without a low-g environment for longer flight periods, resolution of thresholds and response mechanisms cannot be reliably determined. In fact, the most fundamental research requirements to addressing these issues are a "quiet" on- orbit environment on long-term flights and the use of an on-board centrifuge for threshold and control. STATUS OF DISCIPLINE It has been demonstrated repeatedly that plants will grow under microgravity conditions. However, there are no quantitative studies regarding the efficiencies of growth processes. It remains to be demonstrated that seeds produced in space are capable of completing a life cycle and producing another
generation of normal plants. For decades, the clinostat (a horizontal rotation device) has been thought of as a means of reducing the effects of the gravitational vector and thus, in the minds of many, a substitute for microgravity. However, recent studies on-orbit have shown that there is an absence of thermal convection in space which results in a lack of movement of materials and mixing of cell contents. Clinostats exaggerate such mixing and have, in fact, been shown to advance plant cell death as a result. Thus long-term studies of plant responses in clinostats can no longer be viewed as equivalent to experiments in microgravity. The results of the two types of study may sometimes appear superficially to be the same, but will have been derived from virtually opposite physical effects. Since 1980, investigators supported by NASA have published 335 papers about plants and space biology in reviewed scientific literature. Most have dealt with plant hormones and gravitational responses at the levels of cells, organs, and whole plants. There have also been many phenomenological studies concerned with movement and orientation. Our understanding of plant signal transduction remains scant. However, such constituents as G-proteins, phosphoinositides, actin, and calmodulin also occur in plant cells and may have active roles. In addition, since 1987, great strides have been made in plant molecular biology. Many of these advances can be applied to problems in plant growth and development, and will be useful in trying to understand plant responses to the space environment. Of particular relevance are advances in our understanding of signal transduction mechanisms. In the last few years there has been a great deal of elegant work on receptor molecules and signal transduction mechanisms for a variety of responses to hormonal and environmental stimuli in animal cells, bacteria, and yeast. Physiological, biochemical, molecular, and genetic techniques are being exploited, with the most significant advances coming from combinations of these approaches. Some investigators are now beginning to apply these techniques to signal transduction in plants, and it is clear that much can be learned. Particularly promising are recent experiments with auxin receptors and mutants with altered geotropic responses. It is reasonable to suppose that cloned genes for receptors will be available soon, and that genetic analysis of gravitropism will lead to an eventual understanding at the molecular level. Such experiments will also provide characterized genetic material, which may be used in future flight experiments to determine whether genes involved in sensing or responding to gravity on Earth will affect plant growth under microgravity conditions. MAJOR GOALS
The Goldberg Strategy calls for understanding the effects of gravity or its absence on broad categories of plant responses and processes. Here, building on the Goldberg Strategy with issues raised in other reports, the committee has taken a somewhat "global" approach in detailing major goals for plant reproduction, growth, and development in space, which will have overlapping criteria and priorities, rather than address all of the specific proposals suggested in the Goldberg Strategy. Experimentation is needed to identify those impediments to sexual and asexual reproduction that have been suggested in some space studies. The single biggest practical question is whether plants are capable of multiple generations in microgravity. Seeds germinated on-orbit would have ground-born flowers and thus will produce partially with ground-born seeds (since seeds are composed in part of material tissues). Only second generation seeds would produce flowers and seeds from tissues exposed only to microgravity. So, in the end, only plants produced from a third planting of seeds would be entirely free of any prior gravitational influence. Thus, the definitive experiment will not be a seed- to-seed experiment, but a seed-to-seed-to-seed experiment. Fundamentally important experimentation in plant development must include the range from cell to organism and must account for interactions of microgravity with other environmental factors. It has not been clearly shown whether microgravity affects all cells or if some cell types acclimate to gravity deprivation. Some space studies suggest that chromosome behavior is fundamentally changed in weightlessness. If so, what are the consequences for cell development? Biological clocks are important regulators of plant development, but it is not known if they function reliably in space. Radiation in space may also have major developmental consequences and may interact with microgravity effects. For example, mechanisms for repairing radiation-induced genetic damage may be affected by microgravity. The lack of thermal convection in space may affect many aspects of short- and long-distance transport phenomena in plants. For example, the functions of cell membranes, the pathways for ion uptake and nutrient absorption, water relations, and the transport of organic and inorganic molecules must be investigated with regard to any ways by which they are affected by weightlessness. Some spaceflight results indicate that microgravity affects metabolic products in other ways as well. For example, photosynthesis and pathways of carbohydrate and lipid affect plant growth and/or nutritional quality. Also, protein metabolism may be affected by microgravity. Supporting structures of lignin and cellulose may be modified in ways analogous to the loss of bone density in space. A major goal should be to gain an appreciation for the magnitude of such effects and their impact on plant function. The need for extensive study of plant gravitational responses has been identified in all previous reports. Because plants are normally stationary, they are more suitable than animals for studies of gravity perception. Many of these experiments must be done in space, since ground-based horizontal rotation techniques do not properly mimic microgravity. At present, the most pressing
need is for a definition of the minimum gravitational fields required for responses at the molecular, cellular, and whole plant levels. As emphasized in the Goldberg Strategy, such experiments will require the use of an on-orbit centrifuge to provide various levels of partial gravity levels and normal gravity on-orbit controls. PROGRESS Fragmentary Soviet experimentation involving growing plants (Arabidopsis) to the flowering stage in space indicates that reproductive events are most affected by microgravity; fruit set is decreased over ground controls and percent seed germination back on Earth is also below control levels. Experiments studying single cells on the D-1 Mission indicate both stimulatory and inhibitory effects of spaceflight on growth and development. Of note were the observations that the single cell green alga, Chamydamonas, maintained a well-expressed circadian rhythm, which did not differ significantly from ground controls, over a 6-day period in space. Cultured cells of the plant Anise turned into embryoids and developed leaf and root primordia and chlorophyll faster in space than did ground-based controls. Observations of cell division in microgravity indicate a generally slower cycle, and several observations of the mitotic sequence indicate abnormalities in chromosome behavior. Other studies have indicated that there are gravity-induced changes in the distribution, and very likely the synthesis, of cellulose (a structural counter to gravity on Earth) and starch (which makes up the gravity sensing statolith (amyloplast)). Added to the latter observation is the increase in numbers and size of lipid bodies in the cells where the amount of starch is decreased. There seems little doubt left that the amyloplast is the gravity sensor on Earth. On the D-1 Mission, on-orbit seedling roots show no evidence of amyloplast sedimentation, and roots simply grow straight out from their position in the seed. On-orbit 1-g centrifuge controls behaved as if on Earth. Ground-based investigations have indicated calcium, auxin (a plant hormone), and membrane transport processes in gravity perception. Although much more needs to be done before a satisfactory general model can be formulated, specific hypotheses can now be tested. For example, one can ask the question of whether the sedimentation of starch grains causes changes in ion transport at the cell membrane, and if so, whether such changes are required for gravity sensing. LACK OF PROGRESS To date, flight experiments have indicated a variety of differences between the behavior of plants in microgravity and those on Earth. However, both
a lack of flight opportunities and the inability to carry out on-orbit centrifuge controls have severely restricted progress in basic plant biology. As a result, we lack information on both direct and indirect effects of microgravity. The magnitude of long-term microgravity effects on such fundamental processes as cell growth and chromosome behavior remains unknown. It is also unclear to what extent changes in convection, surface tension, cohesion, and so on, may influence photosynthetic gas exchange or the transport of water and nutrients. Long-term flight experiments are necessary to establish if plants grow or can be made to grow under conditions of near-weightlessness. Can they go through a life cycle? Several life cycles? We simply do not know. At present we can only speculate about possible problems. Until there are long-term flight data on plant performance, such questions will remain unanswered. CONCLUSIONS In addition to basic scientific curiosity, the desire of mankind for a long- term presence in space is now driving researchers to learn how to grow plants in space as the fundamental components of bioregenerative life support systems. (For a more detailed discussion of this topic, see Chapter 7, Closed Ecological Life Support Systems.) To compress the photosynthetic life support system that sustains all life on Earth into small spacecraft or even a lunar base is a formidable challenge that will require a much more precise understanding of plant capabilities and performance than has ever before been necessary. Plants will need to grow at maximum yields, in exotic environments, with unfailing reliability to play a role in space biohabitats. An unprecedented goal of coupling engineering to plant production will demand reproducible data about plant performance in space and maximum rigorous scientific precision. In order to approach the goals mentioned above, and those previously emphasized in the 1987 and 1988 CSBM reports, the discipline of space plant biology must have available more frequent flight opportunities, much longer microgravity exposure, and on-orbit controls. Only adequate, replicated, and controlled experiments contribute to science. A series of unrelated anecdotal information does not.
