A Strategy for Research in Space Biology and Medicine in the New Century (1998) / Chapter Skim
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4 Plants, Gravity, and Space
4 Plants, Gravity, and Space
Pages 49-62
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From page 49... ...
SPACE HORTICULTURE Reasons for Studies on Space Horticulture There are three reasons for studying space horticulture. First, if the microgravity environment of spacecraft is to be used to carry out scientific experiments on basic mechanisms of plant responses to gravity, the plants must be grown under optimal conditions.
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From page 50... ...
A major problem has been how to close the plant growth unit completely to the outside environment. A second major problem has been developing the lighting systems to provide maximum photosynthetically active radiation with minimum power.
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From page 51... ...
It will be necessary, then, to focus attention initially on small plants such as Arabidopsis or on plants that can thrive under low light intensities, because power will be a limiting factor on the ISS. The engineering problems associated with developing a totally closed plant growth system are not trivial, but the need is pressing.
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From page 52... ...
Similarly, the lack of gravity may adversely affect root aeration, because in the absence of gravity water tends to pool in large, unbroken masses, causing an excess of water in some places with a concomitant lack of oxygen. A second problem is that the plant growth unit in which this seed-to-seed experiment is conducted must be as "artifact-free" as possible.
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From page 53... ...
Another abnormality is the apparent failure of decapped maize roots to reform caps while in space.~7 The balance between starch and sugar in plant leaves has also been reported to be altered in plants grown in microgravity compared with the ground controls, but this may also be the result of secondary effects of low gravity. One approach to the study of the role of gravity has been to examine plant response on a horizontal clinostat.
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From page 54... ...
However, a definitive experiment will only be possible when the plant growth apparatus has been sufficiently perfected so that extraneous stresses do not interfere with the detection of any gravity-requiring processes. At the present time at least four competing plant growth units are being designed and supported by NASA.
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From page 55... ...
The graviresponses of plants are a powerful system with which to determine the mechanisms controlling plant development, information that will be of fundamental value in agricultural sciences. In general, gravitropism can be broken down into four steps: perception, intracellular transduction, translocation, and reaction.
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From page 56... ...
Possible involvement of localized cytoplasmic calcium and cytoskeletal elements such as actin have been suggested.27 To understand how this gravitropic response is controlled, the process of tip growth will need to be studied in greater detail to learn what controls the location of this growth. Root hairs and pollen tubes also grow by tip growth, so learning about the control of tip growth in these gravitropic systems could have major benefits to other areas of plant science.
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From page 57... ...
Starchless mutants of Arabidopsis retain some ability to undergo root gravitropism,32 as do roots in which amyloplast-containing root cap cells have been eliminated by laser ablation.33 This suggests that roots may have a second, residual method for detecting the direction of the gravity vector. The importance of the dense starch grains in graviperception can be partly assessed by comparing the gthreshold for starch-containing versus starchless Arabidopsis roots.
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From page 58... ...
The mechanism by which auxin controls the rate of cell enlargement has been extensively studied. There is mounting evidence that auxin stimulates elongation of stem and coleoptile cells in part by promoting the export of protons from the cells, with the lowered wall pH activating wall loosening proteins such as the expansins.54 But why does auxin slow root cell elongation?
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From page 59... ...
The power of genetics to dissect the steps in gravitropism is immense, and the information that could be obtained by sequencing genes directly involved in this process could lead to an understanding of how the pieces fit together. Using complementation analysis, it should be possible to locate agravitropic mutants that are blocked specifically in each of the four main steps in gravitropism.
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From page 60... ...
Recommendation As a lower priority, NASA should consider supporting research on algal gravitaxis. Effects of Gravity-induced Tissue Stresses on Plant Development The above-ground organs of multicellular plants can undergo considerable stress in their tissues due to suspended weight.
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From page 61... ...
1996. Gravity related features of plant growth behavior studied with rotating machines.
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From page 62... ...
1992. The mechanism by which an asymmetric distribution of plant growth hormone is attained.
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