Plants have three major responses to changes in the gravity vector. First, plant stems and roots can alter the direction of their growth in relation to the direction of gravity (gravitropism). An organ will assume a specific direction at some defined angle to the gravity vector, and if displaced from that direction, will curve until the original direction is resumed. Gravitropism is by far the most extensively studied plant response to the gravity vector. In the second response (gravitaxis), the swimming direction of some unicellular algae is directed by the gravity vector. Finally, plant development can be influenced by the direction of the gravity vector. For example, the weight of a tree limb alters the pattern of formation of new wood to provide additional support for the limb. This extra strengthening, called reaction wood, is in response to the tissue stresses, which are directly related to the direction of the stresses within the tissues, as well as the direction of gravity itself.
A second approach is to use mutant plants that are insensitive to specific environmental stresses—for example, using ethylene-insensitive mutants to eliminate complications from exogenous ethylene. Another possible solution is to use indicator Arabidopsis plants that have been transformed with indicator genes coupled to specific stress-induced promoters. These specific promoters have already been or are being developed for stresses such as temperature extremes, ethylene, water stress, vibration, and anaerobiosis. It should be possible to detect and perhaps measure the amount and types of stresses the space-grown Arabidopsis undergoes by use of these plants.
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Stresses can trigger similar or opposing movements: for example, drought induces closure of stomata, whereas many pathogens exploit stomata and cause them to open to facilitate entry into plant tissues.
-Explained by Acid Growth Hypothesis:
In both systems, it appears that the direction of gravity is perceived by the sedimentation of statoliths, tethered by the cytoskeleton. The result is a change in the location of the tip growth by which such cells elongate. The steps in between are largely a mystery. Possible involvement of localized cytoplasmic calcium and cytoskeletal elements such as actin have been suggested. 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. To do this, the enzymes, cell wall components, and vesicle trafficking involved need to be identified. It would help greatly if mutants altering the gravitropic process in tip-growing cells were available and if the genetics of these organisms were better understood. Nevertheless, these tip-growing gravitropic systems hold great promise for some comprehensive study of the cell biology involved and may prove to be the systems that lead to an understanding of gravitropism in more complex, multicellular systems.
emerged during the early colonization …
Some developmental abnormalities have been recorded with plants grown in in space. For example, a number of root cells, fixed either in space or after return to Earth, showed chromosomal abnormalities. Whether these were a response to the lack of gravity or to some other stress the plants experienced while on board the spacecraft, such as elevated levels of ethylene or CO2, cannot yet be determined. Another abnormality is the apparent failure of decapped maize roots to reform caps while in space. 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.