Contribution of Space Experiments to our Knowledge of Plant Development

The analysis of the development in space has shown that germination is normal in microgravity. However, even during the first steps of root growth, some differences can be observed between plants grown in microgravity or in 1 g. The orientation of the root tip during germination in space depends upon the orientation of the embryo within the seed (Volkmann et al. 1986). In lentil seedlings, there is a nastic movement, due probably to the fact that the embryo is curved in the dry seed. When the root germinates, its extremity bends first away from the cotyledon and then straightens out. This nastic movement is not clearly displayed on the ground because gravitropism is stronger than the nastic movement in orienting the root tip.

After germination in microgravity, roots can grow straight if they are in humid air, but if they are growing on agar or between agar plates, the orientation of their tip is random (Johnsson et al. 1996), at least for several days. After stimulation on a centrifuge in space, a gravitropic curvature occurs in microgravity for several hours, but it tends to disappear afterward (Stankovic et al. 1998a, 1988b). This phenomenon is called autotropism and cannot be observed on the ground because of the presence of gravity.

Although the morphology of the primary root is not strongly modified during the first two days of growth, there is a change in the cell cycle in the root meristem. In lentil roots, the first cell cycle appears to be longer in microgravity than in 1 g. After several cycles the delay seems to increase because the mitotic index in roots grown in microgravity is lower than in 1 g. In lentils, cell cycle is not changed when the roots are grown on a clinostat with exactly the same conditions of growth (same containers and so on). In this particular case, it is clear that clinostat cannot simulate the effects of microgravity, which shows the limits of the simulation.

The reproductive phase is completed in microgravity when the culture conditions are correct (Figure 6-23). A lot of problems encountered in growing plants in space are related to the fact that the physical environment is different in microgravity (Porterfield 2002), like the absence of convection and it is clear that the limitation of gas exchanges greatly influences plant growth (Musgrave et al. 1997).

Thus, it is clear that microgravity has a great impact on the development of plants. However, it remains to demonstrate whether it is due to (a) indirect effects on plant growth (for instance, lack of convection); or (b) direct effects (for instance, on cell cycle). It is now necessary to analyze these effects at the molecular level and in a well-monitored environment to remove the indirect effects of microgravity. EMCS (European Multi-Cultivation System) is a facility that will be used for plant growth in microgravity on board the ISS. It will have the advantage of monitoring gases (02, C02, ethylene) and to carry out experiment on a onboard 1-g centrifuge.

In microgravity, like on the clinostat, the apical dominance of the primary root over the secondary roots is reduced (Kordyum 1997, Aarrouf et al. 1999). The morphology of the root system is different from that observed in the vertical controls. In particular, there are a greater number of secondary roots and these roots grow faster.

The loss of apical dominance has been well documented with clinostat experiments (Driss-Ecole et al. 1994, Aarrouf et al. 1999). It is linked to the modification of the hormonal balance in the primary root. Experiments in space should be done to confirm that the reduced apical dominance results from the hormonal content in roots. Once again Arabidopsis harboring the DR5::GUS construct should be used to analyze auxin distribution in space grown seedlings.

Cell cycle has been intensively studied in plants in the last decade (for review, see Inze 2005) and plant molecular biologists have the opportunity of using many molecular tools (Paul and Ferl 2002) to analyze plant growth in space. It should be important to confirm that gravity has an influence on the G2/M transition, as hypothesized by Yu et al. (1999). This transition corresponds to a phase of checking which takes place just before the mitosis.

It is clear that we are far from understanding the causes of the changes in the development of plants in space. Many pioneering experiments have been done in space without monitoring gas composition, temperature and so on, so that the conclusion of their authors must be questioned since plants are very sensitive to external factors. More clear-cut results have been obtained on board Space Shuttle flights or the International Space Station, since dedicated facilities providing onboard 1-g controls and better culture conditions have been developed. The experience gained from the past studies will be useful for the future. Undoubtedly, future research on board the International Space Station will provide new insights on the role of gravity on plant growth and development (Figure 6-24).

Figure 6-24. This photograph shows a close-up view of sprouts on the Russian plant growth experiment performed by Expedition-6 crewmembers during their stay on board the International Space Station. Photo courtesy of NASA.
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