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Plant Gravitropism

Example of gravitropic plant growthPlants have always had a prominent role in gravity related biological research, due to the fact their ability to detect gravitational fields underlies their ability to grow and reproduce.

Terrestrial (earth based) plants grow roots, which normally bend downwards (termed a positive gravitropic response) and shoots, which characteristically bend upwards (negative gravitropic response), a plants ability to do this correctly is fundamental to its survival, leading us to the conclusion that plants can not only sense the direction of gravitational fields but also send signals to the growing areas of the shoot and root that change the direction of growth accordingly. Our understanding of this system is at an advanced stage, due to the successful collaboration of ground and space based biological research programs.

At a cellular level, a collection of cuboid shaped cells in the plants root sense changes in its orientation using cellular inclusions called Statoliths. The plant then produce signaling molecules, which carry the signal from the root cap at the end of the root (where the signal is sensed) to the area of growth. A class of plant signaling molecules called auxins (which play a similar role to hormones in the human body) carry the signal from the root cap to the zone of elongation; representing an important component of the gravitropic growth response. The gravitational curvature (a growth curve due to gravity) is accomplished by differing the growth rate of each side of the root or shoot, causing it to bend upwards or downwards.

One area of particular interest to researchers looking at gravitropic responses and plant signaling in general is the way these auxins are transported across plant membranes. The model organism for most biological study is the plant Arabidopsis, its genome is relatively small and has been fully sequenced and the use of mutant strains of the plant has greatly facilitated genetic studies.

In the future it is hoped that space based research programs can continue to complement ground-based studies; and micorgravity research facilities such as those on the International Space Station will provide a base for cutting edge research that would not have been possible on earth; over time this research will help us continue to push back the frontiers of the genetic and molecular processes that underlie plant biology. The ultimate goal would be a complete understanding of plant growth, which would have massive implications both in space and on earth, especially in developing nations where plants represent a cheap way of providing everything from food and clothing to the basis of an economy, at a lower cost to the environment and the nation than man made materials; whether we achieve this depends in part to how willing we are to realize the huge potential offered by the unique research environment of space and microgravity.