Gardening in space has been part of the International Space Station (ISS) from the beginning -- whether peas grown in the Lada greenhouse or experiments in the Biomass Production System. The space station offers unique opportunities to study plant growth and gravity, something that cannot be done on Earth, NASA said in a statement.

The latest experiment is Hydrotropism and Auxin-Inducible Gene expression in Roots Grown Under Microgravity Conditions, known as HydroTropi. Operations were conducted October 18-21, 2010, HydroTropi is a Japan Aerospace Exploration Agency (JAXA)-run study that looks at directional root growth.

In microgravity, roots grow latterly or sideways, instead of up and down like they do under Earth’s gravitational forces. Plants are fundamental to life on Earth, converting carbon dioxide and light into oxygen and food. Plant growth may be an important part of human survival in exploring space, as well.

Using cucumber plants (scientific name Cucumis sativus), investigators look to determine whether hydrotropic -- plant root orientation due to water -- response can control the direction of root growth in microgravity. To perform the HydroTropi experiment, astronauts transport the cucumber seeds from Earth to the space station and then coax them into growth.

The seeds, which reside in Hydrotropism chambers, undergo 18 hours of incubation in a Cell Biology Experiment Facility or CBEF. Then the crew members activate the seeds with water or a saturated salt solution, followed by a second application of water 4 to 5 hours later.

The crew harvests the cucumber seedlings and preserves them using fixation tubes called Kenney Space Center Fixation Tubes or KFTs, which then store in one of the station MELFI freezers to await return to Earth.

It is thought that on Earth mainly gravity determines the orientation of root growth. Under microgravity conditions onboard the Space Shuttle, however, water seemed the only factor that determined the directions in which roots grew.

It is difficult to verify the idea on Earth, as the effect of gravity on root growth direction cannot be completely eliminated. Therefore, the relationship between water and root growth in microgravity will be studied onboard the ISS.

The experiment will demonstrate a plant’s ability to change growth direction in response to gravity (gravitropism) versus directional growth in response to water (hydrotropism).

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The results from HydroTropi, which returns to Earth on STS-133, will help investigators to better understand how plants grow and develop at a molecular level. Investigators will learn about plants inducible gene expression, by looking at the reaction of the plants to the stimuli and the resulting response of differential auxin -- the compound regulating the growth of plants.

In space, investigators hope HydroTropi will show them how to control directional root growth by using the hydrotropism stimulus; this knowledge may also lead to significant advancements in agriculture production on Earth.

NASA is set to launch its space shuttle Discovery on STS-133 mission from Kennedy Space Center in Florida to the ISS on Feb. 24. The crew will deliver and install the Permanent Multipurpose Module (PMM) Leonardo, the Express Logistics Carrier 4 and also provide critical spare components to the International Space Station.

The STS-133 crew members are Commander Steven Lindsey, Pilot Eric Boe and Mission Specialists Alvin Drew, Michael Barratt, Timothy Kopra and Nicole Stott. The mission will feature two spacewalks, which is expected to be done by Timothy Kopra and Alvin Drew, to do maintenance work and install new components. Robonaut 2 (R2) will be the first human-like robot in space when it flies on Discovery inside the PMM to become a permanent resident of the station.