The ISS has what is, essentially, a biolab onboard. Called the Advanced Biological Research System, the locker-like enclosure fits on both the Space Shuttle and the ISS. It has two separate compartments in which scientists can independently control temperature, light, and atmosphere. Its purpose is to grow plants, microorganisms, and arthropods in zero gravity.
In one experiment done inside the ABRS from October 2009 to September 2010, scientists grew a garden of thale cress (a flowering plant found mostly in Asia). In order to best understand how the stress of zero gravity was affecting the plants, scientists genetically modified it to glow when it was unhappy. This was helpful because it meant they could examine the plant without dissecting it.
What they found was that the glow gene (which is triggered by a plant hormone that is involved with growth) didn’t brighten up in the areas of the plant they were expecting. Scientists were sure they’d see the plant glow in the tips of its roots where plants sense gravity. Instead, it showed up in the stem area between the roots and the leaves. Also surprising, the plant roots followed the same movement patterns they do on earth when they use gravity to sense their way down into the ground. The scientists were shocked by the result because they expected the roots to only grow downwards in a gravity environment--but here they were using light as a substitute for gravity to tell the difference between up and down.
You can learn more about the ABRS and the thale cress experiment here.