Some of the challenges that scientists might face in space when growing plants would be the fact that plants must have carbon dioxide to carry out their process, which is why they are grown in special chambers. It is true that the astronauts would put off some carbon dioxide, but perhaps not enough. You would also be trying to grow plants in a zero gravity environment, which they aren't accustomed to.
The astronauts would have to be able to provide an extremely large amount of food in order to meet their nutrition needs. There would have to be enough room to grow an adequate amount of food aboard the ship. The plants would also need to be able to produce more than one crop per growing.
If scientists could genetically modify plants to help them grow better in space, they would need to make them able to thrive in a low gravity setting. They would also need to try and engineer them to need less water to survive. They would need to be strong enough to survive the lift off when going into space. The plants would also need to be able to regenerate and produce vast quantities of food.
