You might haven’t even ever thought about the reason why plant shoots always grow up and roots down, ultimately it is the way it has worked all the time. Now that you are thinking about it, you might say that it is obviously because of plants reacting to light and it is the key to orientating in the space. It is not a bad guess, but the truth is somewhere else. If you grow microgreens by yourself, you probably cover your seeds for first few days to let them attach to the ground properly. You, therefore, limit plant’s light reach as well as put pressure from both sides on it, and yet the plant knows the correct direction. Realising this fact moves us closer to the most probable solution. The gravity.
To further acknowledge this assumption, there have been tests carried out on the International Space Station, a place with almost no gravity (there is still a microgravity). The control sample was left undisturbed and the test sample was put into a centrifuge. This experiment showed that seeds in the untouched sample grew in somewhat random direction, whereas the control sample had its roots in the direction out of the centrifuge (= a direction of the force of gravity).
This plant’s response to a change in its orientation relative to gravity is called the gravitropism. Roots respond positively to gravitropism, meaning that they grow down towards the gravity vector, and shoots respond negatively, growing away from the gravity vector. In order to achieve the growth in a specific direction, plants use an elongation of cells on the side opposite to the desired path. All of this conscious movement is happening in the middle part of plant cell groups called the zone of elongation. Cells in this area are able of increasing in size up to 10 or 20 times. To complete your idea of plant structure: there is also the zone of maturation, where cells assume their final form, and the zone of cell division, the only part of the root where cells actually divide actively and there is no difference among types of cells. Apart from these three zones, there is also a root cap, a part that protects the embryonic cells which are the only cells capable of renewing. It also produces a sort of lubricant that allows the root to be pushed deeper into the ground.
The mentioned root cap is the essential component to sensing gravity. Interestingly enough, the method is very similar to human sensing (although in detail it should not be compared as such). Our body position sensors are located inside our ears. Simply put, we have three canals in all 3 planes connected by a vessel, all filled with some fluid and some crystals. As we move around, the crystals flow through these canals and are detected by hairs on the inside wall of this organ. Taking all this back to plants, we discovered that cells in the root cap contain amyloplasts, small organelles similar to chloroplasts, and they can be found on the bottom side of cells, defining the down side for the plant.
Another thing that we should get to know now is what causes the elongation. Turns out there is a plant hormone called auxin, which stimulates growth and is also involved in phototropism, gravitropism, branching, cell elongation and other functions. It is produced at the tip of a sprout, but usable at most parts. The action of the hormone is therefore dependent on location, timing, and concentration. The last named attribute, the concentration, is quite interesting to say at least. It has been monitored that very high auxin concentration, in fact, inhibits elongation, whereas lower concentration promotes elongation, and in the very bottom part of a plant with almost zero concentration it causes inhibition again.