Hi enaa,
I did some reading on plant gravitropism and I got a feeling of where the field is right now in terms of understanding the signaling pathways. The synthesis and localization of auxin seems to be the key in regulating the selective growth response to gravity that occurs in the roots and shoots; but auxin’s role in plant growth has been known for fifty years or more. What I find more interesting is how the seedling perceives the pull of gravity. I read that there is a specific type of gravity-sensing cell called a statocyte that contains starch bodies known as amyloplasts. The amyloplasts apparently come loose in the cytoplasm and move downward under the influence of gravity. As they move, they cause some stresses on the actin cytoskeleton that leads to calcium release, activation of a calcium-dependent kinase and a protein called ARG for ‘altered response to gravity’, among other things. This cascade of events stimulates localized auxin production in the root that makes it grow down. What I didn’t find was whether the same process occurs in shoots and if so, how does it cause them to grow up rather than down?
If you are a student of botany, you know that Arabidopsis thaliana is the test plant for a lot of genetic experiments where various proteins are either over-produced or lacking to see what happens to a certain signaling process. That’s one way to study gravitropism. I also read that the gravitropism signaling pathway can be studied in cultured cells. That would seem to me to be a quicker way to do some preliminary experiments since you would not need to get mutant Arabidopsis plants and have to wait while they grow.
One area that I did not see anything published about was gravitropism in seedlings. It has to be occurring because regardless of which way the seed is oriented in the ground, its roots always grow down and its cotyledon grows up. This might be a ‘fertile’ area for you to do some research.
Botany is not my specialty—I’m an immunologist—so I don’t have a feel for where more data is needed about gravitropism; but I do know about the importance of the actin cytoskeleton and I would be inclined to study that as one of the early points in the seedling where the gravitropism process is strong. If you could get some seeds from A. thaliana mutants deficient, for example, in ARG or CAM kinase or some enzyme in the actin polymerization pathway, you could germinate them, make sections through the roots and shoots, stain the actin cytoskeleton and analyze the differences among the mutants using a microscope and try to correlate them with changes in the gravitropic response. There are also some enzyme inhibitors you could add to the germination medium to see what effect they would have on the cytoskeleton.
I hope my suggestions are helpful. You’ll have to do a lot of reading and planning before you start this project. If you work with a lab that is studying plant tropisms, they will be able to help you much more than I can. If you have questions about experimental design or statistics, I can help you there. Let us know what you decide.
Here are a couple of the references I read:
http://www-ncbi-nlm-nih-gov.ezproxy.hsc ... PMC280565/
http://www-ncbi-nlm-nih-gov.ezproxy.hsc ... MC3808318/
Good luck!
