Plants Defying Gravity !!!
I love nature so much. Nature brings me such a joy. You will always have something to learn from nature if you look around and meditate. Today I share with you a question that I have hard time understanding.
Since I was little, I wondered... how the plants grow up !?!
The roots go down and the stems grow up !!!
It is so easy to understand how the roots grow, however, stems was very hard for me to understand. I mean, the stems grow "UPWARDS"...What about gravity !!!
I watch those videos showing the process of plant growth and I am amazed. The stems are growing and they have no problem with gravity at all.
Well, It is a phenomenon that is called "Gravitropism". Gravitropism is either positive which means "with gravity" or negative gravitropism which means against gravity. It is said that plant and fungi both have negative gravitropism i.e. both of them grow upwards. The first scientist to notice this phenomenon was Charles Darwin. Darwin couldn't understand or explain this phenomenon.
WELL !!!
Some scientists try to explain negative gravitropism by the action of "auxin". Auxin is a plant hormone that has many functions; one of these functions is controlling which side of the cell elongates during the plant growth. Furthermore, auxin controls the shape, direction and force of growth as well. When explaining auxin function like that, it seems so easy to understand.
But, wait a minute...that is not all. Let me explain it further...so I can confuse you as well :)
Auxin hormone by the action of "GRAVITY" is always in the bottom of the cell. It is function however is opposite in roots than stems. In roots it prevents cellular elongation against gravity and in stems it promotes it !!!
So much for the explaination !!!
Well, if you have time may be you can ask an expert your self. There are many agricultural conferences in Africa this month; in South Africa "Agriculture Investment Summit 2011" or may be in Egypt "International Conference for Agriculture and Irrigation in the Nile Basin Countries" or yet in South Africa in the very famous "Sun City" "allFresh! 2011 Agricultural Conference and Expo".
Or , may be you have any other theory explaining this. Because my question is yet to be answered !?!
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Image Credit: by US Environmental Protection Agency via Wikimedia Commons.
WOW !!!
Thanks a lot Abdelaziz, finally my question is answered :)
Thank you for your great input. These gravity sensors are really interesting and yes I agree with you definitely the cross reactions between hormones might be the reason for this opposed actions.
Thank you very much for such interesting topic.
In fact, many models had been evolved recently to explain the phenomena of gravitropism, however many points are still poorly understood too.
What you explained, was Cholodny-Went model, in which the horizontally oriented organs has higher concentration of auxin in the lower surface in response to gravity, then auxins have inhibitory function in roots on the contrary of their function in stems. so, unequal expansion or growth occur which lead to bending downword or upword in case of root and shoot respectively.
So far this Idea is partially correct, However there are much more complications in the mechanism of gravitropism.
In fact, gravity sensors were recently discovered in specialized cells. Large, dense amylopasts (starchy) molecules have higher density than cytosol called (Statolith) can be settled on sides of the cell wall (Cytoskeleton) according to the orientation of the cell. So, in vertically oriented organs these statolithes settle on the bottom of the cell, while in the horizontally oriented ones they start to settle on one side of the cell wall. Interstingly, there is a specific structured Endoplasmic reticulum in a petal-like form, you can imagine like a cup, when these statolithes settle down on it, it triggers a calcium channels fluxes act as a secondary messenger for gravity signal. Subsequently, a specific auxin efflux carrier protins (PIN protein family) are triggered to be relocalized on the cell memberane they become organized on the lower side of the cell, thus accumulating auxin in the lower side of the organ. So, we can conclude that auxin itself does`t respond to gravity. But it`s unequal distribution in the horizontally oriented organs is a result of multiple events started with statolithes settlement; the gravity sensors.
However, I could`t find a clear reason for the contrast between the auxin function in root and shoot. But as far as I know, the inhibitory action of auxin in root growth may be evolved due to the cross talk between auxin and other phytohormones like ethylene or other organo-specific regulations that I really don`t know.
Thanks for your time
Thanks again for the interesting topic.
References:
1- Plant Physiology, 3rd edition by Lincolin Taiz and Eduardo Zeiger. Chapter 19: Auxin the growth homone, pg 443.
2- Kamil Růžičkaa, Karin Ljungc, Steffen Vannested, Radka Podhorskáf, Tom Beeckmand, Jiří Frimlb and Eva Benková (2007). Ethylene Regulates Root Growth through Effects on Auxin Biosynthesis and Transport-Dependent Auxin Distribution. Plant cell 19:2197-2212.
@Terri, you are right, the reason is obvious, plants surely seeks the sun for photosynthesis...the problem is how the plant does it ignoring gravity like that !!!
@Marwa, it is really interesting, that a single hormone in the same plant will act that way.... in the same plant...not in another spieces, or another climate...the same plant...so strange !!!
interesting Doaa .. I never thought about that .. i am impressed that the same "Auxin" has a dual function!!!
Plants grow upward, defying gravity, toward the light or the sun.
