How does a seed know which direction is up and which down so the plant always grows to the surface but not the roots?
In: Biology
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Gravity, which is why growing plants on spaceships or the space station is rather difficult. https://youtu.be/3q6C8coA5yc
They’ve germinated seeds in space, sans gravity, they use light to orient their growth.
https://www.nasa.gov/content/growing-plants-in-space
This movement is called **Phototaxis**. I will try my best to make it as clear as possible. Bear with me…
When a seed germinates, it goes through four processes:
1. Gravity sensing:
> Some plant cells have a colorless organelle called ‘Amyloplasts’.
> A special type of amyloplasts, called statolith, is present in the cell alongside chloroplasts (the green pigment).
> When a seed starts to germinate, the statolith, being denser, settles down, and chloroplasts float above it (almost like water and oil). This causes a signal.
> Now here is where the magic happens. We all know that due to gravity, the denser material always settles down *below…* The same happens in plants, and so by sensing where statolith is, the plant knows what direction gravity is in. This is called **Gravitropism**.
2. Signal generation
Statoliths are sitting on a bed of Actin, a highly sensitive protein that sends a signal through the plant the moment it gets an idea as to which side is down and which side is up. This signal is carried throughout the plant as an electric charge.
3. Signal transmission and 4. Plant Elongation:
> Now, most green plants produce a growth hormone called **Auxin**. Now the special thing about this hormone is that it’s photosensitive, i.e. it can detect sunlight and when it does, it starts reproducing rapidly.
> So when our signal is transmitted throughout the plant, some of it reaches the Auxin flow, causing a lot of redistribution of auxin from root to shoot.
> As auxin undergoes cell division and grow outwards (towards sunlight.), the shoot grows larger while the root continues its regular growth.
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