Plant & zo
The science of plants and more
Searching for light
After germinating, the plant searches its way to the light. There it can start photosynthesis to obtain sugars, energy. To find light the plant stretches its hypocotyl, the part of the stem between the root and the embryogenic leaves, as far as it can. This is enabled by energy that is stored in the seed. For the seedling to stop stretching and to start photosynthesis, it is important that the plant knows when it has found light.
The plant uses HY5 as an indication that there is light. In the dark this protein is instable, making it a good signal for the amount of light the plant perceives. The higher the levels of HY5 the more light there is. And indeed, when in the light without breakdown of HY5 the hypocotyl stops stretching.
That sound easy, HY5 present, light is there, stop stretching. But there is more going on. Arrives a seedling above ground but in the shade, then the hypocotyl keeps stretching. And when researchers are feeding the seedling sugar, then it keeps stretching for longer. What is going on? This is what Israelian researchers were asking themselves.
To find out how light and sugar influence the stretching of the hypocotyl, the researchers looked at three proteins HY5, PIF4 – this protein is stimulating stretching of the hypocotyl, and HXK – this protein plays a role in processing sugars. They found that the presence of HXK or PIF4 stimulates stretching of the hypocotyl. It turned out that HXK is actually promoting the accumulation of PIF4 which in turn promotes the stretching of the hypocotyl. HY5 is doing the opposite, it stops the stretching of the hypocotyl.
HXK and HY5 are not directly influencing each other, but when there is more HY5 then HXK the stretching stops. From this research it is not clear if the amount of HXK is a signal for the amount of available sugar. If this is the case, then the seedling will be stretching itself as far as possible, with the energy it got from its seed, to get to the optimal amount of light. This makes it possible that a seedling keeps stretching its hypocotyl when in emerges from the ground in the shade, or when it got some extra energy from researchers. Searching for the brightest light.
Plant en Zo 
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