Plant & zo
The science of plants and more
How light gets to the root
The waiting starts, after sowing seeds, for the first leaves to stick out of the ground. A just germinated plant, that found its way in the dark to the light. In the dark the main way of growing for a plant is through stretching its cells. Not by making new cells. This changes the moment the leaves stick out of the ground. Then light tells the cells of the plant: Start dividing. Start photosynthesis. Start using energy. But how does light tell this to all the cells of the plant, not only those in the leaf, but also those in the root?
The plant hormone auxin has a part in this. Auxin has more or less a role in everything in the plant. But one of its important tasks is the promotion of cell division. In a seedling that is still searching for the light, auxin is located mostly at the top of the stem. But as soon as the plant finds light, auxin moves to the tip of the root. Here cell division starts in the meristem, the root grows.
But this does not happen in a mutant, kai2, that had trouble with the transition from dark to light. The order from light: Start growing, use energy! Does not come through correctly. The tip of the root does not start growing, the seedling makes lots of side-roots instead. Auxin, so noticed the researchers, was not in the tip of the root but at the top, there were the root meets the stem. Here auxin accumulates, and does what auxin does best, stimulate cell division.
With use of auxin transferring PIN-channels, the plant is getting auxin in the right spot. At least when everything works as it should. Not so much in the kai2 mutant. The researchers had a look at the PIN-channels in kai2. In the dark, by just germinated seedlings, the PIN-channels in kai2 are at the same spot as in normal plants. But, whereby normal plants the PIN-channels quickly get to their new spot, those of kai2 lag behind. Auxin can not get quickly to the right place.
This shows that using KAI2, light can tell the whole plant ‘Start dividing, start photosynthesis, start using energy’. KAI2 in turn gives an order to the PIN-channels to change places. Giving auxin the chance to get to the right spot to stimulate cell division. Now there is a start of the answer how lights gets to the root.
Hamon-Josse, M., Villaécija-Aguilar, J.A., Ljung, K., Leyser, O., Gutjahr, C. and Bennett, T. (2022) KAI2 regulates seedling development by mediating light-induced remodelling of auxin transport. New Phytol. https://doi.org/10.1111/nph.18110