Plant & zo
The science of plants and more
How plants avoid salt
Plants don’t like high salt levels in their cells. They do therefore everything they can to avoid this. As mentioned in earlier posts plants have a number of strategies for this. However, the best strategy is not taking up to much salt in the first place. Being sensitive to salts, plants can sense differences in salt concentrations. When the roots grow in a patch of soil high in salt their growth is slower than it would have been when growing in soil low in salt. However, when growing in soil low in salt, but coming across a patch of soil high in salt the roots will adjust their direction of growth to avoid the high salt soil. To understand how plants do this you will need to how root growth is regulated.
Simply said, root growth is a product of increasing the number of cells and increase of the length of the cells. Root growth is regulated by distribution of the plant hormone auxin. Auxin is made in the shoot and transported via the phloem to the tip of the root. Here it creates an auxin maximum, which inhibits cell differentiation and elongation, but creates a zone with fast dividing cells. From the tip of the root auxin is actively transported up again creating a reverse fountain effect. With auxin concentration decreasing the further away the cells are from the tip. This enables the plant to control the differentiation and elongation of the cells, the cells just above the zone with fast dividing cells still have a slightly lower level of auxin, resulting in inhibition of division and elongation, but promotion of differentiation. In the cells above that the auxin level is lower again, now enabling elongation, the lengthening of the cells. When the root is simply growing down the reverse fountain of the auxin gradient is symmetric.
But when a salt gradient is sensed, the auxin gradient becomes asymmetrically, with higher levels of auxin at the side with less salt. This higher auxin level inhibits elongation in the cells away from the salt. While at the same time at the side with more salt the auxin levels are lower. These lower auxin levels promote elongation in the cells near the salt. This asymmetric lengthening of the cells bends the root away from the salt. Enabling the root to grow away from the salt.
Active auxin transport is needed for the creation of the reverse auxin fountain. It was found that transporters called PIN are responsible for auxin transport. Under normal circumstances they are mainly located in the plasma membrane at the top of the cell. When exposed to salt the cell use endocytosis to remove these PIN proteins from the plasma membrane, reducing the auxin flow. So when the exposure to salt is asymmetrically, there will be more PIN proteins in the plasma membranes of cells on the side with less salt than in the plasma membrane of the cells on the side with more salt. Resulting in the asymmetric auxin gradient, and subsequent bending of the root.
