High levels of sodium-containing salts in the soil are a problem for many plants: as a result, they do not grow well, or not at all. Soil salinization is seen as one of the greatest threats to feeding the world’s population, as it makes soils increasingly poor, especially in arid regions. A research team of Chinese, German and Spanish researchers, including Professor Jörg Kudla and his team at the University of Meinastre in Germany, has now discovered a mechanism in Arabidopsis thaliana that enables plants protect sensitive stem cells in the root tip meristem against salt stress. The meristem, which ensures the constant formation of new cells in the root and thus growth, is particularly sensitive: In contrast to fully formed plant cells, it has no vacuoles inside the cells that can process harmful substances.
Researchers were surprised to find that plants can provide protection to individual populations of cells against the stress of toxic salts. While we already know that plants have multiple mechanisms that allow them to cope with high salt levels in soil water, one is the active transport of salt in the cells, and the other is the mechanical maceration of specific cell layers in the root system. But what we didn’t know was that plants also specialize in protecting the stem cells in their roots “The signaling pathway we discovered, combining components of known salt stress signaling pathways with signaling proteins that control root development, has the additional purpose of detoxifying the plant,” says Jörg Kudla.
The mechanism: A specific enzyme, a receptor-like kinase called GSO1, transports sodium out of the cells of the meristem. To this end, GSO1 activates the kinase SOS2 (SOS stands for “salt oversensitivity”), which in turn activates the transporter SOS1, which pumps sodium ions outward through the cell membrane and, in turn, transports protons into the cell. Under salt stress, the formation of GSO1 increases, especially in meristem cells.
In addition, the team demonstrated that GSO1 also helps prevent excess salt from seeping into the vascular tissue of the root. This vascular tissue is located inside the plant and transports water and minerals from the roots to the leaves. Water seeps into it in an uncontrolled manner through a mechanical barrier, the Casparis strip, which prevents dissolved minerals in the soil. The researchers also demonstrated that in cells that form Casparian stripes, GSO1 levels increase due to salt stress.
“GSO1 is a well-known receptor kinase in plant developmental biology,” says Jörg Kudla. “It plays an important role in various stages of plant development. Now, for the first time, we have been able to show that it also plays a role in salt tolerance. And activate the ‘sodium pump’ through another signaling pathway that may not depend on calcium.” Calcium signaling in cells plays a key role in other known adaptive responses of plants to salt stress.
Regarding the method: The team discovered the importance of GSO1 by comparing numerous mutants of various receptor-like kinases in thale clothing. By studying the protein-protein interactions, they identified the enzyme’s reaction partners in the signaling pathways that protect the meristem and form Casparian strips. Methods for further investigation include mass spectrometry and high-resolution microscopy.
