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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 salinizationis 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.

Wang Chengshu's research group from the Center for Excellence in Molecular Plant Science, Chinese Academy of Sciences published a paper entitled "A bacterial-like Pictet-Spenglerase drives the evolution of fungi to produce b-carboline glycosides together with separate genes" in the Proceedings of the National Academy of Sciences (PNAS). This study reveals the molecular mechanism of the entomopathogenic fungus Beauveria bassiana acquiring a bacterial-derived horizontal transfer (HGT) gene to synthesize β-carbolin alkaloids and their glycosides.

Different prokaryotes and eukaryotes can synthesize β-carbolin alkaloids with various structures, which have important biological activity or medicinal value respectively. Many β-carbolin alkaloids from fungi have been reported, but the synthesis mechanism is rarely resolved. Different Pictet-Spenglerase (PS) enzymes have been identified in different bacteria, plants and animals, which can synthesize β-carbolin skeleton with tryptophan as a substrate, but PS enzymes derived from fungi are not clear.

This study found a HGT gene highly homologous to the marine bacterial PS enzyme in the genome of Beauveria bassiana, and named it Fcs1. Through heterologous expression in Escherichia coli and yeast, it was found that the gene can synthesize β-carbolin skeleton; Fcs1 was overexpressed in Beauveria bassiana, the products were isolated, purified and identified to obtain a series of β-carbolin alkaloids and their Glycoside compounds, most of which are new structural compounds. The study compared the transcriptome analysis between the wild strain and the Fcs1 overexpression strain, and obtained multiple differentially expressed potential P450 genes. At the same time, yeast expression and gene deletion analysis proved that a CYP684B2 family gene (named Fcs2) can oxidize the β-carbolin skeleton at multiple sites, and found the cofactor gene Fcs3 of Fcs2. Further gene function verification showed that a pair of tandem glycosyl-methyltransferase genes were responsible for the sugar methylation modification of carboxyl and multi-site hydroxyl groups to generate β-carbolin glycosides with different structures, which were named bassicarbosides.

Different from the structural characteristics of typical fungal secondary metabolic gene clusters, the different functional genes identified in this study are located on different chromosomes of Beauveria bassiana, but the CYP684A2 family genes adjacent to Fcs1 do not have the oxidized β-carbolin skeleton activity, but has a high degree of similarity with Fcs2, showing the evolutionary characteristics of clustering around the Fcs1 gene. In addition, the analysis found that when the homologous gene of Fcs1 in the relative species of Beauveria bassiana does not exist, the P450 highly homologous to Fcs2 in the genome does not have oxidative activity. Molecular docking and site mutation proved that there were differences in some key sites of Fcs2 homologous proteins in fungi without Fcs1.

This study enriched the types of PS enzymes derived from fungi, revealed that the acquisition of key HGT genes can promote the functional evolution of associated genes, and obtained transitional evidence for the formation of fungal secondary metabolic gene clusters. The research work is supported by the National Natural Science Foundation of China Innovative Research Group Project and the Key Research Program of Frontier Science of the Chinese Academy of Sciences.

Background reading

Beauveria Bassiana Products

It's a fungus that grows in soils all over the world. It is widely used as a sprayed biological insecticide to control a wide range of pests such as bed bugs, termites, thrips, whiteflies, aphids, and various beetles. It acts as a parasite on various arthropod species, causing white muscardine disease.