Phosphorous is required for all life. However, many people think that the reserves of phosphorous may become depleted as the use of phosphates in commercial fertilizer increases. Researchers are therefore working to improve phosphorous uptake by plants, and the fungi in their root network are key to this. New results indicate a central mechanism in the symbiosis between fungi and plants that, if adjusted, may reduce the use of phosphates in fertilizer.
Our bones, teeth, genes and the membranes of all our cells highly depend on phosphorus. However, if people continue to use phosphates in fertilizer, the reserves may become depleted. We do not know when this will happen, but fertilizer use can be drastically reduced by improving plants’ ability to take up the phosphorus already present in the soil. A key factor is the symbiotic interaction between arbuscular mycorrhizal fungi (AMF) and plants that colonize the roots when the plants lack phosphorus.
“Some nutrients in fertilizer, such as phosphorus, are fixed in the soil. The AMF can transfer otherwise inaccessible phosphorus to the plant. We have found an important regulator in the interaction between plants and AMF. Strengthening this symbiosis early in the life of the plant can ensure more efficient uptake of phosphorus, reducing the need for phosphate fertilizers. This can prolong our access to phosphorus fertilizer and help avoid harmful runoff of phosphates into streams and rivers,” explains Thomas Christian de Bang, Assistant Professor, Department of Plant and Environmental Sciences, University of Copenhagen.
Strengthening symbiosis
To investigate how plants regulate their interaction with AMF, the researchers used the plant Medicago truncatula (barrelclover), a small annual legume often used in basic research. The aim was to determine which genes are expressed at low versus high phosphorus levels.
“Plants use energy to interact with AMF, and they therefore only do this when it´s necessary. When phosphorus levels are low, the plant strengthens the symbiosis with AMF. Conversely, when phosphorus levels are high, the plant reduces the symbiosis to save energy,” says Thomas Christian de Bang.
The researchers therefore compared which genes are activated at high versus low levels of phosphorus to find clues about which genes are important for the fungal symbiosis. After analysing a group of potential candidates, the researchers found a candidate among a group of small peptides from the CLE family of genes that specifically form in the roots of plants.
“The barrelclover genome encodes 52 CLE peptides, each with a specific function, but the results clearly showed that the MtCLE53 peptide played a special role. When we reduced phosphorus, less MtCLE53 was expressed, but at high phosphorus levels, we saw higher expression of MtCLE53. MtCLE53 levels were also high in plants colonized by AMF but with low phosphorus levels and, overall, this indicated that MtCLE53 played an important role in reducing the symbiosis with the fungus,” explains Thomas Christian de Bang.
To confirm that MtCLE53 actually shuts down the symbiosis, the researchers repeated the same experiments with plant mutants lacking the SUNN receptor to which MtCLE53 likely binds and the protein RDN1, which could add sugar molecules to MtCLE53. In both cases, the symbiosis was not reduced.
“The evidence therefore suggests that the MtCLE53 peptide plays a crucial role in this symbiosis, so if we can understand exactly how and if we can easily downregulate this peptide, we might be able to strengthen the symbiosis between the plant and the fungi,” says Thomas Christian de Bang.
Conserving phosphates and improving the environment
The early stages of crop growth constitute an important window for strengthening the symbiosis between plants and fungi. While the plants are small, the soil around them has plenty of phosphorus, but as they grow, they need to absorb phosphorus that is further away.
“Ample access to phosphorus in the early stages of plant growth is especially important for high yields, since phosphorus indirectly controls how many shoots a plant puts out, and thus how many seeds the plant can produce. MtCLE53 slows down the symbiosis with the fungi at high phosphorus levels, which is a paradox in modern agriculture, since this delays the plant’s uptake of distant phosphorus via the AMF. Effective symbiosis with AMF will improve phosphorus uptake in agriculture, avoiding unnecessary use of this limited resource and phosphate runoff into the aquatic environment when heavy rain occurs,” explains Thomas Christian de Bang.
The researchers already have a good idea of how they can change plants to regulate the production of MtCLE53. They have therefore applied for funding for proof-of-concept trials in crop plants. Using CRISPR technology, they will investigate whether the plants attain the right properties so that they can improve the effectiveness of the symbiotic interactions and thereby become more efficient at absorbing phosphorus.
“We can quantify AMF colonization using microscopy, measure gene expression levels and measure the uptake of phosphorous to determine whether the symbiosis improves. Since we are not allowed to use CRISPR or genetically engineered plants for cultivation in Europe, we will seek to recreate the changes through traditional breeding techniques, so that we ensure phosphorus retention and spare the environment from unnecessary phosphate runoff,” says Thomas Christian de Bang.
“The CLE53-SUNN genetic pathway negatively regulates arbuscular mycorrhiza root colonization in Medicago truncatula” has been published in the Journal of Experimental Biology. In 2017, the Novo Nordisk Foundation awarded a grant to Thomas Christian de Bang for the project BIOPEP – Identification of Soil Microorganisms Stimulating Root Growth to Improve Phosphorus Uptake in Plants. The project also received funding from Brødrene Hartmanns Fond and the University of Copenhagen.
