Phosphate in exchange for carbon

Arbuscular mycorrhizas are a type of fungi that can penetrate the roots of plants to form a symbiosis with the plant. Plants are in need of phosphorus to grow, but many species cannot take this up efficiently from soil. The fungus can, thereby helping the plant. In return, the plant provides carbon that is necessary for the fungus. The phosphorous status of a plant can regulate the rate of symbiotic development and maintenance. When phosphate is high, plants suppress development and retention of symbiotic relationships. In low phosphorous conditions, plants stimulate these processes. This is communicated to the root cells via signalling pathways. But which ones?

Of hormones and CLE peptides

Strigolactones are a group of hormones produced by plant roots. They were identified as candidate for regulating the symbiotic process, because phosphorous-deprived plants upregulate strigolactone synthesis and export, while elevated phosphorous levels suppress strigolactones. Moreover, fungi colonisation of the root itself can suppress symbiotic development. The researchers investigated the relationship between these processes by comparing gene transcription in roots colonized by a fungus versus gene transcription in non-colonised control roots. A special focus was on CLE peptides: small, secreted peptides present in many plant species. They have a role in plant response to a changing environment, such as temperature or drought, as well as fungi and phosphate levels. The screen showed that some CLE peptides indeed respond to colonisation by fungi. Using genetic tools such as screening in plants mutant for specific genes, they further elucidated which genes are responsible. They found that in plants high in fungi and phosphate levels, the CLE peptides bind to a specialized signalling protein, a so-called receptor-like kinase, SUNN. The interaction blocks genes involved in strigolactone synthesis, thereby blocking further symbiotic development. This is a negative feedback loop; it leads to less symbiosis.

The importance of balance

But why would a plant need to downregulate a seemingly beneficent relationship? Because the symbiosis comes with a cost. Plants need phosphate to grow, but they also need carbon. The latter is usually no problem, since plants perform photosynthesis to create enough carbon with surplus for the fungus. But as the symbiont grows, it takes up more carbon. At some point, the balance can tip, making the costs outweigh the benefits of the symbiosis. Knowing how to regulate the symbiosis can help in restoring the balance, leading to the best growing conditions for the plant. And since the CLE-SUNN-strigolactone feedback seems to be conserved in different plant species, this might be interesting for optimizing growth in many crops.

Publication

L.M. Müller et al., A CLE–SUNN module regulates strigolactone content and fungal colonization in arbuscular mycorrhiza, Nature Plants, 2 September 2019, doi: https://doi.org/10.1038/s41477-019-0501-1