Recent research has uncovered the critical mechanism involved in the symbiotic relationship between fungi and trees. As it turns out, the fungus is able to take control of its tree host by injecting a small protein that neutralizes the tree’s immune defenses thereby allowing the fungus to colonize the plant. This is a critical advance in our understanding of the coevolution and functions of symbiosis between fungi and plants. These relationships are vital to the health and sustainability of natural ecosystems.
Scientists at the Institut National de Recherche Agronomique (INRA) and Lorraine University in France have been researching this mutually beneficial interaction and its role in the ecosystem. Their results were published in the May edition of the Proceedings of the National Academy of Sciences. Questions they have specifically pursued include how hosts differentiate between parasitic and symbiotic fungi and how exactly are fungi able to bypass the plant’s immune defense when establishing their relationship?
Apparently a lot can be deciphered by the biochemical dialogue which occurs between the organisms and thanks to global collaboration among researchers from INRA, Lorraine University, Oak Ridge National Laboratory (ONRL) and the University of Western Sidney in Australia, some of the molecular language used by mycorrhizal fungi was partially deciphered. While the rhizosphere is awash in the complexities of soil organisms competing for limited resources plant roots are putting out signal molecule which fungi are able to perceive and enable growth towards the root tissue.
The presence of the plant root and its signals triggers the release of small proteins from the fungus. Generally known as “effectors” these proteins are bioactive molecular signals which begin to prepare the plant tissue for a symbiotic relationship. It was the this mechanism; how the molecular signals actually prepared the plant, that was unknown up to this point.
The research collaboration established that one of these effectors, MiSSP7, binds a molecular switch which controls plant immunity. Usually when a plant is targeted by a disease a battery of defense reactions is triggered to kill off the invaders, jasmonic acid is one of the main hormones involved in this defense. MiSSP7 directly targets the jasmonic acid-associated control switch effectively neutralizing the defense response. Hindering the immune response allows the fungus to establish its relationship and begin the exchange of nutrients. These findings change the way this symbiotic relationship is viewed, rather than playing nice, the fungus basically force the host plant into a mutually beneficial relationship under false pretenses. Research continues aiming to identify if the other effectors of mycorrhizal fungi act similarly to control the host plant and force symbiosis.
Jonathan M. Plett, Yohann Daguerre, Sebastian Wittulsky, Alice Vayssieres, Aurelie Deveau, Sarah J. Melton, Annegret Kohler, Jennifer Morrell-Falvey, Annick Brun, Claire Veneault-Fourrey, Francis Martin. The Effector MiSSP7 of the Mutualistic Fungus Laccaria bicolor Stabilizes the Populus JAZ6 Protein and Represses JA-responsive Genes. PNAS, May 2014