The term ectomycorrhiza describes symbiotic relationships between the roots of certain trees and soil fungi. Ectomycorrhizas are common in temperate and boreal forests. They are essential for the tree's nutrition—water absorption in particular—and furthermore beneficial for its ability to adapt to the environment. Despite their importance, how ectomycorrhizas come to be has remained largely mysterious.
To bring light to the subject, researchers from the University of Orléans, Inrae and the University of Lorraine decided to explore the epigenetic mechanisms driving the development and control of this tree-fungus partnership. Working with colleagues from CEA-Jacob's CNRGH department and teams from Spain and the United States, they studied the role of DNA methylation in mycorrhization in Populus tremula × Populus alba, a model poplar line. DNA methylation is an epigenetic phenomenon that affects gene expression without changing the gene sequence. It plays a vital role in the gene activity of each type of cell as a function of its environment.
Thus, to better understand this epigenetic mechanism's role in mycorrhization, the researchers compared fungi colonization responses in poplar lines with modified DNA methylation to those in wild-type poplar lines. The results left little doubt: the tree lines with low DNA methylation showed lower mycorrhization rates (up to 40% lower in one line), suggesting that the host tree's DNA methylation indeed plays a central role in its ability to provoke mycorrhization and thus ensure its healthy development. The authors also reported that host trees appeared to influence DNA expression in the fungus.
Beyond opening new research vistas to better understand symbiotic relationships among living organisms, this pioneering study also hints at possible applications in forest management to address droughts and other challenges in the era of climate change.