Laccaria bicolor pectin methylesterases are involved in ectomycorrhiza development with Populus tremula x Populus tremuloides

O_LIThe development of ectomycorrhizal (ECM) symbioses between soil fungi and tree roots requires modification of root cell-walls. The pectin-mediated adhesion between adjacent root cells loosens to accommodate fungal hyphae in the Hartig Net, facilitating nutrient exchange between partners. We investigated the role of fungal pectin modifying enzymes in Laccaria bicolor for ECM formation with Populus tremula x Populus tremuloides. C_LIO_LIWe combine transcriptomics of cell-wall related enzymes in both partners during ECM formation, immunolocalization of pectin (Homogalacturonan, HG) epitopes in different methylesterification states, pectin methylesterase (PME) activity assays and functional analyses of transgenic L. bicolor to uncover pectin modification mechanisms and the requirement of fungal pectin methylesterases (LbPMEs) for ECM formation. C_LIO_LIImmunolocalization identified remodelling of pectin towards de-esterified HG during ECM formation, which was accompanied by increased LbPME1 expression and PME activity. Overexpression or RNAi of the ECM-induced LbPME1 in transgenic L. bicolor lines led to reduced ECM formation. Hartig Nets formed with LbPME1 RNAi lines were shallower, whereas those formed with LbPME1 over-expressors were deeper. C_LIO_LIThis suggests that LbPME1 plays a role in ECM formation potentially through HG-de-esterification, which initiates loosening of adjacent root cells to facilitate Hartig Net formation. C_LI