Regulation of pectin remodeling and ectomycorrhiza development by cell wall localized proanthocyanidins in hybrid aspen roots

Ectomycorrhiza (ECM) formation in tree roots requires pectin de-esterification and degradation in the middle lamella for fungal entry into the root apoplast. Proanthocyanidins (PAs) interact with pectin in vitro , however their function in roots is unknown. We hypothesized that root PAs regulate ECM formation in planta by impeding pectin degradation. PA staining and microscopy on hybrid aspen roots in interaction with the fungus Laccaria bicolor revealed that cell wall PAs decrease during colonization, further confirmed by metabolomics and consistent with reduced transcription of PA-biosynthesis genes. Transgenic PA-overproducer trees formed fewer ECM and fungal penetration was shallower. Immunolocalization revealed highly methylesterified pectin in their middle lamella, suggesting hindered pectin de-esterification by high PA-levels. Conversely, pectin in transgenic PA-decreased trees was less esterified indicating looser cell-to-cell adhesion consistent with more frequent and accelerated ECM formation. These findings suggest that ECM formation is regulated by in planta PA-pectin interaction and downregulates PA biosynthesis.