The cell death phenomenon during Tuber ectomycorrhiza morphogenesis

Cell death phenomenon was investigated during the formation and establishment of Tuber ectomycorrhiza (ECM) with host trees, both in vitro and in pot culture using terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) reaction, Transmission electron microscopy (TEM) and Enzyme-linked immuno sorbent assay (ELISA). Tuber borchii mycelium and plantlets of Tilia plathyphyllos were used for in vitro ECM synthesis, whereas T. melanosporum, T. aestivum and T. borchii spores and seedlings of Corylus avellana, Quercus pubescens and T. plathyphyllos were employed in pot cultures. Non-mycorrhizal roots showed TUNEL-positive nuclei at the level of cap cells and tracheary elements as a result of physiological root morphogenesis. In contrast, during the pre-symbiotic phase and the following ECM developmental stages, progressive accumulation of tannin/polyphenol deposits developed in epidermal and cortical cells, leading to the cell death but without TUNEL positivity. After this necrosis, a further unexpected autophagic cell death was observed in apparently healthy mycorrhizae, first affecting mycoclena and then the Hartig net hyphae. This series of cell death phenomena involving both root cells and fungal ectomycorrhizal hyphae points to the existence of a genetic orchestration between the two symbiotic partners during ECM morphogenesis and deserves further investigation to elucidate the underlying molecular mechanisms and signaling pathways.