Ultrastructural study on the interaction between Physcomitrella patens and Botrytis cinerea

Little is known about the interactions of bryophytes with their pathogens compared with those of flowering plants. This study used the moss Physcomitrella patens and fungal pathogen Botrytis cinerea to investigate their interaction at the ultrastructural level. Infection behaviour of B. cinerea in moss tissues was observed for 1–7 days after inoculation. Some appressoria directly penetrated P. patens cytoplasm through leaf cell walls, and others entered the intercellular spaces of leaves and infected cytoplasm. Infection hyphae were observed in sclerenchyma cells of P. patens stems for the first time. The results demonstrated that the ratio of hyphae‐inoculated leaf cells to leaf numbers could be used to quantify the disease development process at 24 h after inoculation. The ultrastructural studies revealed two responses of P. patens to B. cinerea inoculation: reinforcement of cell walls, including papillae formation; and cell death. This was the first observation of papillae for P. patens–B. cinerea interaction at the ultrastructural level. Papillae in P. patens had a similar ultrastructure to those of higher plants, and may be involved in the defence response to B. cinerea infection. Thus papillae formation is probably an evolutionarily conserved defence mechanism from the early land colonization by plants. Cell death during the P. patens–B. cinerea interaction had some features of programmed cell death, with hydrogen peroxide produced in cytoplasm membranes, suggesting something like a hypersensitive response. These ultrastructural studies suggest that P. patens could be a useful system for studying phytopathogens and could provide theoretical bases for defence mechanisms in evolutionary development.