Cross walls in arbuscular trunk hyphae form after loss of metabolic activity.

•  A time-course of Allium porrum colonized by Glomus coronatum examined the formation of cross walls in arbuscular trunk hyphae in relation to the development and senescence of arbuscules. •  Mycorrhizal structures visualized using nitroblue tetrazolium as a vital stain indicated metabolically active arbuscules and intercellular hyphae with counterstaining by acid fuchsin. The intrahyphal location and intact nature of cross walls was examined using confocal microscopy. Their presence was correlated with the metabolic status of the mycorrhizal unit (i.e. arbuscule branches, arbuscular trunk hypha and intercellular hypha) using an interdependence magnified intersects technique (IMIT). •  Loss of metabolic activity was observed first from arbuscule branches, then from trunk hypha and lastly from intercellular hyphae. Cross walls were seen in trunk hyphae of many inactive arbuscules. The incidence of cross walls increased with time and was associated with loss of activity. •  Observations suggest cross walls appeared after loss of metabolic activity in arbuscule branches in several plant-fungal combinations. The implications of cross-wall formation for mechanisms of transfer of nutrients between fungus and plant are discussed.