Paired arbuscules in the Arum-type arbuscular mycorrhizal symbiosis with Linum usitatissimum

Experiments were conducted to investigate the "paired" arbuscules characteristic of Arum-type mycorrhizal colonization in Linum usitatissimum L. The development and senescence of arbuscular structures were followed in a time course study. Roots were freeze-sectioned longitudinally and mycorrhizal structures visualized using nitroblue tetrazolium, a vital stain to indicate metabolically active arbuscules and intercellular hyphae, followed by acid fuchsin counterstaining. Arbuscules were imaged using laser scanning confocal microscopy. The volume and surface area of each arbuscule of a developing paired structure were measured using three-dimensional imaging software. Arbuscules occurred in pairs in adjacent cortical cells arising from a single, radial intercellular hypha. These "paired" arbuscules often appeared to be at different developmental stages. Logistic regression and measurement of surface area indicated that there was a delay in initiation of the second arbuscule.