Ultrastructure of thick-walled hyphae of Glomus fasciculatum in old roots of white clover (Trifolium repens)

Transmission electron microscopy reveals many thick-walled intercellular and intracellular arbuscular trunk hyphae in old roots of white clover. Wall thickness varies among hyphae, from those enclosed by only a primary wall to those in which various layers of secondary wall material occur. The total thickness of the wall usually lies between 1 and 2 μm. The secondary wall shows a lamellar organization and occasionally may almost completely obliterate the protoplast. In some walls, closely packed lamellae form dark-staining bands separated by lighter staining bands of wider spaced lamellae. Cytochemical staining indicates that the primary and secondary walls contain polysaccharide material. Several thick-walled hyphae show broken layers in the outermost regions overlying deeper layers of uninterrupted wall. The ruptured wall appears to have been caused by local expansion of the hypha during the phase of extensive secondary wall thickening. It is suggested that the thick-walled hyphae may provide propagative structures on disintegration of the host cortex. Another hypothesis is that these hyphal walls might provide a pathway for apoplastic transport. Solutions contained in mature tissues of the root might move via the walls for utilization by the endophyte in those regions of the root where the growing hyphae are more active metabolically.