Development of a vesicular-arbuscular mycorrhiza in bean roots

Abstract

The ontogeny of the mycorrhizal association of a Glomus species in bean roots was studied. Electron-dense particles adjacent to external hyphae bind iron, as shown by X-ray microprobe analysis, when roots are fixed with glutaraldehyde – ferric chloride. This material is thought to be phenolic in nature. External spores, each with a subtending hyphal stalk, ranged in colour from hyaline to black. Intercellular hyphae, which are initially densely cytoplasmic, become progressively more vacuolate, with some vacuoles containing electron-dense particles. These hyphae form peg-like projections which cause the cell wall to be stretched inwards. Penetration into the inner cortex is followed by the formation of a simple haustorial trunk which dichotomizes successively until fine hyphae fill the cell. The endophyte is at first enclosed by a wall layer but the finer branches are covered only with an extrahaustorial matrix and cortical cell plasma membrane. Cortical cell cytoplasm changes in response to the fungus. Most arbuscules show some evidence of collapse or degeneration in the fine branches. Degeneration within the endophyte follows a pattern; initially the cytoplasm loses its well-defined appearance and small vacuoles develop. These small vacuoles then fuse to form larger vacuoles until the cytoplasm is reduced to a thin dense band around the periphery. The hyphae then collapse and coagulate into successively larger clumps, until only a dense residual mass remains in the cell. A membrane encloses the developing hyphal clumps and the cortical cytoplasm degenerates as clumping advances. No cell wall matrix is seen around the clumped endophyte.