Ultrastructural studies of the mode of penetration by Phoma macdonaldii insunflower seedlings

An ultrastructural investigation of the artificial inoculation of sunflower with Phoma macdonaldii conidia was undertaken using light, scanning, and t ransmission electron microscopy to elucidate the host-parasite relationship . The behavior of the conidia deposited on the cotyledon petiole was invest igated at various time intervals after inoculation. Conidia adhesion and ge rmination were observed first. The cotyledon petiole was invaded by the fun gus directly through the cuticle and via stomata. Externally, the spore and germ tube were covered with a mucilaginous polysaccharide sheath of a cott on-like appearance and of variable thickness. At the time of penetration, t he host cuticle was perforated mechanically. The cuticle was slightly depre ssed and no enzymatic alteration could be observed. The fungus did not form appressoria on the surface of the host tissues but developed an infection peg. As soon as the cuticle barrier was crossed, the fungus rapidly coloniz ed the host parietal layer. in a first step, the plasmalemma of the host ce ll appeared to be stuck against the cell wall. As soon as the fungus passed through the epidermal cell wall to reach the host cytoplasm, the plasmalem ma was disrupted, and the subsequent rapid breakdown of cell integrity favo red the colonization of the tissues by the pathogen.