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Ultrastructural Studies of the Mode of Penetration by Phoma macdonaldii in Sunflower Seedlings

First, second, and sixth authors: Laboratoire de Biotechnologie et Amélioration des Plantes, Ecole Nationale Supérieure Agronomique de Toulouse, Avenue de l'Agrobiopole, Auzeville-Tolosane, BP 107, F-31326 Castanet-Tolosan Cedex, France; fourth and fifth authors: Equipe de Mycologie Végétale, UMR-CNRS 5546 Université Paul Sabatier, 24 chemin de Borde Rouge, BP 17, Auzeville-Tolosane, F-31326 Castanet-Tolosan Cedex, France; third author: Unité de Science du Sol, INRA, Centre de Recherche de Versailles, Route de Saint-Cyr, F-78026 Versailles Cedex, France

An ultrastructural investigation of the artificial inoculation of sunflower with Phoma macdonaldii conidia was undertaken using light, scanning, and transmission electron microscopy to elucidate the host-parasite relationship. The behavior of the conidia deposited on the cotyledon petiole was investigated at various time intervals after inoculation. Conidia adhesion and germination were observed first. The cotyledon petiole was invaded by the fungus directly through the cuticle and via stomata. Externally, the spore and germ tube were covered with a mucilaginous polysaccharide sheath of a cotton-like appearance and of variable thickness. At the time of penetration, the host cuticle was perforated mechanically. The cuticle was slightly depressed 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 colonized the host parietal layer. In a first step, the plasmalemma of the host cell 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 plasmalemma was disrupted, and the subsequent rapid breakdown of cell integrity favored the colonization of the tissues by the pathogen.

Additional keywords: black stem, Helianthus annuus, host-pathogen interactions, subcuticular hyphae.