Nuclear division and morphogenesis during appressorium formation and plant penetration. (A) Appressorium morphogenesis. (a) Growth arrest and swelling of the hyphal tip (beginning of appressorium differentiation). Note that the nucleus is still in the hypha out of the incipient appressorium. (b-d) Time lapse of nuclear division within the appressorium: (b) the nucleus migrates from the hypha to the appressorium neck; (c) the nucleus divides within the appressorium neck; (d) one nucleus remains inside the appressorium and the other moves back into the hypha. (B) Effect of inhibitors on appressorium formation: (a) no treatment; (b) HU (fully developed appressorium develops without nuclear division); (c) benomyl. (C) Spores of strain H1-13 were inoculated onto onion epidermis. Pictures were taken after 24 h. (a) A hypha and an appressorium develop on the surface; a primary hypha is formed inside the plant under the appressorium. Note that the primary hypha is formed before nuclear division (arrow). (b, c) Nuclear division occurs in the appressorium after primary hypha formation. A single nucleus remains inside the appressorium: (b) projections of optical sections; (c) side view of the same sample. (d) A picture showing the spore from which infection originated, a hypha that developed on the leaf, an appressorium, and the underlying developing primary hyphae with several nuclei (arrow). Note that the spore and appressorium, which are on top of the onion epidermis, contain intact nuclei. Picture is a projection of optical sections. The scale bar is 5 μm.