Initiation of appressorium formation in Uromyces appendiculatus: organization of the apex, and the responses involving microtubules and apical vesicles

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Abstract
Urediospore germlings of Uromyces appendiculatus sense topographical signals inherent in host stomata over which they develop appressoria. The site of signal recognition by the fungus is invariably within the apical 6 μm of the germling. The organization of the germling apex before and during initiation of appressorium formation was examined by both electron microscopy and laser scanning confocal microscopy. Most notable were changes in the distribution and organization of the microtubules and apical vesicles. Microtubules were oriented parallel to the inductive ridge, along which expansion of the germling apex occurred. In germlings that contacted artificial inductive topographies in vitro, the distribution of apical vesicles in germlings did not change within the first 3–4 min. After 6–8 min of contact, most apical vesicles became repositioned near the ridge. The apical cluster in developing appressoria was closely associated with that population of cytoplasmic microtubules that developed parallel to the inductive ridge. Coincident with growth of the germling apex over an inductive ridge was a physical deformation of the cell by the ridge, indenting both the cell wall and plasma membrane. Key words: bean rust, cytoskeleton, infection structure, Spitzenkörper, urediospore.

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