Cellular differentiation in Venturia inaequalis ascospores during germination and penetration of apple leaves

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Abstract
The penetration of the apple leaf cuticle by ascospores of Venturia inaequalis was studied by using ultrastructural and cytochemical techniques. A time-course developmental sequence is presented. Attachment of ascospores to the leaf surface appears to be by a mucilaginous substance and is followed by germination and penetration pore formation. The penetration pore is a circular opening adjacent to the leaf. It appears to form by degradation of the fungal wall and is bordered by a thickening of fungal wall material. Above the penetration pore, an infection sac forms from invagination and extension of the fungal plasmalemma. This structure has been reported only in V. inaequalis. Infection sac development initiates when the fungal plasmalemma appears to be forced into a dome shape. The dome flattens out and the membrane folds back upon itself, circumscribing the margin of the pore. The folded membrane becomes apposed, resulting in a circular belt of junctional structures which morphologically resemble molluscan septate junctions. As the infection sac enlarges, additional junctions form wherever the infection sac membrane abuts with the fungal plasmalemma. Selective staining with phosphotungstic acid revealed that the infection sac membrane becomes differentiated from the fungal plasmalemma. The infection sac enlarges and accumulates a dense matrix that appears to penetrate into the cuticle, causing ultrastructural changes in host tissues. The infection hypha, which is an extension of the infection sac membrane, breaches the cuticle without any apparent mechanical pressure.