Intracellular junctional structures in germinating ascopores ofVenturia inaequalis

Summary

An ultrastructural study of the fungal plant pathogen,Venturia inaequalis (Cke)Wint., has revealed the presence of junctions that form within germinated ascospores as they penetrate the cuticle of apple leaves. This is the first report of junctional structures in fungi or in any organism with a cell wall. Morphologically they resemble septate junctions of invertebrate tissues, but developmentally they differ in origin. Junctions inV. inaequalis form intracellularly during formation of the infection sac by an invagination and subsequent folding back of the plasma membrane around the penetration pore. The apposed plasma membranes form junctions and result in a junctional belt around the penetration pore. The plasma membrane and the junctional structures stain positively with phosphotungstic acid and the confluent infection sac membrane remains unstained. As the infection sac grows, additional junctions form between the infection sac membrane and plasma membrane. The intermembrane space of these junctions measures 17 nm, and the septa occur at regular 2.5 nm intervals. The junctions appear to provide adhesive support and structural rigidity for the developing infection sac and may also facilitate differentiation between the infection sac membrane and the plasma membrane. Similarities and differences between these junctions and invertebrate septate junctions are discussed.