ULTRASTRUCTURE OF THE VASCULAR-RESPONSES OF COTTON TO VERTICILLIUM-DAHLIAE

Ultrastructural examination of cotton plants infected with Verticillium dahliae showed responses in the vascular tissue similar to those reported earlier in cotton infected with Fusarium oxysporum f.sp. vasinfectum. Contact cells reacted vigorously to infection, but the nature of the response depended on the location of the cells. The majority of the contact cells adjacent to initially infected vessels showed a reorganization of the cytoplasm with the formation of an apposition layer, a partitioning of the central vacuole into smaller vacuoles, and the appearance of osmiophilic material that was secreted through the plasmalemma and deposited in the apposition layer and in the pit regions of the vessels. The remainder of the contact cells adjacent to initially infected vessels contained a degenerated cytoplasm but no fungal hyphae. Contact cells adjacent to secondarily infected vessels responded with increased cytoplasmic activity and the direct secretion of osmiophilic material into the vessel lumens where it accumulated to form aggregates of various sizes and shapes that occluded the vessels. The fungus in these vessels was coated and embedded in this secreted material.