Ultrastructural observations and DNA degradation analysis of pepper leavesundergoing a hypersensitive reaction to Xanthomonas campestris pv. vesicatoria

Ultrastructural details of the hypersensitive reaction induced by infiltrat ion with avirulent race 2 Xanthomonas campestris pv. vesicatoria in pepper 'Early Calwonder-10R' leaves (incompatible interaction) are reported. Affec ted cells displayed plasmalemma undulations and disruption, lysis of the ch loroplast membrane, degeneration of other organelles, general cytoplasm dis organisation and, often, protoplast shrinkage. The nuclei contained large m asses of electron-dense material, apparently formed by chromatin aggregatio n. In many cases a single chromatin-like layer was deposited on the inner s ide of the nuclear envelope leaving a finely granular matrix in the centre of the nucleus; the nucleolus usually disappeared. The nuclear envelope was sometimes ruptured and the internal matrix leaked into the cytoplasm. The content of many affected cells eventually coagulated and became very electr on-dense. The walls often collapsed. All these alterations were especially visible in spongy mesophyll cells at sites where bacteria occurred in the i ntercellular spaces. Although some of the nuclear and cytoplasmic alteratio ns recall certain aspects of apoptotic cell death, molecular determinations did not reveal any DNA degradation in hypersensitively reacting tissues. T he first cell alterations in leaves infected with the virulent bacterial ra ce 1 (compatible interaction) were observed only 27 h after inoculation, wh en the cytoplasm of some cells showed limited internal disorganisation and plasmolysis at sites where bacterial colonies developed.