Xylem Parenchyma Plasmolysis and Vessel Wall Disorientation Caused byErwinia amylovora

Publisher Website
Google Scholar
Abstract
Ultrastructural changes induced in ''Jonathan'' apple leaf petioles by inoculation with 5 .times. 104 cells of E. amylovora were monitored after 24 and 48 h. Although no external symptoms were visible within 48 h, ultrastructural alterations were detected after 24 h. These included xylem parenchyma plasmolysis, deposition of a vessel-occluding ground substance, and the appearance of brushlike fibrils on the inner surfaces and spiral secondary thickenings of the xylem cell walls. After 48 h, the vessel-occluding substance appeared in more vessels and became more dense and xylem parenchyma protoplast material aggregated and shrank still further. Then the brushlike fibrils appeared to become more granular and seemed to aggregate with bacterial extracellular polysaccharide, possibly accounting for the denser deposits of vessel-occluding material. The electron micrographs suggest that the often-reported lysigenous cavities develop due to the collapse of plasmolyzed xylem parenchyma cells. Release of bacteria into this newly formed intercellular space followed the rupture of xylem vessels that had lost the physical support formerly provided by the xylem parenchyma cells. The xylem vessels also showed what seemed to be wall dissolution. These ultrastructural observations revealed xylem parenchyma plasmolysis to be an early symptom of pathogenesis and substantiated previous reports that xylem vessels are sites for early proliferation of E. amylovora.