Histopathology of tomato plants infected with Pseudomonas solanacearum, with emphasis on ultrastructure

Abstract

Tomato plants grown in nutrient solution were inoculated just behind the root-cap with Pseudomonas solanacearum. Light- and electron-microscopy revealed that initially only small diameter cells adjacent to large vessels were invaded, the vessels remaining bacterium-free. Some of these cells were stimulated to form tyloses which bulged into the vessels. Bacteria migrated into the tyloses, many of which ruptured 48 to 72 h after inoculation, liberating the organisms and non-cellular materials into the vessels. At approximately this time the plants began to show the first signs of wilting. Within vessels, bacterial multiplication and spread was rapid and was accompanied by accumulation of large amounts of fine granular material identified as bacterial extracellular polysaccharide. Various other plugging materials also occurred in the vessels of diseased plants. Bacteria became so numerous that they were compressed into irregular shapes in the root vessels. A relatively thick layer of electron-dense material was deposited on the primary walls of many invaded root and stem vessels. The bacterial cells together with their extracellular materials, released into vessels upon tylosis collapse, subsequently increased in amount and were probably the major cause of wilting. Intact and collapsed tyloses, as well as the layer of electron-opaque material lining invaded vessel walls, probably also contributed to wilting. Host degradation products, arising from bacterial enzyme action on vessel walls, were apparently only minor contributing factors.