Ultrastructural changes in susceptible and resistant plants of citrus following artificial inoculation with Xanthomonas citri (Hasse) Dowson.

Abstract

Electron microscopic observations of leaf tissues of susceptible Citrus natsudaidai injected with Xanthomonas citri suspension were made. The cell wall swelled followed by a separation between wall surface and cell wall, 24-48hr after inoculation incubated at 20C. Bacteria existing between cell wall and wall surface multiplied with a marked decomposition of outer layer of the cell wall and with a formation of fibrils. Plasmalemma separated from wall-inner surface and then degenerated 96hr after inoculation, followed by a marked multiplication of bacteria around these cells. The tonoplast and other cellular organelles disintegrated. Afterward, cell wall was broken and bacteria invaded and multiplied intracellularly. In the similar tissues of resistant C. junos, fibrillar materials coagulated in intercellular spaces and ensheathed bacteria 96hr after inoculation. Plasmalemma separated from the inner cell wall surface. The cytoplasm became electron-dense when the Plasmalemma degenerated. This seemed to be hypersensitive reaction. Around these cells no bacteria multiplied. In diseased tissues of 50-day-old lesions on the resistant host and moderately resistant C. hassaku, intercellular spaces were plugged with electron-dense materials which had immobilized shrunken, inactive bacteria. In contrast, the electron-lucent intercellular spaces in the susceptible diseased tissues contained intact bacteria. The facts suggest that fibrillar coagulation with bacteria encapsulation is the most important resistant reaction.