Ultrastructural responses of vessel contact cells in cotton plants resistant or susceptible to infection by Fusarium oxysporum f. sp. vasinfectum

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When vascular elements of cotton plants became infected with Fusarium oxysporum f. sp. vasinfectum , contact parenchyma cells surrounding the initially infected vessels responded with changes in their ultrastructural characteristics that could be categorized into three groups. In one group, contact cells were invaded by the fungus and rapidly disintegrated within 24 h after inoculation. In the second group, the cytoplasm in contact cells degenerated within 48 h after inoculation but without apparent hyphal invasion. The evidence suggests that this cytoplasmic degeneration may represent a type of defence response. In the third group, which comprised the majority of contact cells, the cytoplasm remained uninvaded and appeared healthy after inoculation; however, it showed ultrastructural evidence of cytoplasmic reorganization and increased metabolic activity that resulted in the deposition of a complex wall apposition, as well as the development of osmiophilic droplets in the cytoplasm and their apparent secretion through the plasmalemma into the apposition layers and vessel lumens. Lateral colonization by the fungus into the vascular parenchyma cells in both cvs Seabrook Sea Island SBSI and Rowden seemed to be prevented by these cellular responses. The responses, however, were produced faster and were more pronounced in the contact cells of the resistant cultivar than in the susceptible cultivar. Thus is appears that quantitative differences in these reactions may contribute to the degree of resistance to Fusarium .