Effects of Plasmids

The R. solani isolate RI-64 carrying plasmids grew slowly, was weakly pathogenic, produced oxalic acid, and was unable to form sclerotia. It is unlikely, however, that the 2.7-kb plasmids carry four genes that control these four phenotypes independently. To obtain isolates that have lost plasmids, or isolates altered in some characteristics associated with the plasmids, protoplasts of isolate RI-64 were regenerated and characterized. Of 205 isolates from the protoplasts, five showed better growth and formed sclerotia, but carried plasmids (Shimma et al. 1988). When a larger number of isolates was examined, there was no correlation between plasmid and hypoviru-lence (Miyasaka et al. 1990). Jabaji-Hare et al. (1994) compared the degree of pathogenicity of plasmid-carrying and plasmid-free isolates of R. solani and demonstrated that the plasmids do not have a role in pathogenicity.

The formae speciales of F. oxysporum that harbor the pFOXC plasmids are classified by their ability to cause disease in specific crucifers. Yet, the isolation of plasmid-free strains and transmission of pFOXC1 and pFOXC2 into different F. oxysporum backgrounds via protoplast fusion experiments demonstrated that the plasmids are not determinants of the host range or pathogenicity (Momol and Kistler 1992).

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