Isolation of a DNA plasmid in the fungus Rhizoctonia solani.

A DNA plasmid, designated pRS64, was detected in three isolates of anastomosis group 4 (AG-4) of Rhizoctonia solani Kühn by biophysical methods. The plasmid was a linear double-stranded DNA with a molecular weight of 1.68 +/- 0.06 X 10(6) or 2617 +/- 87 bp. Weakly pathogenic isolates of R. solani, 1668 RI-1, 1271 RI-64 and 1272 RI-1, which showed abnormally slow growth, contained the plasmids, but pathogenic isolates, 1668, 1271 and 1272, showing normal growth, contained no detectable plasmid DNA.

Stimulation of the extension of validamycin-inhibited hyphae by the hyphal extension factor present in Rhizoctonia solani.

Validamycin inhibited the hyphal extension of Rhizoctonia solani without growth inhibition. Hyphal inhibition was more pronounced in the main hyphae than in the primary or secondary branches of R. solani. Validamycin inhibition was antagonized by the hyphal extract of R. solani which stimulated hyphal extension. From the hyphal extract, the hyphal extension factor antagonizing the action of validamycin was isolated as a syrup after passing through an Amberlite IR-120 B column, adsorption on an Amberlite IRA-410 column, elution with 0.2 N NH4Cl and concentration of the eluate. The hyphal extension factor stimulated only the extension of the validamycin-inhibited hyphae and not that of the normal hyphae.