Identification and characterization of plasmids from the western aster yellows mycoplasmalike organism.

Supercoiled double-stranded DNA molecules (plasmids) were isolated from plants infected with three laboratory strains of western aster yellows mycoplasma-like organism (AY-MLO) by using cesium chloride-ethidium bromide density gradients. Southern blot analysis, using plasmids from the severe strain of AY-MLO (SAY-MLO) as the probe, identified at least four plasmids in celery, aster, and periwinkle plants and in Macrosteles severini leafhopper vectors infected with either the dwarf AY-MLO, Tulelake AY-MLO, or SAY-MLO strain. Plasmids were also detected in two California field isolates of AY-MLO but not in plants infected with the beet leafhopper-transmitted virescence agent, western X, or elm yellows MLOs. SAY-MLO plasmids were 5.2, 4.9, 3.4, and 1.7 kilobase pairs in size. Plasmids isolated from dwarf AY- and Tulelake AY-MLOs were 7.4, 5.1, 3.5, and 1.7 kilobase pairs in size. No evidence was obtained for integration of SAY-MLO plasmids into the MLO chromosome.