Isolation and characterization of derivative lines of the onion yellows phytoplasma that do not cause stunting or phloem hyperplasia

Two lines of onion yellows phytoplasma. producing milder symptoms were isol ated from the original line (OY-W). One has an additional characteristic, n on-insect-transmissibility (OY-NIM), compared with the other (OY-M). OY-M w as established after maintaining OY-W for 11 years on a plant host (Chrysan themum coronarium) with an insect vector (Macrosteles striifrons), and OY-N IM was isolated after subsequent maintenance of OY-M in plants by periodic grafting. Polymerase chain analysis suggested that OY-NIM cannot traverse t he gut or survive in the hemolymph of the leafhopper. OY-W results in witch es'-broom formation and stunted growth in the host plant. In contrast, OY-M and OY-NIM do not cause stunting in the host plant, although they result i n witches'-broom. Histopathological analysis of these lines revealed that t he hyperplastic phloem tissue and severe phloem necrosis seen in OY-W did n ot exist in OY-M and OY-NIM. This was attributed to a reduction in the popu lation of phytoplasma in tissues in both OY-M- and OY-NIM-infected plants. The results suggest that the cause of stunting and phloem hyperplasia may b e genetically different from the cause of witches'-broom. Pulsed field gel electrophoresis analysis showed that OY-M had a smaller genome size (approx imate to 870 kbp) than OY-W (approximate to1,000 kbp). Thus, some of the OY -W genes responsible for pathogenicity may not be present in OY-M.