MOLECULAR CHARACTERIZATION AND TISSUESPECIFIC COPY NUMBER OF THREE PLASMIDS FROM WHEAT BLUE DWARF PHYTOPLASMA
W. Chen, Y. Li, Y.F. Wu
Three plasmids of wheat blue dwarf (WBD) phytoplasma, named pWBD1, pWBD2 and pWBD3, were cloned and confirmed by southern blot. They were 3449 bp, 3601 bp and 3844 bp in length, respectively, and encoded fifteen proteins in all, which included five secreted proteins and five membrane proteins, as well as the replication-associated protein (Rep), copy number control protein (Cop) and single-stranded DNA-binding protein (SSB). Phylogenetic analysis based on the sequences of Rep proteins from 16 phytoplasma type II extrachromosomal DNA molecules (EC-DNAs) and geminiviruses indicated that pWBD1 and pWBD2 were most closely related to pPaWBNy-1 from paulownia witches’ broom phytoplasma Nanyang strain, while pWBD3 was most closely related to pPaWBNy-2. Real time PCR analysis revealed that the copy numbers of WBD phytoplasma plasmids had tissue specificity in infected periwinkle and changed in the course of infection. The copy numbers of plasmids were higher in the tissues below the graft union, especially in the roots. pWBD1 copy number changed little during infection, whilst pWBD2 and pWBD3 copy number peaked three months after symptom appearance.