16S rDNA sequence analysis of witches' broom phytoplasma isolates from Chinese jujube in North China

Phytoplasmas are agriculturally important pathogens for more than one thousand plant species. Jujube witches' broom (JWB), a plant disease caused by phytoplasma, is highly fatal in Chinese jujube, an important fruit tree and medicinal crop in Asia. The 16S rDNA of 15 JWB isolates from 4 provinces in North China were amplified, cloned and sequenced. The isolates were over 99.8 % homologous, indicating that JWB phytoplasma is highly conserved. Phylogenetic analysis of the 16S rDNA sequences from those isolates, together with the representative phytoplasma sequences from other host species, showed that all JWB isolates belong to a subgroup of 16Sr V group and are closely related to Elm yellows (EY) phytoplasma. Virtual RFLP and single-nucleotide polymorphism (SNP) analysis of 16S rDNA also revealed that JWB isolates have a relatively low level of genetic diversity and that their SNP sites exhibit a strong bias toward A/G variation when compared with other phytoplasmas. The high similarity among JWB isolates from different regions and cultivars is consistent with the conserved symptoms of the disease and the wide infectivity of JWB phytoplasmas. This result provides valuable information to better understand the genetic diversity of JWB phytoplasmas.