Group 16SrXII phytoplasma strains, including subgroup 16SrXII-E (‘Candidatus Phytoplasma fragariae’) and a new subgroup, 16SrXII-I, are associated with diseased potatoes (Solanum tuberosum) in the Yunnan and Inner Mongolia regions of China

Abstract

In China, potato is widely cultivated economic crop. Recently, potato diseases with characteristic symptoms of phytoplasma infections were found in potato fields. In 2006 and 2007, samples exhibiting symptoms including rosette and upright growth, upward rolling, yellowing and purpling of leaves, shortened and thickened internodes and formation of aerial tubers were collected from plants in Yunnan and Inner Mongolia and analyzed for the presence of phytoplasmas. DNA was extracted from tissues of 63 symptomatic and 10 asymptomatic plants. Phytoplasma 16S rRNA was amplified by PCR with primer pair P1/P7, followed by nested PCR with P1A/P7A, P1A/16S-SR or R16F2n/R16R2n. Twenty nine symptomatic plants (46 %), but no asymptomatic plants, tested positive for phytoplasmas. Nested PCR products were cloned and sequenced. Sequence analysis indicated that the phytoplasmas from diseased potatoes shared 98.1–99.8 % similarity with ‘Candidatus Phytoplasma fragariae’ (16SrXII-E) and other strains in 16SrXII subgroups. RFLP and phylogenetic analyses also indicated that phytoplasmas of group 16SrXII were associated with phytoplasma infected potatoes in China; these strains are most closely related to subgroup 16SrXII-E. Our results showed that five strains belonged to 16SrXII-E; 11 strains were designated as a new 16SrXII subgroup, 16SrXII-I; and subgroup affiliations of all others were not determined. The genetic diversity of the strains was corroborated by sequence analysis of ribosomal protein genes, the elongation factor Tu gene (tuf) and the pre-protein translocase membrane subunit gene (secY). The results illustrated the complexity and diversity of phytoplasmas associated with potatoes in China.