Occurrence of an aster yellows (16SrI) group phytoplasma associated with a leaf roll disease of shinyleaf yellowhorn in China

Shinyleaf yellowhorn (Xanthoceras sorbifolia), family Sapindaceae, is a common ornamental and afforestation plant widely grown in northwest China because it is well adapted to cold, arid and barren environments. There have been no previous reports of phytoplasma infections in this species. During a survey in 2008, 9% of shinyleaf yellowhorn plants in the ornamental gardens of Northwest A&F University displayed symptoms resembling phytoplasma infection. Affected plants showed leaf roll, as well as little leaf symptoms, which greatly reduced the ornamental value of the plants. On the basis of symptoms and transmission electron microscopy observations, nested PCR assays were used to verify association of phytoplasma with the disease. Total nucleic acids were extracted from the leaf veins of eight symptomatic plants and six symptomless plants, using the modified CTAB method (Angelini et al., 2001). Using phytoplasma universal primer pair R16mF2/R16mR1 followed by R16F2n/ R16R2, PCR products of 1.4-kb and a 1.2-kb respectively, were amplified from symptomatic plants only. After sequencing of the 16S rDNA fragment from one sample (GenBank Accession No. FJ459913), a BLAST search determined that the shinyleaf yellowhorn leaf roll (SYLR) phytoplasma shared closest homology with 16SrI-D aster yellows group members (Lee et al., 1998). Furthermore, phylogenetic analysis of 16S rDNA sequences revealed SYLR phytoplasma to be most similar (99.68%) to paulownia witches' broom (PaWB, FJ263621). No differences were detected between the phytoplasmas from the eight symptomatic shinyleaf yellowhorn plants by restriction fragment length polymorphism (RFLP) analysis of nested rDNA products (1.2 kb) using endonucleases MseI, TaqI, RsaI, AluI, HpaII,and EcoRI. To our knowledge, this is the first report of an aster yellows (16SrI) group phytoplasma associated with leaf roll disease of shinyleaf yellowhorn in China.