Aster Yellows Group 16SrI Research Articles

First Report of a 16SrI-C Group Phytoplasma Associated With a Yellows-Type Disease Affecting Willow Plants in China

In May of 2008, a phytoplasma-like disease was observed on willows (Salix babylonica Linn) grown in the Shaanxi Province. Affected plants showed yellowed leaves with green veins and dieback. Incidence of the disease was less than 10%. Samples were collected from 10 symptomatic and five asymptomatic willow plants from five different areas in Shaanxi Province. Total DNA was extracted from 0.5 g of leaf midrib and stem phloem tissue with a modified cetyltrimethylammoniumbromide (CTAB) method (3). Resulting DNA extracts were analyzed by a nested PCR assay using phytoplasma 16S rRNA gene primer pairs R16mF2/R16mR1 followed by R16F2n/R16R2 (1), which amplified a 1,452- and a 1,246-bp product, respectively. Sequences of amplicons were almost the same. Restriction fragment length polymorphism (RFLP) analysis of the nested 1.2-kb 16S rDNA products with AluI, MseI, HhaI, HpaI, RsaI, HinfI, and TaqI endonucleases (2) indicated that all symptomatic plants were infected by a phytoplasma belonging to aster yellows group (16SrI) subgroup C (16SrI-C) 'Candidatus Phytoplasma asteris'. None of the symptomless plants tested positive. Nucleotide sequence analysis of cloned 16S rDNA (GenBank Accession No. FJ179166) confirmed the results on the basis of RFLP analyses. Subsequently, the presence of the phytoplasmas in symptomatic plants was also confirmed by transmission electron microscopy. To our knowledge, this is the first molecular evidence of the presence of a phytoplasma associated with a yellows-type disease of willows in northern China and its association with aster yellow group 16SrI, subgroup 16SrI-C.

First Report of "Candidatus Phytoplasma asteris"-Related Strains in Brassica rapa in Saskatchewan, Canada.

"Candidatus phytoplasma asteris" and related strains (i.e., aster yellows group 16SrI) have been associated with diseases of numerous plant species worldwide. Symptoms of aster yellows (AY) have been reported on rapeseed/canola (Brassica napus and B. rapa) crops in Saskatchewan (SK) and Manitoba, Canada since 1953 (2). Symptoms generally include stunting, virescence, leaf yellowing or purpling, phyllody, and formation of bladder-like siliques. A total of 120 mature B. rapa cv. AC Sunbeam plants exhibiting AY symptoms were collected in commercial fields near Medstead, SK during 2003 and 2004 (one field per year). As described previously (4), total genomic DNA was extracted from leaf, stem, roots, and seeds collected from the 120 plants, from seeds from the seed lots sown in 2003 and 2004, and from leaf and stem tissue of 20 greenhouse-grown plants from each seed lot. The latter DNA samples were assayed for phytoplasma DNA by a nested polymerase chain reaction (PCR) assay incorporating phytoplasma universal 16S rRNA primer pairs P1/P6 (1) followed by R16R2/R16F2 (4). Seed samples analyzed from the 2003 and 2004 seed lots and tissues of the 40 greenhouse-grown plants all tested negative for phytoplasma DNA using this assay. Leaf, stem, and/or root tissues of all plants collected in the field in 2003 (60 plants) and 2004 (60 plants) and 71.1% (315 of 443) of seed samples (five seeds per sample) tested positive for the presence of phytoplasma DNA, as evidenced by the presence of an expected band of 1.2 kb on the gels after the second amplification with primers R16R2/R16F2. Nested PCR products from plant samples collected in 2003 were cloned, sequenced, and compared with phytoplasma sequences archived in the GenBank nucleotide database. On this basis, phytoplasmas detected in plants or their seeds collected in 2003 were found to be most similar (98.8%) to CHRY (Accession No. AY180956), a 16SrI-A subgroup strain, or were most similar (98.9%) to isolate 99UW89 (Accession no. AF268407), a known 16SrI-B subgroup strain. Sequences of phytoplasmas detected in plants or their seeds in 2004 were obtained by direct sequencing of rRNA products amplified from samples using PCR incorporating primer pairs P1/P6 and P4/P7 (3). Analysis of sequence data revealed that phytoplasmas in these plants were all most similar (99.5%) to AY-WB (Accession no. AY389828), a 16SrI-A subgroup member. The nucleotide sequences have been deposited with GenBank under Accession nos. DQ404346, DQ404347, and DQ411470. To our knowledge, this is the first report of 16SrI-A and 16SrI-B subgroup phytoplasmas infecting plants and seed of B. rapa in Saskatchewan.