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First Report of Oat as Host of a Phytoplasma Belonging to Group 16SrI, Subgroup A

Diseased plants of oat (Avena sativa L.) exhibiting abnormal proliferation of spikelets were observed in the field in Raseniai, Lithuania. The possible association of a phytoplasma with the disease, termed oat proliferation (OatP), was determined using polymerase chain reaction (PCR) for amplification of phytoplasmal ribosomal (r) RNA gene (rDNA) sequences from template DNA extracted from the diseased oats. DNA extractions and nested PCRs were conducted as previously described (2). In the nested PCRs, the first reaction was primed by phytoplasma-universal primer pair P1/P7, and the second (nested) PCR was primed by primer pair R16F2n/R16R2 (F2n/R2). Phytoplasmal rDNA was amplified in the nested PCR, indicating that the plants contained a phytoplasma, designated oat proliferation (OatP) phytoplasma. The OatP phytoplasma was identified and classified according to the system of Lee et al. (2) through restriction fragment length polymorphism (RFLP) analysis of 16S rDNA amplified in the PCR primed by F2n/R2. On the basis of collective RFLP patterns of the 16S rDNA, the OatP phytoplasma was classified as a member of group 16SrI (group I, aster yellows phytoplasma group). The RFLP patterns of the 16S rDNA were indistinguishable from those of 16S rDNA from tomato big bud (BB) phytoplasma and other phytoplasmas classified in group I, subgroup A (subgroup I-A, tomato big bud phytoplasma subgroup). The 1.8-kbp rDNA product of PCR primed by primer pair P1/P7 was cloned, and its nucleotide sequence was determined. The sequence was deposited in GenBank under Accession No. AF453416. Results from putative restriction site analysis of the cloned and sequenced rDNA were in excellent agreement with the results from enzymatic RFLP analysis of uncloned rDNA from OatP-diseased oat plants. Sequence similarity between the 1.8-kbp rDNA of OatP phytoplasma and that of BB phytoplasma (GenBank No. AF222064) was 99.2%; 9 of the 14 base changes were in the 16S-23S rRNA intergenic spacer region. The base differences in rDNA may signal that the OatP and BB phytoplasmas are mutually distinct in their biologies. Phytoplasmas classified in subgroup I-A have previously been reported in a broad range of plant species in North America and Europe, although there are no previous definitive reports of oat as a host of a subgroup I-A phytoplasma (3,4). In 1977, Fedotina (1) reported electron microscopy of a mycoplasma-like organism (phytoplasma) in pseudorosette-diseased oat plants in Siberia, but the identity of that phytoplasma remains unknown. Subgroup I-A phytoplasma strains are geographically widespread and have been found in numerous plant species (3,4). The discovery reported here, of a subgroup I-A phytoplasma in diseased oats in Lithuania, provokes questions concerning possible impacts of this phytoplasma on oat cultivation in central Europe and other regions.

References: (1) V. L. Fedotina. Arch. Phytopathol. Pflanzenschutz 13:177, 1977. (2) I.-M. Lee et al. Int. J. Syst. Bacteriol. 48:1153, 1998. (3) C. Marcone et al. Int. J. Syst. Evol. Microbiol. 50:1703, 2000. (4) D. Valiunas et al. Plant Dis. 85:804, 2001.