Detection of Poinsettia mosaic virus by RT-PCR in Euphorbia spp. in New Zealand.

Euphorbia pulcherrima (poinsettias) are commonly infected with Poinsettia mosaic virus (PnMV), which resembles the Tymovirus genus in its morphology and viral properties (2) but is closer to the Marafivirus genus at the sequence level (1). Symptoms induced by PnMV range from leaf mottling and bract distortion to symptomless (2). The presence of PnMV in plants imported into New Zealand had never been proven. Leaves of 10 E. pulcherrima samples and six samples from other Euphorbia spp. (E. atropurpurea, E. lambii, E. leuconeura, E. mellifera, E. milii, and E. piscatorial) were collected in the Auckland area, North Island in 2002. Isometric particles of 26 to 30 nm in diameter were observed with electron microscopy in 3 of 10 E. pulcherrima samples. These three samples produced systemic chlorosis and crinkling symptoms on mechanically inoculated Nicotiana benthamiana, which tested PnMV positive by double-antibody sandwich (DAS)-ELISA (Agdia, Elkart, IN). No particles or symptoms on N. benthamiana were observed with the other Euphorbia spp., which were also PnMV-negative by DAS-ELISA. A reverse transcription-polymerase chain reaction (RT-PCR) was developed to further characterize PnMV. Specific primers were designed from the PnMV complete genome sequence (Genbank Accession No. AJ271595) using the Primer3 web-based software (4). Primer PnMV-F1 (5'-CCTGTATTGTCTCTTGCCGTCC-3') and primer PnMV-R1 (5'-AGAGGAAAGGAAAAGGTGGAGG-3') amplified a 764-bp product from nt 5291 of the 5'-end RNA polymerase gene to nt 6082 of the 3'-untranslated region (UTR). Total RNA was extracted from leaf samples using the Qiagen Plant RNeasy Kit (Qiagen Inc., Chastworth, CA). RT was carried out by using PnMV-R1 primer and MMLV reverse transcriptase (Promega, Madison, WI). The PCR was performed in a 20-μl volume reaction containing 2 μl cDNA, 1× Taq reaction buffer, 1.5 mM MgCl₂, 0.2 mM dNTPs, 0.2 μM PnMV-F1 primer, and 1 U of Taq polymerase (Promega) with a denaturation step (94°C for 5 min), 30 amplification cycles (94°C for 30 s; 55°C for 30 s; 72°C for 1 min), and a final elongation (72°C for 5 min). The sequence of the RT-PCR product (Genbank Accession No. DQ462438) had 98.7% amino acid identity to PnMV. PCR products were obtained from two of three PnMV ELISA-positive E. pulcherrima and three of three PnMV ELISA-positive symptomatic N. benthamiana. The failure to amplify the fragment from all ELISA-positive PnMV is likely because of the presence of inhibitors and latex in E. pulcherrima (3) that make the RNA extraction difficult. Thus, while RT-PCR may be useful for further characterizing PnMV isolate sequences, ELISA may be more reliable for virus detection. In conclusion, to our knowledge, this is the first report of PnMV in E. pulcherrima but not in other Euphorbia spp. in New Zealand. E. pulcherrima plants have been imported into New Zealand for nearly 40 years, and the virus is probably widespread throughout the country via retail nursery trading. References: (1) B. G. Bradel et al. Virology 271:289, 2000. (2) R. W. Fulton and J. L. Fulton. Phytopathology 70:321, 1980. (3) D.-E. Lesemann et al. Phytopathol. Z. 107:250, 1983. (4) S. Rozen and S. Skaletsky. Page 365 in: Bioinformatics Methods and Protocols: Methods in Molecular Biology. S. Krawetz and S. Misener, eds. Humana Press, Totowa, NJ, 2000.