Orychophragmus violaceus, a new host of Potato virus X, reported from China

Orychophragmus violaceus (Cruciferae) is a biennial herbaceous ornamental plant and has been widely cultivated in gardens or at the roadside in China. Recently we found that about 70% of surveyed O. violaceus plants in Beijing showed viral disease symptoms including mosaic, malformation and colour-breaking (Fig. 1). Turnip mosaic virus (TuMV) was previously isolated from diseased O. violaceus plants showing mosaic and distortion in eastern China (Chen & Li, 1999). To determine the possible causal agent of O. violaceus in Beijing, some samples (collected in April, 2007) were tested by ACP-ELISA using polyclonal antibodies against Cucumber mosaic virus, Potato virus X (PVX), Potato virus Y, Sugarcane mosaic virus, Tobacco mosaic virus, Tomato mosaic virus, TuMV and Zucchini yellow mosaic virus. The samples reacted positively only with the antibodies against PVX.

To confirm that the diseased O. violaceus plants were infected by PVX, total RNA of the infected plants were extracted and RT-PCR performed using potexviruses-specific degenerate primers Potex 1RC and Potex 5 (van der Vlugt & Berendsen, 2002). A PCR product of the expected size (ca.730bp) was visualised by agarose gel electrophoresis. In addition a pair of PVX-specific primers (5′-ATG TCA GCA CCA GCT AGC AC-3′ and 5′-TGG TGG TGG GAG AGT GAC AAC-3′) were designed to amplify a section of the coat protein (CP) gene. A PCR product of the expected size of 711 bp was cloned and sequenced (GenBank accession number EF624257). Sequence comparison with other sequences of the PVX CP gene deposited in GenBank showed the greatest homology with the PVX-Shandong isolate from China (AF528555), with a nucleotide sequence identity of 99%.

In addition, the sap of O. violaceus plants that reacted with the PVX antibodies were used for mechanical inoculation onto Chenopodium amaranticolor, and necrotic local lesions were observed on inoculated leaves 7 days after inoculation (Fig. 2). Subsequent ELISA testing of inoculated plants showing local lesions confirmed the presence of PVX. PVX is widespread on potato, tobacco and tomato in China (Du & Chen, 2003; Qu et al., 2003), thus O. violaceus could serve as the virus reservoir in winter when the crops are harvested. To our knowledge, this is the first report of natural occurrence of PVX on O. violaceus.