First report of Beet mosaic virus infecting lettuce, in China

Beet mosaic virus (BtMV) is a species of the biggest plant virus genus, Potyvirus. It is widespread and mainly infects plants belonging to the families Chenopodiaceae, Solanaceae and Fabaceae. In 1981, BtMV was found in spinach (Spinacia oleracea) in Beijing, China (Liu et al., 1981). So far, only a few isolates from USA, Slovakia, Germany, China and UK have been molecularly characterized, mostly from beet (Beta vulgaris) (Glasa et al., 2003; Xiang et al., 2007).

From 2004 to 2006, a series of field surveys were conducted in Tai'an, Shandong Province, China, to investigate the viruses infecting vegetables. Lettuce plants (Lactuca sativa) showing symptoms of mosaic, puckering and severe stunting were found in several fields, with an incidence of 30-40%. These samples showed positive reactions with antisera to Potato virus Y (PVY) and Turnip mosaic virus (TuMV), and negative reactions with antisera to Cucumber mosaic virus (CMV), Tobacco mosaic virus (TMV), and Potato virus X (PVX), in SDS-agar gel double immunodiffusion tests, indicating that a potyvirus was present. The 3'-terminal region of the virus was amplified by RT-PCR using primers specific to potyviruses (Chen et al., 2001). The resultant 1629 bp fragment (GenBank Acc. No. EF633501, isolate named BtMV-SD), included 630 bp encoding partial NIb, 831 bp encoding CP, and 168 bp for the 3′-UTR. BLAST results indicated that the virus is BtMV, which was further confirmed in western blotting with antiserum provided by Prof. C Han of China Agricultural University. Sequence alignment showed that the 3'-terminal genomic sequence of BtMV-SD shared 91.1%-98.8% identity at nucleotide level with other BtMV isolates available in the GenBank database, while the CP-encoding sequence shared 92.4%-99.0% identity at nucleotide level and 97.5%-100.0% identity at amino acid level with other BtMV isolates. As shown by Xiang et al. (2007) BtMV isolates can be divided into two groups, a Euroasia group and an America group. BtMV-SD clusters with the Euroasia group and forms a common branch with isolate REN-1 from Slovakia (AF363639). Lettuce is reported to be susceptible to more than 50 viruses. However, to our knowledge, this is the first report that BtMV can naturally infect lettuce.