In January 2012, lettuce (Lactuca sativa L.) plants (19 out of 38) of one of the accessions (EC687345, variety NVRS-10:001818) exhibiting mild mosaic and stunted growth symptoms were observed at the Indian Agricultural Research Institute (IARI) experimental farm, New Delhi. Similar disease symptoms in lettuce plants in India were previously described (3) and the associated virus was characterized for host range, dilution end point, thermal inactivation point, and longevity in vitro. In this study, definitive molecular evidence is presented for the presence of Lettuce mosaic virus (LMV) infecting lettuce in India. Analysis of preparations from leaves of symptomatic samples with an electron microscope revealed flexuous virus particles measuring 750 × 13 nm, suggesting the association of a potyvirus (4). To identify the potyvirus infecting these lettuce plants, the 3′ terminal portion of the genome including the part of the nuclear inclusion b (NIb), complete coat protein (CP) region, and 3′ untranslated region (UTR) was amplified by RT-PCR, cloned, and sequenced. Total RNA was extracted from infected leaves using an RNeasy Plant Mini Kit (Qiagen, Valencia, CA) and subjected to RT-PCR using potyvirus specific forward (5′ ACCACAGGATCCGGBAAYAAYAGYGGDCARCC 3′) and reverse (5′ CACGGATCCCGGG(T17)V 3′) primers (2). PCR products (~1.8 kb) were cloned into pGEM-T Easy vector (Promega, Madison, WI) and sequenced (GenBank Accession No. JQ794776). Sequence comparisons revealed the CP of the virus infecting lettuce (834 bp) shared 96 to 100% nucleotide and deduced amino acid sequence identity with the corresponding regions of LMV isolates AJ306288 and AJ297630 from the United Kingdom, CAA46603 and NC003605 from the United States, AJ278854 and AJ278854 from Brazil, and AJ488153 from China, thus complying with the cut off range of 90 to 99% for identifying isolates/strains of the same virus (1). Similarly, 99 to 100% nucleotide sequence identity was observed with the corresponding region of the 3′UTR (245 bp) while 93 to 96% nucleotide identity of NIb region (654 bp) with LMV isolates. These results confirm that the virus infecting the symptomatic lettuce plants was an isolate of LMV. The amino acid sequences (DAG and WCIEN) conserved among majority of potyviruses were also present. Since the virus is aphid transmissible, its natural infection on other hosts and spread can't be ruled out.
References: (1) M. J. Adams et al. Arch Virol. 150:459, 2005. (2) A. Gibbs and A. Mackenzie. J. Virol. Methods 63:9, 1997. (3) T. K. Nariani and P. S. Pathanian. Indian Phytopathol. 13:172, 1960. (4) D. D. Shukla et al. The Potyviridae, page 338, 1994.