Molecular Virology Laboratory, National Botanical Research institute, Lucknow-226001.
Cucumber mosaic virus (CMV), the type species of genus Cucumovirus in the family Bromoviridae, has polyhedral virions which encapsidate three linear, plus sense, single stranded genomic RNAs. CMV encodes five proteins which are distributed on three genomic RNAs. CMV is economically important due to its large host range among plant viruses infecting more than 1000 species. The 3b or coat protein (CP) of CMV is encoded by RNA3 but is expressed from the subgenomic RNA4. The CP is required for host range, encapsidation, systemic virus movement and aphid transmission. Based on phylogenetic analysis of the CP ORF and rearrangements in the 5’ nontranslated region (NTR) of RNA 3, CMV strains were divided into three subgroups: IA, IB, and II. In our study, molecular characterization of an Amaranth isolate of CMV was attempted based on RNA3. The complete CP and MP ORFs were cloned and sequenced. The 657 bp region of CP gene and the 840 bp region of MP gene encode 218 and 276 amino acids, respectively. The CP nucleotide sequence showed 90–98% sequence identity with the CMV subgroup I, and less than 80% with the CMV subgroup II; and at amino acid level 92–96% identity with the subgroup I and 74–87% with the subgroup II. The nucleotide and amino acid sequence identities of MP ranged in 91–94% and 92– 96%, respectively with the subgroup I, but in 81–83% with the subgroup II. Phylogenetic trees generated from nucleotide and amino acid sequences of both CP and MP genes identified the virus strain as a member of the subgroup IB. Molecular similarity among CMV isolates from Amaranthus tricolor, Datura innoxia and Hyoscyamus muticus was also investigated. The sequence analysis and phylogenetic trees generated by nucleotide and amino acid sequence alignments placed Indian isolates into subgroup IB. They also showed a high molecular similarity among themselves and appeared as a distinct cluster within subgroup IB, indicating their common origin in relation to other members of the subgroup. Most of the Asian isolates of CMV were found to belong to subgroup IB. For management of CMV ELISA, Western immuno assay, RT-PCR and nucleic acid probe based diagnostic tools have been developed for sensitive and reliable diagnosis of CMV in Amaranths, Banana, Tomato, Tobacco, Gladiolus and Chrysanthemum plant species. RT-PCR was found to be more reliable and sensitive as compared to ELISA. The CP genes of CMV have been utilized for development of transgenic lines of tobacco and tomato. The transgenic lines expressing high copy number of the CP have been found resistant to these viruses (TLCV and CMV) under glass house conditions. Further, the findings on theses approaches will be discussed.
