Department of Plant Molecular Biology, South Campus, New Delhi.
Abstracts of Research Papers Presented during the National Symposium of Indian Virological Society at Unit of Plant Virology, Division of Plant Pathology, Indian Agricultural Research Institute, New Delhi-110 012, October 14–1.
An economically devastating disease of rice in South and Southeast Asia known as rice tungro disease is caused by a complex of two viruses an RNA virus, Rice tungro spherical virus (RTSV) and a DNA virus Rice tungro bacilliform virus (RTBV). The complex of RTSV and RTBV are transmitted by a specific leafhopper vector. The pathogenic relationship between the two viruses is complex as RTSV confers leaf hopper transmissibility to RTBV which provokes major symptoms of the disease. RTSV from South Asian countries has been reported to contain a poly-adenylated RNA genome approximately 12 kb in length with one large open reading frame (ORF) at the 5’ end and two short ORFs at the 3’ terminal region. Even though several tungro epidemics have been reported from India not much information about the nucleotide sequence of Indian RTSV is available. To obtain nucleotide sequence information of RTSV from India, infected material was obtained from Orissa. Central Rice Research Institute, Cuttack, and provisionally named as OR. The virus was purified and viral RNA used as template for amplification of its genome by RT-PCR. Using RTSV-specific oligos, several cDNA fragments covering entire genome were obtained and cloned in cloning vectors like Topo TA, pBluescript, PGEM. Sequence information obtained from the overlapping cloned fragments was assembled to reveal the length of genome as 12.12 kb long with one large open reading frame starting from first AUG at position 516 and encoding a putative polyprotein of 386KD. The sequence comparison of OR isolate of RTSV with the Phillippines type strain A revealed that they isolates share a high homology at the amino acid level were 90% identical at the the nucleotide level and 97% similarity at the amino acid level within the coding region, comprising the coat proteins1,2,3, Q1-NTP-Q2 region, protease and replicase. The 5’- leader region showed 88% identity at amino acid level with region from 65–98 amino acid being completely different from RTSV A. The 3’ end has several small ORFs, Also the two sORFs reported in other RTSVs are missing with just a 5KD protein being made by the sequence showing similarity to sORFIII. As opposed to Indian isolates of RTBV, which shows high divergence from previously reported Southeast Asian isolates, RTSV showed high homology, indicating possibly that the two viruses which are causative agents for tungro in rice have dissimilar rates of evolution.
