Department of Plant Molecular Biology, University of Delhi South Campus, New Delhi-110021, India.
Rice tungro disease is the most important viral disease in rice, prevalent in South and Southeast Asia. Two viruses are associated with Rice tungro disease, Rice tungro bacilliform virus (RTBV), member of the Genus “RTBV-like” of the family Caulimoviridae and Rice tungro spherical virus (RTSV), member of the Genus Waikavirus, family Sequiviridae. We have cloned and sequence analyzed two complete 8 kb genomes of RTBV; one each from West Bengal and Andhra Pradesh and two partial genome fragments; one each from Punjab (4.2 kb) and Orissa (2.7 kb). Comparison of the above sequences with those from Southeast Asia indicates that, within India, the isolates are highly conserved. However, the Southeast Asian sequences form a distinct cluster from that of the Indian sequences. This difference is especially noticeable in the ORF IV, absent from all other Badnaviruses, and intergenic region, bearing control elements important in viral gene expression. RTSV has a single-stranded RNA genome of approximately 12 kb. Overlapping cloned cDNA fragments representing isolates from West Bengal and Orissa were obtained, which were cloned and sequenced. The analysis suggests a very high degree of conservation among all RTSV sequences across South and southeast Asia. Both the Indian RTSV sequences lack the two downstream ORFs reported from one isolate from Philippines. The noticeable difference in sequence conservation between the Philippine and Indian isolates of RTBV and RTSV are intriguing, both being propagated together in rice. The implications of the above studies for control of tungro disease will be discussed.
