Unit of Plant Virology, Division of Plant Pathology, Indian Agricultural research institute, New Delhi-110012, India.
Abstracts of the papers presented at the International Conference of Indian Virological Society on “Emerging and Re-emerging viral Diseases of the Tropics and Subtropics” at Indian Agricultural Research Institute, New Delhi, India, December 11–14, 2007.
Citrus tristeza virus(CTV), a member of genusClosterovirus is a brown citrus aphid (Toxoptera citricidus) borne, flexuous, filamentous plant virus having positive sense, single stranded RNA molecule of 19.3 kb long. Historically, CTV has been known as destructive pathogen in all citrus growing countries worldwide causing decline and death of 50 millions of citrus trees including more than one million trees in India till to date. Occurrence of a large number of biological and genetic variants of CTV have been reported worldwide Northeast Himalayan region (NeHR) is considered as one of the major mandarin growing areas of India and CTV causes serious yield losses in mandarin in this region. About 34 CTV isolates were collected, established in planta, biological and genetic diversities of these isolates were studied. Biological reaction of CTV isolates on different citrus species exhibits a wide range of diversity, and 4 different groups and one unassigned group were formed based on severity. The 672 nt full length coat protein (CP), and a 404 nt length fragment of 5’ ORFIa variable region (positioned from 5’ 1074-1478 3’ nt) gene of CTV genome were amplified using the specific primers, cloned and sequenced. Sequence analysis of CP gene showed that the present CTV isolates shared 91–98% identity among them and phylogenetic tree generated showed that they fell into four different clusters viz., Kpg1, Kpg2, Kpg3 and CTK27. Further, sequencing of 5’ ORFIa variable region gene showed present CTV isolates sharing 80–98% identity among them, and phylogenetic tree determined four different clusters viz., Kpg1, Kpg2, CTK7and CTK14. The diversity of CTV populations was also determined by deduced amino acid sequences and software based restriction map analysis. There was no definite correlation between the CTV groups obtained based on CP gene sequence, 5’ ORFIa variable region gene sequence and biological reactions. Many of the NeHR CTV isolates were found to be genetically related to exotic CTV isolates like T30 (Florida, mild), T385 (Spain, severe), SY568 (California, severe), PB61 (Australia), VT (Israel, severe). While data presented in this report from the limited number of CTV isolates from NeHR, it is clearly indicated that diversified CTV populations occurs in this region and this region is the origin of several variants of CTV.
