Abstract
BackgroundEmerging whitefly transmitted begomoviruses are major pathogens of vegetable and fibre crops throughout the world, particularly in tropical and sub-tropical regions. Mutation, pseudorecombination and recombination are driving forces for the emergence and evolution of new crop-infecting begomoviruses. Leaf curl disease of field grown radish plants was noticed in Varanasi and Pataudi region of northern India. We have identified and characterized two distinct monopartite begomoviruses and associated beta satellite DNA causing leaf curl disease of radish (Raphanus sativus) in India.ResultsWe demonstrate that RaLCD is caused by a complex of two Old World begomoviruses and their associated betasatellites. Radish leaf curl virus-Varanasi is identified as a new recombinant species, Radish leaf curl virus (RaLCV) sharing maximum nucleotide identity of 87.7% with Tomato leaf curl Bangladesh virus-[Bangladesh:2] (Accession number AF188481) while the virus causing radish leaf curl disease-Pataudi is an isolate of Croton yellow vein mosaic virus-[India] (CYVMV-IN) (Accession number AJ507777) sharing 95.8% nucleotide identity. Further, RDP analysis revealed that the RaLCV has a hybrid genome, a putative recombinant between Euphorbia leaf curl virus and Papaya leaf curl virus. Cloned DNA of either RaLCV or CYVMV induced mild leaf curl symptoms in radish plants. However, when these clones (RaLCV or CYVMV) were individually co-inoculated with their associated cloned DNA betasatellite, symptom severity and viral DNA levels were increased in radish plants and induced typical RaLCD symptoms. To further extend these studies, we carried out an investigation of the interaction of these radish-infecting begomoviruses and their associated satellite, with two tomato infecting begomoviruses (Tomato leaf curl Gujarat virus and Tomato leaf curl New Delhi virus). Both of the tomato-infecting begomoviruses showed a contrasting and differential interaction with DNA satellites, not only in the capacity to interact with these molecules but also in the modulation of symptom phenotypes by the satellites.ConclusionThis is the first report and experimental demonstration of Koch's postulate for begomoviruses associated with radish leaf curl disease. Further observations also provide direct evidence of lateral movement of weed infecting begomovirus in the cultivated crops and the present study also suggests that the exchange of betasatellites with other begomoviruses would create a new disease complex posing a serious threat to crop production.
Highlights
Emerging whitefly transmitted begomoviruses are major pathogens of vegetable and fibre crops throughout the world, in tropical and sub-tropical regions
We further demonstrate the complexity of the interactions between RaLCD-associated satellites with radish and tomatoinfecting begomoviruses indicating co-adaptation of weed infecting begomovirus in cultivated crop species like radish, hitherto a non-host
Cloning and genome organization of two begomovirus species associated with RaLCD in India RaLCD symptoms developed on grafted Raphanus sativus plants were identical to those of naturally infected plants
Summary
Emerging whitefly transmitted begomoviruses are major pathogens of vegetable and fibre crops throughout the world, in tropical and sub-tropical regions. Begomoviruses are assumed to have been coevolving with their hosts for a long time, it is in the past two decades, these viruses have become an economically important plant pathogens [3,4,5]. These viruses contain genomes consisting of either one or two similar-size DNA components [6]. The DNA-B, code for two proteins which are involved in movement of the virus between and within plant cells [6] Both components of bipartite begomoviruses are required for systemic infection and symptom induction [9]. Betasatellite is a circular, singlestranded DNA molecule of ~1.35 kb length with a single open-reading frame (ORF) bC1, an adenine-rich region, a satellite conserved region having nonanulceotides (TAATATTAC) and it shares negligible sequence similarity with either DNA-A or DNA-B of bipartite begomoviruses [20,21,22]
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