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Introduction of Cotton leaf curl Gezira virus into the United Arab Emirates

Severe leaf curl and small vein thickening symptoms were observed in okra fields in Al-Ain, United Arab Emirates (UAE) during the winter season, 2013. These symptoms were reminiscent of those often associated with begomovirus infection. Based on the symptoms observed in okra plants growing in adjacent fields (20 × 20 m) on two small holding farms, the disease incidence ranged from 90 to 100%. The fields were infested with the whitefly Bemisia tabaci (Genn.), the insect vector of begomoviruses. Total DNA was extracted from four symptomatic okra leaves collected from two plants per field and used for PCR amplification of the core region of the begomovirus coat protein gene using the degenerate primers AVcore 3′-GCCHATRTAYAGRAAGCCMAGRAT-5′ and ACcore 3′-GGRTTDGARGCATGHGTACANGCC-5′. Amplicons of the expected size (~579 bp) were cloned and sequenced. BLASTn analysis of the partial coat protein sequences against the NCBI database revealed that the closet match to the four okra isolates was the Cotton leaf curl Gezira virus (CLCuGeV). CLCuGeV is a widespread Old World monopartite begomovirus described from the Nile Basin, sub-Saharan Africa, and southwestern Arabia (1). Recently, this virus has been reported in Jordan (GU945265) and Pakistan (3). To obtain the full-length viral genomic DNA for cloning and sequencing, total DNA extracts were enriched for circular dsDNA by rolling circle amplification (RCA) using Illustra TempliPhi (GE Healthcare, Life Sciences, Piscataway, NJ). The RCA products from each sample were digested with PstI, resulting in ~2.7 and 1.3 kbp molecules, respectively, and cloned into the pGEM plasmid vector (Promega, Madison, WI), linearized with PstI. Two inserts were selected from each cloning event and subjected to DNA sequencing using primer walking. The four resultant sequences of the 2.7 kbp (identified as virus genome) and the 1.3 kbp (betasatellite) inserts shared 99 to 100% nucleotide (nt) identity with each other, respectively. Therefore, only two representative genome and betasatellite sequences of 2,772 bp (KJ939446) and 1,356 bp (KM279620) were deposited in GenBank. Analysis using the Species Demarcation Tool (SDT) (v.1.0) (2) showed that the CLCuGeV UAE isolate sequence shared its highest nt identity (96 to 97%) with isolates from Egypt (AF155064), Pakistan (FR751142), and Jordan (GU945265). In contrast, it was only 93% identical to an isolate of CLCuGeV (HG530540) from the west coast of Saudi Arabia, nonetheless indicating they are all isolates of the same species. Analysis of the CLCuGeV sequence indicated that it was like other monopartite begomoviral genomes, containing a predicted hairpin, REP-binding iterons (GGTACTCA), and a TATA-box in the intergenic region. The genome contained six open reading frames encoding proteins with high homology to other CLCuGeV isolates. The 1,356-bp betasatellite shared its highest nt identity, at 97%, with the Okra leaf curl Oman betasatellite (KF267444) reported from infected okra plants in a neighboring country, Oman. The recent practice of transporting plants between the Gulf countries represents an important means and route for introducing begomoviruses among neighboring countries, compared to the long-distance aerial dispersal of viruliferous whiteflies, which is less likely because the Arabian desert poses a major barrier to long-distance whitefly flights.

References: (1) A. Idris et al. Viruses 6:1219, 2014. (2) B. M. Muhire et al. Arch. Virol. 158:1411, 2013. (3) M. N. Tahir et al. PLoS ONE 6:E20366, 2011.