Biological and Molecular Properties of Sweet Potato Leaf Curl Virus.

In 1994, a sweet potato sample showing leaf curl symptoms was collected from the field in Louisiana. When graft-inoculated, Ipomoea setosa reacted with mild upward leaf curling while I. aquatica reacted with a bright yellow mottle. The putative virus was transmitted by the sweet potato whitefly, Bemisia tabaci biotype B, and it was designated the United States isolate of Sweet potato leaf curl virus (SPLCV-US). Four independent Southern blot hybridizations using the DNA-A of different begomoviruses as probes confirmed the presence of begomovirus-like DNA in the infected plants. Fibrillar inclusions and granular aggregates of geminivirus-like particles were observed in the nuclei of infected I. cordatotriloba. These results suggested that SPLCV-US belongs to the genus Begomovirus. The complete nucleotide sequence of the SPLCV-US DNA-A genomic component was determined from three overlapping polymerase chain reaction (PCR) clones. The genome organization of the virus was similar to that of monopartite begomoviruses, with six open reading frames (ORF) and an intergenic region containing a stem-loop motif present in all geminiviruses. The iterative elements located within the intergenic region of SPLCV-US have sequences and arrangement similar to those of the Old World begomoviruses. The presence of the AV2 ORF supports the relationship between SPLCV-US and begomoviruses from the Old World. Phylogenetic analyses using the AV2 sequences revealed relationships between SPLCV-US and begomoviruses from the East Asia. The presence of the virus DNA in plant samples was detected using PCR with SPLCV-specific primers. From more than 150 sweet potato and Ipomoea spp. samples collected between 1997 and 2000, only thirteen samples were infected with SPLCV. The virus also was detected in the total DNAs extracted from five I. setosa grafted with sweet potato from four different countries. The amplified viral DNA fragments of these samples were cloned and sequenced. Based on the partial AC1 ORF sequences, the phylogenetic relationships among SPLCV isolates was determined. The results indicated that SPLCV isolates clustered into three groups, and all of them might have evolved from the same common ancestor possibly from the Old World