Structure and Genome Organization of a Novel Fiji Strain of Sweet Potato Vein Clearing Virus Identified by High-Throughput Sequencing

ABSTRACT The complete genome of a Sweet potato vein clearing virus (SPVCV) isolate infecting a quarantined sweet potato accession from Fiji was determined. Sequence comparisons revealed the highest nucleotide sequence identity of 94.6% with that of the SPVCV type species, an isolate from the Dominican Republic. The virus was mechanically transmitted to Nicotiana bigelovii plants.

Temporal and Spatial Appearance of Recombinant Viruses Formed Between Cauliflower Mosaic Virus (CaMV) and CaMV Sequences Present in Transgenic Nicotiana bigelovii

Cauliflower mosaic virus (CaMV) strain CM1841 is able to recombine with a CaMV transgene sequence present in Nicotiana bigelovii. In the present study we have characterized the temporal and spatial appearance of recombinant viruses formed between CM1841 and the transgene within individual transgenic plants. CM1841 infections were initiated by mechanical inoculation and by agro-inoculation to nontransformed N. bigelovii and transgenic N. bigelovii that expressed the gene VI product of CaMV strain D4. In agroinoculated transgenic plants, inoculated leaf tissue turned necrotic around the point of agroinocu-lation, while chlorotic lesions appeared in the leaves inoculated with CM1841 virions. The first systemic symptoms in both agroinoculated and mechanically inoculated transgenic N. bigelovii consisted of necrotic patches. The predominant type of virus recovered from the inoculated and first systemically infected leaves was the wild-type CM1841 rather than a recombinant. As the infection progressed in the transgenic plants, symptoms changed from necrosis in the lower leaves to a chlorotic mosaic in the upper leaves. This shift in symptom type was associated with the recovery of recombinant viruses, indicating that the recombinants predominated only in later stages of pathogenesis.

Region VI of cauliflower mosaic virus encodes a host range determinant.

A domain of cauliflower mosaic virus (CaMV) which controls systemic spread in two solanaceous hosts (Datura stramonium and Nicotiana bigelovii) was mapped to the first half of open reading frame 6. Whereas ordinary strains of CaMV are unable to infect solanaceous species except to replicate locally in inoculated leaves, a new CaMV strain (D4) induces chlorotic local lesions and systemically infects both D. stramonium and N. bigelovii. To determine which portion of the CaMV genome controls systemic spread of the virus in solanaceous hosts, nine recombinant genomes constructed between D4 and two ordinary strains of the virus were tested for their ability to infect solanaceous hosts. A 496-base-pair DNA segment comprising the first half of open reading frame 6 specified the type of local lesions and systemic spread of the virus in solanaceous hosts. Exchange of this segment of the genome between strains of CaMV converted a compatible host reaction to an incompatible (hypersensitive) one in response to infection. This suggests that the gene VI protein interacts with the plant to suppress hypersensitivity, the normal response of solanaceous hosts to CaMV infection.