When some virus- and disease-free Musa spp. (banana and plantain) are propagated by tissue culture, the resulting plants develop infections with banana streak badnavirus (BSV), a pararetrovirus. In sharp contrast to the virion DNA recovered from natural infections, the virion DNA from tissue culture-associated infections of different Musa spp. was highly similar if not identical. Although BSV does not employ integration during the infection cycle, BSV DNA was found to be integrated into the Musa genome. While one integration consisted of a partial BSV genome, a second contained more than one complete genome that was almost identical to BSV recovered from tissue culture-derived plants. The arrangement of this integrated BSV DNA suggests that it can yield an infectious episomal genome via homologous recombination. This report documents the first instance of integrated DNA of a nonintegrating virus giving rise to an episomal viral infection and identifies tissue culture as a possible trigger for the infection, raising the question of whether similar activatable viral sequences exist in the genomes of other plants and animals.

A previously undescribed caulimo-like virus was identified in the hybrid tobacco species Nicotiana edwardsonii, and was named tobacco vein clearing virus (TVCV) after the symptoms associated with its occurrence in this plant. The virions of TVCV are 50 nm in diameter and are composed of a 45 kDa capsid protein and a 7767 bp dsDNA genome. Each strand of the genome is interrupted by a site-specific discontinuity. In genome sequence and arrangement of ORFs TVCV was most similar to cassava vein mosaic virus, indicating that TVCV is a pararetrovirus. No serological relationship was detected between TVCV and any other caulimoviruses, including petunia vein clearing virus, which has similar biological properties. In N. edwardsonii TVCV was seed-transmitted to 100 % of progeny plants, but was not transmitted by mechanical inoculation, grafting or Myzus persicae to any of seven other Nicotiana spp. Genomic DNA of TVCV hybridized to genomic DNA of N. edwardsonii and of N. glutinosa, its male parent, but not to genomic DNA of N. clevelandii, the female parent. TVCV has 78 % sequence identity with pararetrovirus-like sequences that are present in high copy number in the N. tabacum genome, and TVCV genomic DNA hybridized to genomic DNA of N. tabacum and N. rustica. These observations suggest that the episomal form of TVCV may arise from integrated pararetroviral elements present in N. edwardsonii, that these integrants were inherited from the male parent N. glutinosa, and that these elements are related but not identical to pararetroviral elements occurring in other Nicotiana spp.

The effect of temperature on symptom expression and detection of banana streak badnavirus (BSV) by immunosorbent electronmicroscopy (ISEM) and enzyme-linked immunosorbent assay of 12 in vitro-propagated plantain hybrids (genome AAB × AA), 3 ABB cooking banana, and 3 AAB plantain landraces was studied. Experiments were done for 2 years under two temperature regimes, 28 to 35°C in a screenhouse and 22°C in a temperature-controlled room. Most BSV-infected plants of plantain hybrids expressed symptoms under both conditions. Symptom expression was enhanced when plants were continuously grown at 22°C, but later became indiscernible when plants were continuously grown at 28 to 35°C. Plants grown at 22°C and showing severe symptoms contained significantly higher virus titer than plants grown at 28 to 35°C. When asymptomatic plants with very low virus titer at 28 to 35°C were transferred back to 22°C, there was a significant increase in both symptom severity and concentration of virus (greater than 3 to 5 times) in leaf tissues after 9 months. In contrast, the concentration of virus and symptom severity decreased in plants after transfer from 22°C to 28 to 35°C. Micropropagated plants of AAB plantain landrace cv. Mimi Abue and ABB cooking bananas (cvs. Bluggoe, Cardaba, and Pelipita) did not express visible symptoms under either temperature regime, but BSV was detected by ISEM in 23% of the plants. After 2 years at 22°C, virus was detected in 64% of the plants, but the concentration of virus remained low. Implications of these results on quarantine screening of in vitro plants and virus diagnosis are discussed.

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Micropropagated plants of 36Musa genotypes with diverse genetic backgrounds, including 14 tetraploid plantain (TMPx) and banana (TMBx) hybrids, were evaluated for their response to banana streak badnavirus (BSV) infection under three environments from 1995 to 1997 in Nigeria. The characteristics evaluated were the natural incidence of BSV based on symptoms and virus indexing, relative concentration of BSV antigens in leaf tissues determined by ELISA, and some growth and yield descriptors. Virus occurrence and symptom expression, as well as the relative concentration of BSV antigens, fluctuated greatly between seasons during the cropping cycle, being high during the rainy season and low or negligible during the hot dry season. The natural incidence of plants with symptoms and BSVinfected plants varied between genotypes. Incidence of BSV on most International Institute of Tropical Agriculture (IITA) TMPx hybrids and three Fundació n Hondureò a de Investigació n Agri`cola (FHIA) hybrids was high in the three environments, with some variation. Most landraces and some FHIA or Empresa Brasileira de Pesquisa Agropecuaria (EMBRAPA) hybrids were not BSV-infected under either environment at Onne. However, a few expressed some foliar symptoms at Ibadan and indexed BSV positive. The relative concentration of BSV antigens in leaf samples was also high in most TMPx and some FHIA hybrids, but low in most landraces. While BSV infection had no significant effect on most growth characteristics, it had a highly variable effect on bunch weight loss among the genotypes. There was no relationship between the natural incidence of BSV, concentration of viral antigen and bunch weight loss among the 11 TMPx hybrids, three FHIA hybrids and three plantain landraces. Despite the high natural BSV incidence and the high relative antigen concentration in their leaf tissue, TMPx 548-9, TMPx 2637-49, TMPx 7002-1 and FHIA 21 suffered less than 15% bunch weight loss, and TMPx 548-4 and FHIA 22 suffered no loss. These results suggest that under the conditions specified in this study, these hybrids could be tentatively classified as 'field tolerant' to BSV.

Summary Banana streak virus strain OL (BSV-OL) commonly infects new Musa hybrids, and this infection is thought to arise de novo from integrated virus sequences present in the nuclear genome of the plant. Integrated DNA (Musa6+8 sequence) containing the whole genome of the virus has previously been cloned from cv. Obino l'Ewai (Musa AAB group), a parent of many of the hybrids. Using a Southern blot hybridization assay, we have examined the distribution and structure of integrated BSV-OL sequences in a range of Musa cultivars. For cv. Obino l'Ewai, almost every restriction fragment hybridizing to BSV-OL was predicted from the Musa6+8 sequence, suggesting that this is the predominant type of BSV-OL integrant in the genome. Furthermore, since only two junction fragments of Musa/BSV sequence were detected, and the Musa6+8 sequence is believed to be integrated as multiple copies in a tandem array, then the internal Musa spacer sequences must be highly conserved. Similarly sized restriction fragments were detected in four BB group cultivars, but not in six AA or AAA group cultivars, suggesting that the BSV-OL sequences are linked to the B-genome of Musa. We also provide evidence that cv. Williams (Musa AAA group) contains a distinct badnavirus integrant that is closely related to the 'dead' virus integrant previously characterized from Calcutta 4 (Musa acuminata ssp. burmannicoides). Our results suggest that the virus integrant from cv. Williams is linked to the A-genome, and the complexity of the hybridization patterns suggest multiple sites of integration and/or variation in sequence and structure of the integrants.

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SummaryA Nigerian isolate of banana streak badnavirus (BSV) was purified and a polyclonal antiserum was produced in mice. The antiserum titre was between 1 : 10 000 and 1:40 000 in enzyme-linked immunosorbent assay (ELISA), and showed a good specificity to BSV antigens. Comparative tests were carried out to determine the sensitivity and reliability of BSV antigen detection by double antibody sandwich (DAS)-ELISA, triple antibody sandwich (TAS)-ELISA, antigen coated plate (ACP)-ELISA, and protein-A coated antibody sandwich (PAS)-ELISA. TAS-ELISA using rabbit polyclonal antiserum to trap BSV and mouse polyclonal antiserum to detect the virus particles, was more sensitive than ACP-ELISA and PAS-ELISA and detected BSV in plant extracts from both symptomatic and some asymptomatic plants. However, immunosorbent electron microscopy detected more BSV-infected plants from asymptomatic plant samples than did TAS-ELISA. Results of this study showed that detection of BSV antigens in sap extracts by TAS-ELISA was most efficient with symptomatic tissues which occurred most frequently in the 'cool rainy' season. This suggests that for more reliable BSVindexing of field samples, tissue sampling should be done during the rainy season when most BSV-infected plants express severe symptoms.