As shown in the experiments summarized in Table 2
, when using petunia symptomatic plants of the local cultivar grown in an insect-free greenhouse, both BCTV-Svr and BCTIV were transmitted via seed, whereas control plants of PCSP cultivar grown under the same conditions remained virus-free, despite susceptibility to the viruses.
In sugar beet, Bennet [1
] could not demonstrate BCTV seed transmission, despite the presence of the virus in sugar beet seeds. There are, however, some unconfirmed reports on seed transmissibility of certain geminiviruses, including BCTV [25
] and abutilon mosaic virus [26
]. More recently, however, it was shown that sweet potato leaf curl virus, tomato yellow leaf curl virus (TYLCV), and MBYMV, three distinct begomoviruses, can be transmitted via seeds in sweet potato, tomato, and black gram cultivars, respectively [16
]. There are conflicting reports about the seed transmission of TYLCV to other hosts. While it has been reported that TYLCV is seed-transmissible in soybean (Glycine max
], no evidence of seed transmissibility of TYLCV in the experimental host Nicotiana benthamiana
was found [28
]. In the present study, BCTV-Svr was detected in seeds of naturally infected petunia plants (local cultivar) and their progeny plants, by molecular and serological tests. Because very small size of petunia seeds was a limiting factor for using single seed-tests, such as tissue blot hybridization, presence of viral particle in petunia seeds was tested by IC-PCR. However, lower efficiency of IC-PCR (Figure 3
and Figure 4
f) compared to regular PCR (Figure 1
e) may suggest that only a small portion of the virus in the seed is in the form of whole particles. In Southern blots, there was no signal when 4 µg of DNA extracted from seeds was used, whereas both ssDNA and dsDNA forms were detected by this assay in seedlings of naturally infected petunia plants developed from such seeds (Figure 5
, lane 3). The results of Southern blot analysis revealed a high concentration of the virus DNA in the leaf tissues of young seedlings originated from infected seeds of local cultivar, and also in agroinfected petunia plants of the PCSP cultivar. Failure to detect viral DNA in Southern blots from seeds that were PCR-positive indicates a low concentration of virus in these organs. Southern blot assays, and a look at the results as a whole, indicate very low concentrations of viral genome in the seeds. Kim et al. [16
] showed that the SPLCV in young sweet potato seedlings developed from infected seeds was too low to be detectable by Southern blot assay, but in the later stages, replicating forms of SPLCV DNA were detected by the same method. This suggests some form of latency in virus cycle when transmitted via seed. Similar conclusions can be drawn from our results. However, given that the seeds of mung beans are large enough, confirmation of the presence of the MBYMV virion particles in a single mung bean seed was possible by performing DAS-ELISA and ISEM experiments [18
Seed infection apparently had a detrimental effect on the seed and plant development. In this study, it was observed that many flowers of infected plants failed to produce capsule and/or seeds, and some even died before flowering. Natural incidence of BCTV-Svr in seedlings developed from the seeds of the petunia local cultivar was higher (38.2–78%) than BCTIV (8.8–18.5%), suggesting a higher transmission efficiency for one virus compared to the other. However, in a vertical transmission experiment in three generations, very few seeds were obtained, indicating a strong detrimental effect of virus infection. Considering that several factors may affect transmission of plant viruses through seeds [29
], it is possible that the growing conditions used to produce the original batch of seed were quite different from those in the insect-free glasshouse employed in our experiments. Also, the development of seed may be affected by the time of plant infection.
Seed transmission of viruses may be, in part, responsible for their transmission over long distances, including intercontinental spread. It becomes even more important when considering the wide host range of BCTV and BCTIV, which includes several plant families. Since the first detailed molecular description of BCTV in Iran [7
], it was clear that this virus must have moved intercontinentally, either from America to the Middle East, or vice versa. Further studies will clarify if BCTV can be seed transmitted in beet, but even if the seed transmission is through a different species, such as petunia, once in another continent, the virus can move to beet in the presence of its vector. The fact that, in the case of BCTIV, our results suggest a low seed transmission rate, may in part explain why its geographical distribution is, so far, limited to the Middle East.
Further studies are needed to explore seed transmissibility of these viruses in other hosts. The outcome of this study, and recent reports on seed transmission of some other geminiviruses [16
], could answer questions of how these viruses get around the world by plant materials, in addition to accidental movement of their viruliferous vectors.