Geminiviruses

Genetic diversity of sweet potato begomoviruses in the United States and identification of a natural recombinant between sweet potato leaf curl virus and sweet potato leaf curl Georgia virus

In the United States, two sweet potato begomoviruses, sweet potato leaf curl virus (SPLCV) and sweet potato leaf curl Georgia virus (SPLCGV), were previously identified in Louisiana. In recent years, at least seven additional sweet potato begomoviruses have been identified in other parts of the world. In an effort to determine the genetic diversity and distribution of sweet potato begomoviruses in the U.S., we focused our efforts on molecular characterization of field-collected begomovirus isolates in two states: Mississippi and South Carolina. Using rolling-circle amplification, a total of 52 clones of the full genome were obtained. Initial inspection of alignments of the end sequences in these clones revealed a strong genetic diversity. Overall, 10 genotypes could be assigned. A majority of the isolates (50/52) in eight genotypes were shown to be closely related to SPLCV. A representative clone of each genotype was fully sequenced and analyzed. Among them, four genotypes from South Carolina with 91-92% sequence identity to the type member of SPLCV were considered a new strain, whereas four other genotypes from Mississippi with >95% sequence identity to SPLCV were considered variants. In addition, a member of a proposed new begomovirus species was identified after comparative sequence analysis of the isolate [US:SC:646B-9] from South Carolina with less than 89% sequence identity to any known begomovirus. Hence, the provisional name Sweet potato leaf curl South Carolina virus (SPLCSCV) is proposed. Moreover, a natural recombinant consisting of two distinct parental genomic sequences from SPLCV and SPLCGV was identified in the sample [US:MS:1B-3] from Mississippi. Two recombinant breakpoints were identified, one in the origin of replication and the other between C2 and C4. This knowledge about the genetic diversity of begomoviruses infecting sweet potato will likely have a major impact on PCR-based virus detection and on disease management practice through breeding for virus resistance.

Plasmodesmata: gateways to local and systemic virus infection

As channels that provide cell-to-cell connectivity, plasmodesmata are central to the local and systemic spread of viruses in plants. This review discusses the current state of knowledge of the structure and function of these channels and the ways in which viruses bring about functional changes that allow macromolecular trafficking to occur. Despite the passing of two decades since the first identification of a viral movement protein that mediates these changes, our understanding of the relevant molecular mechanisms remains in its infancy. However, viral movement proteins provide valuable tools for the modification of plasmodesmata and will continue to assist in the dissection of plasmodesmal properties in relation to their core roles in cell-to-cell communication.

The complete nucleotide sequence of a new bipartite begomovirus from Brazil infecting Abutilon

The complete nucleotide sequence of Abutilon mosaic Brazil virus (AbMBV), a new bipartite begomovirus from Bahia, Brazil, is described and analyzed phylogenetically. Its DNA A is most closely related to those of Sida-infecting begomoviruses from Brazil and forms a phylogenetic cluster with pepper- and Euphorbia-infecting begomoviruses from Central America. The DNA B component forms a cluster with different Sida- and okra-infecting begomoviruses from Brazil. Both components are distinct from those of the classical Abutilon mosaic virus originating from the West Indies. AbMBV is transmissible to Nicotiana benthamiana and Malva parviflora by biolistics of rolling-circle amplification products and induces characteristic mosaic and vein-clearing symptoms in M. parviflora.

Experimental evidence indicating that mastreviruses probably did not co-diverge with their hosts

Background

Despite the demonstration that geminiviruses, like many other single stranded DNA viruses, are evolving at rates similar to those of RNA viruses, a recent study has suggested that grass-infecting species in the genus Mastrevirus may have co-diverged with their hosts over millions of years. This "co-divergence hypothesis" requires that long-term mastrevirus substitution rates be at least 100,000-fold lower than their basal mutation rates and 10,000-fold lower than their observable short-term substitution rates. The credibility of this hypothesis, therefore, hinges on the testable claim that negative selection during mastrevirus evolution is so potent that it effectively purges 99.999% of all mutations that occur.

Results

We have conducted long-term evolution experiments lasting between 6 and 32 years, where we have determined substitution rates of between 2 and 3 × 10^-4 substitutions/site/year for the mastreviruses Maize streak virus (MSV) and Sugarcane streak Réunion virus (SSRV). We further show that mutation biases are similar for different geminivirus genera, suggesting that mutational processes that drive high basal mutation rates are conserved across the family. Rather than displaying signs of extremely severe negative selection as implied by the co-divergence hypothesis, our evolution experiments indicate that MSV and SSRV are predominantly evolving under neutral genetic drift.

Conclusion

The absence of strong negative selection signals within our evolution experiments and the uniformly high geminivirus substitution rates that we and others have reported suggest that mastreviruses cannot have co-diverged with their hosts.

Plant geminivirus rep protein induces rereplication in fission yeast

The replication-associated protein (Rep) of geminiviruses, single-stranded DNA viruses of higher plants, is essential for virus replication. Since these viruses do not encode their own polymerases, Rep induces differentiated plant cells to reenter the cell cycle by interacting with the plant homologues of retinoblastoma proteins in order to activate the host DNA synthesis machinery. We have used fission yeast (Schizosaccharomyces pombe) as a model organism to analyze the impact of ectopically expressed African cassava mosaic virus Rep protein on the cell division cycle in closer detail. Upon expression, Rep showed its characteristic DNA cleavage activity, and about 10% of the cells exhibited morphological changes. They were elongated threefold, on average, and possessed a single but enlarged and less compact nucleus in comparison to noninduced or vector-only control cells. Flow cytometry of Rep-expressing cultures revealed a distinct subpopulation of Rep protein-containing cells with aberrant morphology. The other 90% of the cells were indistinguishable from control cells, and no Rep was detectable. Rep-expressing cells exhibited DNA contents beyond 2C, indicating ongoing replication without intervening mitosis. Because a second open reading frame (ORF), AC4, is present within the Rep gene, the role of AC4 was examined by destroying its start codon within the AC1 ORF. The results confirmed that Rep is necessary and sufficient to induce rereplication in fission yeast. The unique potential of this well-investigated model for dissecting the cell cycle control by geminiviral proteins is discussed.