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Posttranscriptional gene silencing (PTGS) in plants isPosttranscriptional gene silencing (PTGS) involves the degradation of viral and cellular mRNAs in a homology-dependent manner, and it is conserved in diverse eukaryotes (15,25). In plants, PTGS functions as a natural antiviral defense because plant viruses are both initiators and targets of PTGS (47). PTGS was first discovered in plants (30); however, a mechanistically similar phenomenon was later described in other organisms: it is called quelling in fungi (8) and RNA interference in Caenorhabditis elegans (11) and in Drosophila melanogaster (16). Recent studies at the molecular level revealed that all of these can be considered to be manifestations of an RNA-targeting pathway. Even though the mechanism by which a virus infection triggers PTGS in plants is not fully understood, double-stranded RNA (dsRNA) has been found to be a strong inducer of PTGS (57). Such a form is produced during replication of an RNA virus or conversion of aberrant single-stranded RNAs into dsRNA in the cell by host-encoded RNA-directed RNA polymerase. These dsRNAs are first processed into 21-to 26-nt short interfering RNAs (siRNAs) by an RNase DICER enzyme and subsequently serve as guides by forming an active multicomplex RNA-induced silencing complex, which cleaves homologous RNA molecules (5). In plants, gene silencing generates an unknown mobile signal that can trigger PTGS in distant tissues and across a graft union (32).In recent years, RNA silencing-inhibiting proteins that counter antiviral RNA silencing have been identified in several plant viruses (47) and in an insect virus (25). These identified silencing suppressor proteins may act at different steps in the PTGS pathway. Three distinct phases have been identified in the RNA-silencing process: initiation, maintenance, and systemic signaling. Thus, (i) the potyvirus helper component proteinase (HC-Pro) interferes with the initiation and maintenance of silencing at a step coincident with or upstream of siRNA production, because it did not prevent the silencing signal from becoming systemic (1,22,26); (ii) the 2b protein of Cucumber mosaic virus (CMV) prevents the initiation of PTGS in newly emerging tissues by inhibiting long-range PTGS-signaling activity (6, 13); and (iii) p25 of Potato virus X (PVX) suppresses the production or accumulation of the mobile silencing signal (54). Recently, the p19 protein of tombusviruses was implicated in inhibiting RNA silencing by physically interacting with siRNAs and thus providing another mechanism to interfere with RNA silencing (43). In geminiviruses, AC2, encoding the transcriptional activator protein (TrAP) of the Kenyan strain of African cassava mosaic virus (ACMV- [KE]), and the product of C2, a positional homologue of AC2 in the monopartite Tomato yellow leaf curl China virus (TYLCCNV) have both been identified as suppressors of PTGS (52,55).In nature, mixed viral infections occur in the same plant,

The molecular variability of cassava geminiviruses occurring in Uganda was investigated in this study. Infected cassava plants and whiteflies were collected from cassava plantings in different geographical areas of the country and PCR was used for molecular characterization of the viruses. Two complete sequences of DNA-A and -B from African cassava mosaic virus (ACMV), two DNA-A sequences from East African cassava mosaic virus (EACMV), two DNA-B sequences of EACMV and the partial DNA-A nucleotide sequence of a new virus strain isolated in Uganda, EACMV-UG3, are reported here. Analysis of naturally infected cassava plants showed various assortments of DNA-A and DNA-B of the Ugandan viruses, suggesting the occurrence of natural inter-and intraspecies pseudorecombinations and a pattern of cassava mosaic disease (CMD) more complex than previously reported. EACMV-UG2 DNA-A, which contains a recombinant fragment between ACMV and EACMV-UG1 in the coat protein gene that resembles virus from Tanzania, was widespread in the country and always associated with EACMV-UG3 DNA-B, which probably resulted from another natural recombination event. Mixed infections of ACMV-UG and EACMV-UG in cassava and whiteflies were detected in most of the regions where both viruses occurred. These mixed-infected samples always showed extremely severe CMD symptoms, suggesting a synergistic interaction between ACMV-UG and EACMV-UG2. The first demonstration is provided of infectivity of EACMV clones to cassava, proving conclusively that the pseudorecombinant EACMV-UG2 DNA-AMEACMV-UG3 DNA-B is a causal agent of CMD in Uganda.

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