Differential Roles of AC2 and AC4 of Cassava Geminiviruses in Mediating Synergism and Suppression of Posttranscriptional Gene Silencing

Abstract

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,

Supervirulent pseudorecombination and asymmetric synergism between genomic components of two distinct species of begomovirus associated with severe tomato leaf curl disease in India

“…Synergism between isolates of two cassava-infecting geminiviruses, ACMV and EACMV, has been observed and the genes mediating synergism have been identified (Fondong et al, 2000;Pita et al, 2001;Vanitharani et al, 2004). Synergism between ACMV and East African cassava mosaic Cameroon virus (EACMCV) is a two-way process, as the presence of the DNA-A component of ACMV or EACMCV enhanced the accumulation of viral DNA of EACMCV and ACMV, respectively, in tobacco BY2 protoplasts (Vanitharani et al, 2004).…”

“…Synergism between ACMV and East African cassava mosaic Cameroon virus (EACMCV) is a two-way process, as the presence of the DNA-A component of ACMV or EACMCV enhanced the accumulation of viral DNA of EACMCV and ACMV, respectively, in tobacco BY2 protoplasts (Vanitharani et al, 2004). ACMV AC4 and EACMCV AC2, the putative synergistic genes, are able to suppress post-transcriptional gene silencing (Vanitharani et al, 2004). In the present study, we demonstrated that NA and NB accumulation was Virulent pseudorecombination and synergism among ToLCVs also increased due to synergism, whilst the level of VA remained unchanged or decreased, indicating a unique asymmetric interaction.…”

“…Harrison et al (1997) suggested the possibility of synergism between isolates of the two cassava begomovirus species, African cassava mosaic virus and East African cassava mosaic virus. Molecular evidence for synergism between the cassava-infecting begomoviruses African cassava mosaic virus from Cameroon (ACMV- [CM:98]) and East African cassava mosaic virus from Cameroon (EACMV- [CM:98]) has been demonstrated (Fondong et al, 2000;Pita et al, 2001;Vanitharani et al, 2004).…”

MicroRNA-binding viral protein interferes with Arabidopsis development

“…Coding sequences of AC2 and AC4 genes from ACMV and EACMCV were fused to the cauliflower mosaic virus (CaMV) 35S promoter (35S) in a binary vector and introduced into Agrobacterium tumefaciens strain GV3101 as described in ref. 18. Transformation of Arabidopsis thaliana ecotype Columbia plants (Col-0) was performed by the floral dipping method (22).…”

“…In a previous study, we identified two types of virus-encoded PTGS-suppressor proteins: (i) the AC4 of African cassava mosaic virus Cameroon Strain (ACMV) and of Sri Lankan cassava mosaic virus (SLCMV) and (ii) the AC2 of East African cassava mosaic Cameroon virus (EACMCV) and of Indian cassava mosaic virus (ICMV). It is worth noting that not all AC4 proteins are PTGS suppressors (AC4ϩ), and that, for example, the AC4 of EACMCV (E-AC4) and ICMV are not PTGS suppressors (AC4Ϫ) (18). In addition, the AC2 of ACMV-Kenyan strain (A-AC2) and mungbean yellow mosaic virus and the C2 of tomato yellow leaf curl China virus were reported to possess silencing-suppression activity (19)(20)(21).…”