Infectivity of okra enation leaf curl virus and the role of its V2 protein in pathogenicity

Differences in the 3' intergenic region and the V2 protein of two sequence variants of tomato curly stunt virus play an important role in disease pathology in Nicotiana benthamiana

Tomato production in South Africa is threatened by the emergence of tomato curly stunt virus (ToCSV), a monopartite Begomovirus transmitted by the whitefly vector Bemisia tabaci (Genn.). We investigated the role of sequence differences present in the 3' intergenic region (IR) and the V2 coding region on the differing infectivity of ToCSV sequence variant isolates V30 and V22 in the model host Nicotiana benthamiana. Using virus mutant chimeras, we determined that the development of the upward leaf roll symptom phenotype is mediated by sequence differences present in the 3' IR containing the TATA-associated composite element. Sequence differences present in the V2 coding region are responsible for modulating disease severity and symptom recovery in V22-infected plants. Serine substitution of V22 V2 Val27 resulted in a significant increase in disease severity with reduced recovery, the first study to demonstrate the importance of this V2 residue in disease development. Two putative ORFs, C5 and C6, were identified using in silico analysis and detection of an RNA transcript spanning their coding region suggests that these ORFs may be transcribed during infection. Additional virus-derived RNA transcripts spanning multiple ORFs and crossing the boundaries of recognised polycistronic transcripts, as well as the origin of replication within the IR, were detected in ToCSV-infected plants providing evidence of bidirectional readthrough transcription. From our results, we conclude that the diverse responses of the model host to ToCSV infection is influenced by select sequence differences and our findings provide several avenues for further investigation into the mechanisms behind these responses to infection.

Artificial microRNA-mediated resistance against Oman strain of tomato yellow leaf curl virus

Tomato yellow leaf curl virus (TYLCV) is a global spreading begomovirus that is exerting a major restraint on global tomato production. In this transgenic approach, an RNA interference (RNAi)-based construct consisting of sequences of an artificial microRNA (amiRNA), a group of small RNA molecules necessary for plant cell development, signal transduction, and stimulus to biotic and abiotic disease was engineered targeting the AC1/Rep gene of the Oman strain of TYLCV-OM. The Rep-amiRNA constructs presented an effective approach in regulating the expression of the Rep gene against TYLCV as a silencing target to create transgenic Solanum lycopersicum L. plant tolerance against TYLCV infection. Molecular diagnosis by PCR followed by a Southern hybridization analysis were performed to confirm the effectiveness of agrobacterium-mediated transformation in T0/T1-transformed plants. A substantial decrease in virus replication was observed when T1 transgenic tomato plants were challenged with the TYLCV-OM infectious construct. Although natural resistance options against TYLCV infection are not accessible, the current study proposes that genetically transformed tomato plants expressing amiRNA could be a potential approach for engineering tolerance in plants against TYLCV infection and conceivably for the inhibition of viral diseases against different strains of whitefly-transmitted begomoviruses in Oman.

The interplay of plant hormonal pathways and geminiviral proteins: partners in disease development

Viruses belonging to the family Geminiviridae infect plants and are responsible for a number of diseases of crops in the tropical and sub-tropical regions of the World. The innate immune response of the plant assists in its defense against such viral pathogens by the recognition of pathogen/microbe-associated molecular patterns through pattern-recognition receptors. Phytohormone signalling pathways play a vital role in plant defense responses against these devastating viruses. Geminiviruses, however, have developed counter-defense strategies that prevail over the above defense pathways. The proteins encoded by geminiviruses act as suppressors of plant immunity by interacting with the signalling components of several hormones. In this review we focus on the molecular interplay of phytohormone pathways and geminiviral infection and try to find interesting parallels with similar mechanisms known in other plant-infecting viruses and strengthen the argument that this interplay is necessary for disease development.

Construction of Infectious Clones of Begomoviruses: Strategies, Techniques and Applications

Simple Summary

Begomovirus has a wide host range and threatens a significant amount of economic damage to many important crops such as tomatoes, beans, cassava, squash and cotton. There are many efforts directed at controlling this disease including the use of insecticides to control the insect vector as well as screening the resistant varieties. The use of synthetic virus or infectious clones approaches has allowed plant virologists to characterize and exploit the genome virus at the molecular and biological levels. By exploiting the DNA of the virus using the infectious clones strategy, the viral genome can be manipulated at specific regions to study functional genes for host–virus interactions. Thus, this review will provide an overview of the strategy to construct infectious clones of Begomovirus. The significance of established infectious clones in Begomovirus study will also be discussed.

Abstract

Begomovirus has become a potential threat to the agriculture sector. It causes significant losses to several economically important crops. Given this considerable loss, the development of tools to study viral genomes and function is needed. Infectious clones approaches and applications have allowed the direct exploitation of virus genomes. Infectious clones of DNA viruses are the critical instrument for functional characterization of the notable and newly discovered virus. Understanding of structure and composition of viruses has contributed to the evolution of molecular plant pathology. Therefore, this review provides extensive guidelines on the strategy to construct infectious clones of Begomovirus. Also, this technique’s impacts and benefits in controlling and understanding the Begomovirus infection will be discussed.

Characterization of Pathogenicity-Associated V2 Protein of Tobacco Curly Shoot Virus

V2 proteins encoded by some whitefly-transmitted geminiviruses were reported to be functionally important proteins. However, the functions of the V2 protein of tobacco curly shoot virus (TbCSV), a monopartite begomovirus that causes leaf curl disease on tomato and tobacco in China, remains to be characterized. In our report, an Agrobacterium infiltration-mediated transient expression assay indicated that TbCSV V2 can suppress local and systemic RNA silencing and the deletion analyses demonstrated that the amino acid region 1-92 of V2, including the five predicted α-helices, are required for local RNA silencing suppression. Site-directed substitutions showed that the conserved basic and ring-structured amino acids in TbCSV V2 are critical for its suppressor activity. Potato virus X-mediated heteroexpression of TbCSV V2 in Nicotiana benthamiana induced hypersensitive response-like (HR-like) cell death and systemic necrosis in a manner independent of V2's suppressor activity. Furthermore, TbCSV infectious clone mutant with untranslated V2 protein (TbCSV∆V2) could not induce visual symptoms, and coinfection with betasatellite (TbCSB) could obviously elevate the viral accumulation and symptom development. Interestingly, symptom recovery occurred at 15 days postinoculation (dpi) and onward in TbCSV∆V2/TbCSB-inoculated plants. The presented work contributes to understanding the RNA silencing suppression activity of TbCSV V2 and extends our knowledge of the multifunctional role of begomovirus-encoded V2 proteins during viral infections.

Tobacco curly shoot virus C3 protein enhances viral replication and gene expression in Nicotiana benthamiana plants

Geminiviruses are single-stranded DNA viruses that cause devastating diseases in many crops worldwide. The replication enhancer proteins (REn), encoded by the C3 (AC3, and AL3) ORFs of geminiviruses, have critical roles in viral DNA accumulation and symptom development in infected plants. In the current study, we have constructed an infectious clone of the Tobacco curly shoot virus (TbCSV) C3 mutant, TbCSVΔC3, that contains two start codon mutations that abrogated C3 ORF expression, but did not alter the amino acid sequence of the C2 ORF. As predicted, the absence of the C3 protein reduced TbCSV DNA accumulation, and over-expression of the C3 protein enhanced TbCSV DNA accumulation in infected leaves of Nicotiana benthamiana. The C3 mutation reduced the expression levels of both virion- and complementary-sense TbCSV genes whereas over-expression of the C3 protein increased TbCSV gene expression. Furthermore, the expression of the wild-type and site-directed mutants of C3 proteins using the potato virus X (PVX) system showed that Y93A mutation reduced the replication enhancement activity of the C3 protein in N. benthamiana. All the available evidence demonstrates that the C3 protein is tightly coupled with TbCSV DNA accumulation. However, the TbCSVΔC3 mutant was nearly as infectious in N. benthamiana as TbCSVWT and only had slightly delayed and attenuated symptom expression. Our findings demonstrate that TbCSV C3 protein enhances viral replication and gene expression, but has only moderate effects on symptom development in N. benthamiana.