A plant-specific syntaxin-6 protein contributes to the intracytoplasmic route for the begomovirus CabLCV

In Situ Protein Microarray for Identifying the Geminivirus-Arabidopsis Interactome

The protein-protein interactions (PPI) by protein array technology complement other PPI assay technologies such as AP-MS and Y2H. The in situ protein array technology (NAPPA) enables low-cost, rapid, and comprehensive protein detection. It allows standardized and simultaneous assay of a wide range of proteins with a broad range of expression in cells. This technology facilitates the detection of protein-protein interactions within species and between heterologous species such as host-microbe. Here, we described the technique that identified a syntaxin-6 protein-mediated begomovirus infection using an array containing 4600 Arabidopsis genes. The protein microarray assay also identified several other viral protein-host protein interactions.

Bimolecular Fluorescence Complementation (BiFC) in Host-Virus Interactions

Bimolecular fluorescence complementation (BiFC) is an assay widely used for studying protein-protein interactions and determining the subcellular localization of proteins. This technique involves fusing the proteins of interest to separate structural domains of a fluorescent protein, followed by transient expression in cells. The interaction between the proteins of interest in vivo allows the reconstitution of the fluorescence that can be visualized by fluorescence microscopy. BiFC has been particularly useful in investigating the interactions between viral and host proteins. Here, we describe the steps involved in preparing expression cassettes that allow the expression of proteins of interest fused to nonfluorescent fragments of yellow fluorescent protein (YFP), Agrobacterium transformations, and agroinfiltration of Nicotiana benthamiana leaves to facilitate virus protein-host protein interactions. Finally, high-resolution images can be obtained by analyzing the leaves under a confocal microscope.

Protein-Protein Interaction via Two-Hybrid Assay in Yeast

The yeast two-hybrid assay enables detecting interactions between proteins, which makes this tool of particular interest for plant-virus interaction studies. Basically, the reporter gene expression (HIS3) is activated by the binding of a transcription factor GAL4, which, like eukaryotic transcription factors, is modular in nature and consists of two structurally independent domains: DNA-binding (DB) and activation (AD) domains. The two proteins under investigation are expressed separately, one fused to the BD domain and the other to the AD domain. In the yeast strain AH109, activation of the reporter gene occurs only in cells that contain interacting proteins, reconstituting the transcription factor GAL4 which then binds to the responsive promoter and results in yeast colony growth.

Replication Assay of Begomovirus in Arabidopsis Protoplasts

Protoplasts are isolated plant cells from which the cell walls have been removed by treatment with fungal cellulase and macerozyme enzymes, which degrade the primary components of the cell wall. The protoplasts are totipotent, sensitive, and versatile; thereby, they have been extensively used to study signal transduction pathways, cell-autonomous responses, and replication of plant viruses. This system has several advantages over the use of whole plants for viral replication, including a high percentage of infected cells and uncoupling virus movement from replication assays. Here, we describe a simple and efficient protocol for begomovirus transfection and replication in Arabidopsis protoplasts. The method consists of four steps, (i) protoplast isolation, (ii) PEG-calcium transfection of begomovirus infectious clones, (iii) elimination of input plasmid DNA by DpnI digestion, and (iv) quantification of the viral newly synthesized DNA by qPCR. The protoplasts can be transformed efficiently with begomovirus infectious clones, and virus replication can be monitored by the accumulation of nascent viral DNA in the infected protoplasts.

Geminivirus C5 proteins mediate formation of virus complexes at plasmodesmata for viral intercellular movement

Movement proteins (MPs) encoded by plant viruses deliver viral genomes to plasmodesmata (PD) to ensure intracellular and intercellular transport. However, how the MPs encoded by monopartite geminiviruses are targeted to PD is obscure. Here, we demonstrate that the C5 protein of tomato yellow leaf curl virus (TYLCV) anchors to PD during the viral infection following trafficking from the nucleus along microfilaments in Nicotiana benthamiana. C5 could move between cells and partially complement the traffic of a movement-deficient turnip mosaic virus (TuMV) mutant (TuMV-GFP-P3N-PIPO-m1) into adjacent cells. The TYLCV-C5 null mutant (TYLCV-mC5) attenuates viral pathogenicity and decreases viral DNA and protein accumulation, and ectopic overexpression of C5 enhances viral DNA accumulation. Interaction assays between TYLCV-C5 and the other eight viral proteins described in TYLCV reveal that C5 associates with C2 in the nucleus and with V2 in the cytoplasm and at PD. The V2 protein is mainly localized in the nucleus and cytoplasmic granules when expressed alone; in contrast, V2 forms small punctate granules at PD when co-expressed with C5 or in TYLCV-infected cells. The interaction of V2 and C5 also facilitates their nuclear export. Furthermore, C5-mediated PD localization of V2 is conserved in two other geminiviruses. Therefore, this study solves a long-sought-after functional connection between PD and the geminivirus movement and improves our understanding of geminivirus-encoded MPs and their potential cellular and molecular mechanisms.

A sword or a buffet: plant endomembrane system in viral infections

The plant endomembrane system is an elaborate collection of membrane-bound compartments that perform distinct tasks in plant growth and development, and in responses to abiotic and biotic stresses. Most plant viruses are positive-strand RNA viruses that remodel the host endomembrane system to establish intricate replication compartments. Their fundamental role is to create optimal conditions for viral replication, and to protect replication complexes and the cell-to-cell movement machinery from host defenses. In addition to the intracellular antiviral defense, represented mainly by RNA interference and effector-triggered immunity, recent findings indicate that plant antiviral immunity also includes membrane-localized receptor-like kinases that detect viral molecular patterns and trigger immune responses, which are similar to those observed for bacterial and fungal pathogens. Another recently identified part of plant antiviral defenses is executed by selective autophagy that mediates a specific degradation of viral proteins, resulting in an infection arrest. In a perpetual tug-of-war, certain host autophagy components may be exploited by viral proteins to support or protect an effective viral replication. In this review, we present recent advances in the understanding of the molecular interplay between viral components and plant endomembrane-associated pathways.

Begomovirus-Host Interactions: Viral Proteins Orchestrating Intra and Intercellular Transport of Viral DNA While Suppressing Host Defense Mechanisms

Begomoviruses, which belong to the Geminiviridae family, are intracellular parasites transmitted by whiteflies to dicotyledonous plants thatsignificantly damage agronomically relevant crops. These nucleus-replicating DNA viruses move intracellularly from the nucleus to the cytoplasm and then, like other plant viruses, cause disease by spreading systemically throughout the plant. The transport proteins of begomoviruses play a crucial role in recruiting host components for the movement of viral DNA within and between cells, while exhibiting functions that suppress the host's immune defense. Pioneering studies on species of the Begomovirus genus have identified specific viral transport proteins involved in intracellular transport, cell-to-cell movement, and systemic spread. Recent research has primarily focused on viral movement proteins and their interactions with the cellular host transport machinery, which has significantly expanded understanding on viral infection pathways. This review focuses on three components within this context: (i) the role of viral transport proteins, specifically movement proteins (MPs) and nuclear shuttle proteins (NSPs), (ii) their ability to recruit host factors for intra- and intercellular viral movement, and (iii) the suppression of antiviral immunity, with a particular emphasis on bipartite begomoviral movement proteins.

Geminiviral betasatellites: critical viral ammunition to conquer plant immunity

Geminiviruses have mastered plant cell modulation and immune invasion to ensue prolific infection. Encoding a relatively small number of multifunctional proteins, geminiviruses rely on satellites to efficiently re-wire plant immunity, thereby fostering virulence. Among the known satellites, betasatellites have been the most extensively investigated. They contribute significantly to virulence, enhance virus accumulation, and induce disease symptoms. To date, only two betasatellite proteins, βC1, and βV1, have been shown to play a crucial role in virus infection. In this review, we offer an overview of plant responses to betasatellites and counter-defense strategies deployed by betasatellites to overcome those responses.

Engineering plant immune circuit: walking to the bright future with a novel toolbox

Plant pathogens destroy crops and cause severe yield losses, leading to an insufficient food supply to sustain the human population. Apart from relying on natural plant immune systems to combat biological agents or waiting for the appropriate evolutionary steps to occur over time, researchers are currently seeking new breakthrough methods to boost disease resistance in plants through genetic engineering. Here, we summarize the past two decades of research in disease resistance engineering against an assortment of pathogens through modifying the plant immune components (internal and external) with several biotechnological techniques. We also discuss potential strategies and provide perspectives on engineering plant immune systems for enhanced pathogen resistance and plant fitness.

Clustered Regularly Interspaced Short Palindromic Repeats-Associated Protein System for Resistance Against Plant Viruses: Applications and Perspectives

Different genome editing approaches have been used to engineer resistance against plant viruses. The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas; CRISPR/Cas) systems to create pinpoint genetic mutations have emerged as a powerful tool for molecular engineering of plant immunity and increasing resistance against plant viruses. This review presents (i) recent advances in engineering resistance against plant viruses by CRISPR/Cas and (ii) an overview of the potential host factors as targets for the CRISPR/Cas system-mediated broad-range resistance and immunity. Applications, challenges, and perspectives in enabling the CRISPR/Cas system for crop protection are also outlined.

Insights into the multifunctional roles of geminivirus-encoded proteins in pathogenesis

Geminiviruses are a major threat to agriculture in tropical and subtropical regions of the world. Geminiviruses have small genome with limited coding capacity. Despite this limitation, these viruses have mastered hijacking the host cellular metabolism for their survival. To compensate for the small size of their genome, geminiviruses encode multifunctional proteins. In addition, geminiviruses associate themselves with satellite DNA molecules which also encode proteins that support the virus in establishing successful infection. Geminiviral proteins recruit multiple host factors, suppress the host defense, and manipulate host metabolism to establish infection. We have updated the knowledge accumulated about the proteins of geminiviruses and their satellites in the context of pathogenesis in a single review. We also discuss their interactions with host factors to provide a mechanistic understanding of the infection process.