AltMV TGB1 Nucleolar Localization Requires Homologous Interaction and Correlates with Cell Wall Localization Associated with Cell-to-Cell Movement

Suppression of Arabidopsis Mediator Subunit-Encoding MED18 Confers Broad Resistance Against DNA and RNA Viruses While MED25 Is Required for Virus Defense

Mediator subunits play key roles in numerous physiological pathways and developmental processes in plants. Arabidopsis Mediator subunits, MED18 and MED25, have previously been shown to modulate disease resistance against fungal and bacterial pathogens through their role in jasmonic acid (JA) signaling. In this study, Arabidopsis mutant plants of the two Mediator subunits, med18 and med25, were tested against three ssRNA viruses and one dsDNA virus belonging to four different families: Turnip mosaic virus (TuMV), Cauliflower mosaic virus (CaMV), Alternanthera mosaic virus (AltMV), and Cucumber mosaic virus (CMV). Although both subunits are utilized in JA signaling, they occupy different positions (Head and Tail domain, respectively) in the Mediator complex and their absence affected virus infection differently. Arabidopsis med18 plants displayed increased resistance to RNA viral infection and a trend against the DNA virus, while med25 mutants displayed increased susceptibility to all viruses tested at 2 and 14 days post inoculations. Defense marker gene expression profiling of mock- and virus-inoculated plants showed that med18 and med25 mutants exhibited an upregulated SA pathway upon virus infection at 2 dpi for all viruses tested. JA signaling was also suppressed in med18 plants after virus infection, independent of which virus infected the plants. The upregulation of SA signaling and suppression of JA signaling in med18 may have led to more targeted oxidative burst and programmed cell death to control viruses. However, the susceptibility exhibited by med25 mutants suggests that other factors, such as a weakened RNAi pathway, might play a role in the observed susceptibility. We conclude that MED18 and MED25 have clear and opposite effects on accumulation of plant viruses. MED18 is required for normal virus infection, while MED25 is important for defense against virus infection. Results from this study provide a better understanding of the role of Mediator subunits during plant-virus interactions, viral disease progression and strategies to develop virus resistant plants.

Alternanthera mosaic potexvirus: Several Features, Properties, and Application

Alternanthera mosaic virus (AltMV) is a typical member of the Potexvirus genus in its morphology and genome structure; still it exhibits a number of unique features. They allow this virus to be considered a promising object for biotechnology. Virions and virus-like particles (VLPs) of AltMV are stable in a wide range of conditions, including sera of laboratory animals. AltMV VLPs can assemble at various pH and ionic strengths. Furthermore, AltMV virions and VLPs demonstrate high immunogenicity, enhancing the immune response to the target antigen thus offering the possibility of being used as potential adjuvants. Recently, for the first time for plant viruses, we showed the structural difference between morphologically similar viral and virus-like particles on AltMV virions and VLPs. In this review, we discuss the features of AltMV virions, AltMV VLP assembly, and their structure and properties, as well as the characteristics of AltMV isolates, host plants, infection symptoms, AltMV isolation and purification, genome structure, viral proteins, and AltMV-based vectors.

Selective Interaction Between Chloroplast β-ATPase and TGB1L88 Retards Severe Symptoms Caused by Alternanthera mosaic virus Infection

The multifunctional triple gene block protein 1 (TGB1) of the Potexvirus Alternanthera mosaic virus (AltMV) has been reported to have silencing suppressor, cell-to-cell movement, and helicase functions. Yeast two hybrid screening using an Arabidopsis thaliana cDNA library with TGB1 as bait, and co-purification with TGB1 inclusion bodies identified several host proteins which interact with AltMV TGB1. Host protein interactions with TGB1 were confirmed by biomolecular fluorescence complementation, which showed positive TGB1 interaction with mitochondrial ATP synthase delta' chain subunit (ATP synthase delta'), light harvesting chlorophyll-protein complex I subunit A4 (LHCA4), chlorophyll a/b binding protein 1 (LHB1B2), chloroplast-localized IscA-like protein (ATCPISCA), and chloroplast β-ATPase. However, chloroplast β-ATPase interacts only with TGB1L88, and not with weak silencing suppressor TGB1P88. This selective interaction indicates that chloroplast β-ATPase is not required for AltMV movement and replication; however, TRV silencing of chloroplast β-ATPase in Nicotiana benthamiana induced severe tissue necrosis when plants were infected by AltMV TGB1L88 but not AltMV TGB1P88, suggesting that β-ATPase selectively responded to TGB1L88 to induce defense responses.