Cauliflower mosaic virus disease spectrum uncovers novel susceptibility factor NCED9 in Arabidopsis thaliana.

Viruses are intimately linked with their hosts and especially dependent on gene-for-gene interactions to establish successful infections. On the host side, defence mechanisms such as tolerance and resistance can occur within the same species, leading to differing virus accumulation in relation to symptomology and plant fitness. The identification of novel resistance genes against viruses and susceptibility factors is an important part of understanding viral patho-genesis and securing food production. The model plant Arabidopsis thaliana displays a wide symptom spectrum in response to RNA virus infections, and unbiased genome-wide association studies have proven a powerful tool to identify novel disease-genes. In this study we infected natural accessions of A. thaliana with the pararetrovirus cauliflower mosaic virus (CaMV) to study the phenotypic variations between accessions and their correlation with virus accumulation. Through genome-wide association mapping of viral accumulation differences, we identified several susceptibility factors for CaMV, the strongest of which was the abscisic acid synthesis gene NCED9. Further experiments confirmed the importance of abscisic acid homeostasis and its disruption for CaMV disease.

Cauliflower mosaic virus protein P6 is a multivalent node for RNA granule proteins and interferes with stress granule responses during plant infection.

Biomolecular condensation is a multipurpose cellular process that viruses use ubiquitously during their multiplication. Cauliflower mosaic virus replication complexes are condensates that differ from those of most viruses, as they are nonmembranous assemblies that consist of RNA and protein, mainly the viral protein P6. Although these viral factories (VFs) were described half a century ago, with many observations that followed since, functional details of the condensation process and the properties and relevance of VFs have remained enigmatic. Here, we studied these issues in Arabidopsis thaliana and Nicotiana benthamiana. We observed a large dynamic mobility range of host proteins within VFs, while the viral matrix protein P6 is immobile, as it represents the central node of these condensates. We identified the stress granule (SG) nucleating factors G3BP7 and UBP1 family members as components of VFs. Similarly, as SG components localize to VFs during infection, ectopic P6 localizes to SGs and reduces their assembly after stress. Intriguingly, it appears that soluble rather than condensed P6 suppresses SG formation and mediates other essential P6 functions, suggesting that the increased condensation over the infection time-course may accompany a progressive shift in selected P6 functions. Together, this study highlights VFs as dynamic condensates and P6 as a complex modulator of SG responses.

Salicylic acid and the viral virulence factor 2b regulate the divergent roles of autophagy during cucumber mosaic virus infection.

Macroautophagy/autophagy is a conserved intracellular degradation pathway that has recently emerged as an integral part of plant responses to virus infection. The known mechanisms of autophagy range from the selective degradation of viral components to a more general attenuation of disease symptoms. In addition, several viruses are able to manipulate the autophagy machinery and counteract autophagy-dependent resistance. Despite these findings, the complex interplay of autophagy activities, viral pathogenicity factors, and host defense pathways in disease development remains poorly understood. In the current study, we analyzed the interaction between autophagy and cucumber mosaic virus (CMV) in Arabidopsis thaliana. We show that autophagy is induced during CMV infection and promotes the turnover of the major virulence protein and RNA silencing suppressor 2b. Intriguingly, autophagy induction is mediated by salicylic acid (SA) and dampened by the CMV virulence factor 2b. In accordance with 2b degradation, we found that autophagy provides resistance against CMV by reducing viral RNA accumulation in an RNA silencing-dependent manner. Moreover, autophagy and RNA silencing attenuate while SA promotes CMV disease symptoms, and epistasis analysis suggests that autophagy-dependent disease and resistance are uncoupled. We propose that autophagy counteracts CMV virulence via both 2b degradation and reduced SA-responses, thereby increasing plant fitness with the viral trade-off arising from increased RNA silencing-mediated resistance.

The Effects of Turnip Mosaic Virus Infections on the Deposition of Secondary Cell Walls and Developmental Defects in Arabidopsis Plants Are Virus-Strain Specific.

Two isolates of Turnip mosaic virus (UK 1 and JPN 1), representative of two different viral strains, induced differential alterations on secondary cell wall (SCW) development in Arabidopsis thaliana, suggesting cell-type specific effects of these viral infections. These potential effects were analyzed in inflorescence stems and flowers of infected plants, together with other possible cellular effects of the infections. Results obtained from macroscopic and histochemical analyses showed that infection with either virus significantly narrowed stem area, but defects in SCW were only found in JPN 1 infections. In flowers, reduced endothecium lignification was also found for JPN 1, while UK 1 infections induced severe floral cell and organ development alterations. A transcriptomic analysis focused on genes controlling and regulating SCW formation also showed notable differences between both viral isolates. UK 1 infections induced a general transcriptional decrease of most regulatory genes, whereas a more complex pattern of alterations was found in JPN 1 infections. The role of the previously identified viral determinant of most developmental alterations, the P3 protein, was also studied through the use of viral chimeras. No SCW alterations or creeping habit growth were found in infections by the chimeras, indicating that if the P3 viral protein is involved in the determination of these symptoms, it is not the only determinant. Finally, considerations as to the possibility of a taxonomical reappraisal of these TuMV viral strains are provided.

Association between flower stalk elongation, an Arabidopsis developmental trait, and the subcellular location and movement dynamics of the nonstructural protein P3 of Turnip mosaic virus.

Virus infections affect plant developmental traits but this aspect of the interaction has not been extensively studied so far. Two strains of Turnip mosaic virus differentially affect Arabidopsis development, especially flower stalk elongation, which allowed phenotypical, cellular, and molecular characterization of the viral determinant, the P3 protein. Transiently expressed wild-type green fluorescent protein-tagged P3 proteins of both strains and selected mutants of them revealed important differences in their behaviour as endoplasmic reticulum (ER)-associated peripheral proteins flowing along the reticulum, forming punctate accumulations. Three-dimensional (3D) model structures of all expressed P3 proteins were computationally constructed through I-TASSER protein structure predictions, which were used to compute protein surfaces and map electrostatic potentials to characterize the effect of amino acid changes on features related to protein interactions and to phenotypical and subcellular results. The amino acid at position 279 was the main determinant affecting stalk development. It also determined the speed of ER-flow of the expressed proteins and their final location. A marked change in the protein surface electrostatic potential correlated with changes in subcellular location. One single amino acid in the P3 viral protein determines all the analysed differential characteristics between strains differentially affecting flower stalk development. A model proposing a role of the protein in the intracellular movement of the viral replication complex, in association with the viral 6K2 protein, is proposed. The type of association between both viral proteins could differ between the strains.

Diverse plant viruses: a toolbox for dissection of cellular pathways.

Research in virology has usually focused on one selected host-virus pathosystem to examine the mechanisms underlying a particular disease. However, as exemplified by the mechanistically versatile suppression of antiviral RNA silencing by plant viruses, there may be functionally convergent evolution. Assuming this is a widespread feature, we propose that effector proteins from diverse plant viruses can be a powerful resource for discovering new regulatory mechanisms of distinct cellular pathways. The efficiency of this approach will depend on how deeply and widely the studied pathway is integrated into viral infections. Beyond this, comparative studies using broad virus diversity should increase our global understanding of plant-virus interactions.

The apparent non-host resistance of Ethiopian mustard to a radish-infecting strain of Turnip mosaic virus is largely determined by the C-terminal region of the P3 viral protein.

Two different isolates of Turnip mosaic virus (TuMV: UK 1 and JPN 1) belonging to different virus strains were tested on three different Brassica species, namely turnip (Brassica rapa L.), Indian mustard (Brassica juncea L.) and Ethiopian mustard (Brassica carinata A. Braun). Although all three hosts were readily infected by isolate UK 1, isolate JPN 1 was able to establish a visible systemic infection only in the first two. Ethiopian mustard plants showed no local or systemic symptoms, and no virus antigens could be detected by enzyme-linked immunosorbent assay (ELISA). Thus, this species looks like a non-host for JPN 1, an apparent situation of non-host resistance (NHR). Through an experimental approach involving chimeric viruses made by gene interchange between two infectious clones of both virus isolates, the genomic region encoding the C-terminal domain of viral protein P3 was found to bear the resistance determinant, excluding any involvement of the viral fusion proteins P3N-PIPO and P3N-ALT in the resistance. A further determinant refinement identified two adjacent positions (1099 and 1100 of the viral polyprotein) as the main determinants of resistance. Green fluorescent protein (GFP)-tagged viruses showed that the resistance of Ethiopian mustard to isolate JPN 1 is only apparent, as virus-induced fluorescence could be found in discrete areas of both inoculated and non-inoculated leaves. In comparison with other plant-virus combinations of extreme resistance, we propose that Ethiopian mustard shows an apparent NHR to TuMV JPN 1, but not complete immunity or extreme resistance.

Virulence evolution of a sterilizing plant virus: Tuning multiplication and resource exploitation.

Virulence evolution may have far-reaching consequences for virus epidemiology and emergence, and virologists have devoted increasing effort to understand the modulators of this process. However, still little is known on the mechanisms and determinants of virulence evolution in sterilizing viruses that, as they prevent host reproduction, may have devastating effects on host populations. Theory predicts that sterilizing parasites, including viruses, would evolve towards lower virulence and absolute host sterilization to optimize the exploitation of host resources and maximize fitness. However, this hypothesis has seldom been analyzed experimentally. We investigated the evolution of virulence of the sterilizing plant virus Turnip mosaic virus (TuMV) in its natural host Arabidopsis thaliana by serial passage experiments. After passaging, we quantified virus accumulation and infectivity, the effect of infection on plant growth and development, and virulence of the ancestral and passaged viral genotypes in A. thaliana. Results indicated that serial passaging increased the proportion of infected plants showing absolute sterility, reduced TuMV virulence, and increased virus multiplication and infectivity. Genomic comparison of the ancestral and passaged TuMV genotypes identified significant mutation clustering in the P1, P3, and 6K2 proteins, suggesting a role of these viral proteins in the observed phenotypic changes. Our results support theoretical predictions on the evolution of virulence of sterilizing parasites and contribute to better understand the phenotypic and genetic changes associated with this process.

Viral Strain-Specific Differential Alterations in Arabidopsis Developmental Patterns.

Turnip mosaic virus (TuMV) infections affect many Arabidopsis developmental traits. This paper analyzes, at different levels, the development-related differential alterations induced by different strains of TuMV, represented by isolates UK 1 and JPN 1. The genomic sequence of JPN 1 TuMV isolate revealed highest divergence in the P1 and P3 viral cistrons, upon comparison with the UK 1 sequence. Infectious viral chimeras covering the whole viral genome uncovered the P3 cistron as a major viral determinant of development alterations, excluding the involvement of the PIPO open reading frame. However, constitutive transgenic expression of P3 in Arabidopsis did not induce developmental alterations nor modulate the strong effects induced by the transgenic RNA silencing suppressor HC-Pro from either strain. This highlights the importance of studying viral determinants within the context of actual viral infections. Transcriptomic and interactomic analyses at different stages of plant development revealed large differences in the number of genes affected by the different infections at medium infection times but no significant differences at very early times. Biological functions affected by UK 1 (the most severe strain) included mainly stress response and transport. Most cellular components affected cell-wall transport or metabolism. Hubs in the interactome were affected upon infection.