Infections by plant virus generally cause disease symptoms by interfering with cellular processes. Here we demonstrated that infection of Nicotiana tabacum ( N.t ) by plant viruses representative of the Tobamoviridae , Potyviridae , and Potexviridae families altered accumulation of certain microRNAs (miRNAs). A correlation was observed between symptom severity and alteration in levels of miRNAs 156, 160, 164,166, 169, and 171 that is independent of viral posttranscriptional gene silencing suppressor activity. Hybrid transgenic plants that produced tobacco mosaic virus (TMV) movement protein (MP) plus coat protein (CP) ^T42W (a variant of CP) exhibited disease-like phenotypes, including abnormal plant development. Grafting studies with a plant line in which both transgenes are silenced confirmed that the disease-like phenotypes are due to the coexpression of CP and MP. In hybrid MPxCP ^T42W plants and TMV-infected plants, miRNAs 156, 164, 165, and 167 accumulated to higher levels compared with nontransgenic and noninfected tissues. Bimolecular fluorescence complementation assays revealed that MP interacts with CP ^T42W in vivo and leads to the hypothesis that complexes formed between MP and CP caused increases in miRNAs that result in disease symptoms. This work presents evidence that virus infection and viral proteins influence miRNA balance without affecting posttranscriptional gene silencing and contributes to the hypothesis that viruses exploit miRNA pathways during pathogenesis.

Vitamin E (VTE) content is a low heritability nutritional trait for which the genetic determinants are poorly understood. Here, we focus on a previously detected major tomato VTE quantitative trait loci (QTL; mQTL_9-2-6) and identify the causal gene as one encoding a 2-methyl-6-phytylquinol methyltransferase (namely VTE3(1)) that catalyses one of the final steps in the biosynthesis of γ- and α-tocopherols, which are the main forms of VTE. By reverse genetic approaches, expression analyses, siRNA profiling and DNA methylation assays, we demonstrate that mQTL_9-2-6 is an expression QTL associated with differential methylation of a SINE retrotransposon located in the promoter region of VTE3(1). Promoter DNA methylation can be spontaneously reverted leading to different epialleles affecting VTE3(1) expression and VTE content in fruits. These findings indicate therefore that naturally occurring epialleles are responsible for regulation of a nutritionally important metabolic QTL and provide direct evidence of a role for epigenetics in the determination of agronomic traits.

The genome of tobacco mosaic virus (TMV) encodes replicase protein(s), movement protein (MP), and capsid protein (CP). On infection, one or more viral proteins direct the assembly of virus replication complexes (VRCs), in association with host-derived membranes. The impact of CP-mediated resistance on the structures of the replication complexes was examined in nontransgenic and transgenic BY-2 cell lines that produce wild-type CP, mutant CP ^T42W , and Ds-Red, which was targeted to endoplasmic reticulum by using immunofluorescence and 3D microscopy. We developed a model of VRCs that shows a clear association of MP with and surrounding the endoplasmic reticulum. Replicase is located within the MP bodies, as well as isolated sites throughout the cell. CP surrounds the VRCs. CP enhances the production of MP and increases the size of the VRC; however, the mutant CP ^T42W reduces the amount of MP and interferes with the formation of VRCs. We propose a regulatory role of the CP in the establishment of the VRC. We suggest that the lack of formation of VRCs restricts the efficiency of virus replication and the formation of virus movement complexes, resulting in restriction of cell–cell spread of infection. This results in higher levels of plant CP-mediated protection provided by CP ^T42W .

Plant viral infections induce changes including gene expression and metabolic components. Identification of metabolites and microRNAs (miRNAs) differing in abundance along infection may provide a broad view of the pathways involved in signaling and defense that orchestrate and execute the response in plant-pathogen interactions. We used a systemic approach by applying both liquid and gas chromatography coupled to mass spectrometry to determine the relative level of metabolites across the viral infection, together with a miRs profiling using a micro-array based procedure. Systemic changes in metabolites were characterized by a biphasic response after infection. The first phase, detected at one dpi, evidenced the action of a systemic signal since no virus was detected systemically. Several of the metabolites increased at this stage were hormone-related. miRs profiling after infection also revealed a biphasic alteration, showing miRs alteration at 5 dpi where no virus was detected systemically and a late phase correlating with virus accumulation. Correlation analyses revealed a massive increase in the density of correlation networks after infection indicating a complex reprogramming of the regulatory pathways, either in response to the plant defense mechanism or to the virus infection itself. Our data propose the involvement of a systemic signaling on early miRs alteration.