Movement proteins are the unsung workhorses of plant virus infection. Unlike animal viruses, which can be released into extracellular spaces and picked up by neighboring cells, plant viruses are trapped by the thick cell walls that surround every cell in a leaf. Their only route out is through plasmodesmata, microscopic channels that pierce the cell wall and connect the cytoplasm of adjacent cells. Movement proteins enlarge the size-exclusion limit of these channels and escort viral genomes, sometimes as intact particles, through the narrow gates. Since the movement protein of tobacco mosaic virus was first characterized decades ago, these proteins have been shown to dictate host range and disease severity, making their subcellular geography a matter of considerable agricultural importance.

Researchers at the CSIR–National Botanical Research Institute in Lucknow, India, led by Susheel Kumar, set out to close this gap for ToBRFV. Writing in Frontiers in Virology, the team describes how they engineered a fluorescent version of the virus’s 266-amino-acid movement protein and tracked its position in living cells of Nicotiana glutinosa, a wild tobacco species long used as a model host in plant virology. The work, published on 24 September 2026, represents the first experimental demonstration of plasmodesmata-associated localization for the ToBRFV movement protein, filling a conspicuous gap in the literature on this emerging pathogen.

The technical design of the study reflects the standard toolkit of modern plant cell biology, executed with careful attention to controls. The movement protein coding sequence, retrieved from the public database under GenBank accession NC_028478, was synthesized as a single open reading frame fused to a small c-Myc epitope tag at its N-terminus and a cyan fluorescent protein reporter at its C-terminus, connected by a flexible five-residue glycine-rich linker known as GGGGS. The entire cassette was placed under the control of the strong CaMV 35S promoter and cloned into the pCAMBIA2300 binary vector, producing a construct designated pCAM-ToBRFV-MP-CFP.

To deliver the construct into plant tissue, the researchers turned to agroinfiltration, a technique that hijacks the natural DNA-transfer machinery of Agrobacterium tumefaciens. The recombinant plasmid was introduced into the LBA4404 strain of the bacterium by freeze-thaw transformation, and bacterial suspensions adjusted to a specific optical density were gently pressed into the undersides of leaves on four- to six-week-old N. glutinosa seedlings using a needleless syringe. Within seventy-two hours, the infiltrated leaf zones were producing the fluorescent fusion protein, while macroscopic inspection confirmed that the tissue remained healthy, with no chlorosis, necrosis, or visible damage that might have confounded the imaging results.

Before trusting any fluorescence data, the team verified that the cells were producing an intact protein rather than fragments. Western blot analysis of total leaf protein, probed with an anti-c-Myc antibody, revealed a single immunoreactive band of approximately 59 kilodaltons, matching the predicted mass of the full fusion protein. Crucially, no smaller bands were detected, indicating that the fusion had not been cleaved or degraded, and no band appeared in control leaves infiltrated with empty vector or buffer alone. This biochemical validation ensured that every cyan fluorescent signal seen under the microscope corresponded to the genuine movement protein.

The microscopy itself delivered the study’s central observation. Under confocal laser scanning microscopy, epidermal cells expressing the MP-CFP fusion displayed a striking punctate fluorescence pattern confined to the inner cell periphery, with bright focal points lining the boundaries between adjacent cells. Weak, diffuse fluorescence occasionally appeared in the cortical cytoplasm, which the authors interpret as the protein in transit toward the plasma membrane, hinting at the intracellular trafficking journey that tobamovirus movement proteins are known to undertake along the endoplasmic reticulum, cytoskeleton, and vesicle transport pathways before reaching their final destination.

To confirm that these peripheral puncta truly marked plasmodesmata, the researchers stained the same leaf tissue with aniline blue, a dye that binds callose, the polysaccharide deposited around plasmodesmal necks. Merged fluorescence images revealed substantial spatial overlap between the punctate MP-CFP signal and the aniline-blue-labeled callose deposits, providing direct visual evidence that the movement protein accumulates at the intercellular channels. Control tissues showed only endogenous callose fluorescence, with no cyan signal, ruling out any contribution from the vector or the infiltration procedure itself.

The qualitative microscopy was reinforced by quantitative image analysis performed in Fiji/ImageJ. By drawing line profiles across adjacent cell boundaries and measuring fluorescence intensity from cytoplasm to cytoplasm, the team calculated the ratio of peripheral to cytoplasmic signal. In cells expressing MP-CFP, this ratio averaged 18.83, compared with only 3.08 in empty-vector controls, an approximately sixfold enrichment of fluorescence at the cell boundary. An unpaired two-tailed Welch’s t-test confirmed the difference was highly significant, with a P value below 0.0001. The pattern was reproduced across three independent biological replicates, with all confocal acquisition settings held constant to allow valid comparisons.

The findings anchor ToBRFV within the well-characterized framework of tobamovirus movement biology while opening several avenues for future work. The authors acknowledge that aniline blue labels callose surrounding plasmodesmata rather than the protein itself, so it cannot fully distinguish plasmodesmata from closely apposed structures such as the plasma membrane. They also note that the N-terminal c-Myc tag, while invaluable for detection, sits in a region of tobamovirus movement proteins thought to carry plasmodesmal targeting determinants, although the reproducible punctate localization suggests the tag did not abolish targeting under these conditions. Future experiments using genetically encoded plasmodesmal markers such as PDLP1, functional transport assays like the 2xGFP movement assay, and systematic mapping of the host proteins commandeered by the movement protein should clarify exactly how ToBRFV traffics between cells and where its dependence on host machinery might be exploited. Because the location of a movement protein determines how efficiently a virus spreads and how virulent the resulting disease becomes, this first map of the ToBRFV movement protein’s address inside the cell offers both a scientific baseline and a practical starting point for engineering durable resistance against a virus that continues to outpace the defenses bred against it.

Subject of Research: Subcellular localization of the tomato brown rugose fruit virus movement protein in plant cells

Article Title: Subcellular mapping of ToBRFV movement protein in Nicotiana glutinosa via confocal laser scanning microscopy and agroinfiltration

Article References: Dwivedi, R., Vishwakarma, T., Rathore, A., Maurya, D. K., Maurya, P. K., Selvaraj, V., Verma, P. C., & Kumar, S. (2026). Subcellular mapping of ToBRFV movement protein in Nicotiana glutinosa via confocal laser scanning microscopy and agroinfiltration. Frontiers in Virology, 6, Article 1870416. https://doi.org/10.3389/fviro.2026.1870416

Image Credits: AI Generated

DOI: 10.3389/fviro.2026.1870416

Keywords: ToBRFV, movement protein, plasmodesmata, Nicotiana glutinosa, agroinfiltration, confocal microscopy, CFP fusion, aniline blue, callose, tobamovirus, plant virology, subcellular localization