ARS | Publication request: The "tobacco mosaic virus" 126-kDa protein associated with virus replication and movement suppresses RNA silencing

Submitted to: American Phytopathological Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 30, 2004
Publication Date: June 1, 2004
Repository URL: http://apsjournals.apsnet.org/doi/pdf/10.1094/MPMI.2004.17.6.583
Citation: Ding, X.S., Liu, J., Cheng, N-H., Folimonov, A., Hou, Y-M., Bao, Y., Katagi, C., Carter, S.A., Nelson, R.S. 2004. The "tobacco mosaic virus" 126-kDa protein associated with virus replication and movement suppresses RNA silencing. The American Phytopathological Society. 17(6):583-592.

Interpretive Summary: Proper gene expression and regulation are critical for plant growth, development, adaptation to environmental changes, and defense to pathogen infection. Recent studies indicate that eukaryotic cells, including plant cells, utilize a sophisticated system to identify and destroy aberrant gene transcript. This process is referred to as RNA interference (silencing). RNA silencing can function in plants as a defense mechanism against virus infection. In contrast, many viruses encode proteins that defeat RNA silencing to cause disease. However, the molecular mechanism underlying the virus-plant interaction is not fully understood. This study demonstrates that a viral protein is able to break down plant’s defense system through blocking RNA silencing activity, whereas a mutated variant of this protein can trigger RNA silencing which results in plant resistance. These findings provide invaluable insights towards manipulating plant resistance through genetically modification.

Technical Abstract: Systemic symptoms induced on "Nicotiana tabacum" cv. Xanthi by "Tobacco mosaic virus" (TMV) are modulated by one or both amino-coterminal viral 126- and 183-kDa proteins, proteins involved in virus replication and cell-to-cell movement. Here we compare the systemic accumulation and gene silencing characteristics of TMV strains and mutants that express altered 126- and 183-kDa proteins and induce varying intensities of systemic symptoms on "N. tabacum". Through grafting experiments, it was determined that M(IC)1,3, a mutant of the masked strain of TMV that accumulated locally and induced no systemic symptoms, moved through vascular tissue but failed to accumulate to high levels in systemic leaves. The lack of M(IC)1,3 accumulation in systemic leaves was correlated with RNA silencing activity in this tissue through the appearance of virus-specific, approximately 25 nucleotide RNAs and the loss of fluorescence from leaves of transgenic plants expressing the 126-kDa protein fused with green fluorescent protein (GFP). The ability of TMV strains and mutants altered in the 126-kDa protein open reading frame to cause systemic symptoms was positively correlated with their ability to transiently extend expression of the 126-kDa protein:GFP fusion and transiently suppress the silencing of free GFP in transgenic "N. tabacum" and transgenic "N. benthamiana", respectively. Suppression of GFP silencing in "N. benthamiana" occurred only where virus accumulated to high levels. Using agroinfiltration assays, it was determined that the 126-kDa protein alone could delay GFP silencing. Based on these results and the known synergies between TMV and other viruses, the mechanism of suppression by the 126-kDa protein is compared with those utilized by other originally characterized suppressors of RNA silencing.