James C. Carrington

A Counterdefensive Strategy of Plant Viruses: Suppression of Posttranscriptional Gene Silencing

Posttranscriptional gene silencing (PTGS) in plants inactivates some aberrant or highly expressed RNAs in a sequence-specific manner in the cytoplasm. A silencing mechanism similar to PTGS appears to function as an adaptive antiviral response. We demonstrate that the P1/HC-Pro polyprotein encoded by tobacco etch virus functions as a suppressor of PTGS. A locus comprised of a highly expressed beta-glucuronidase (GUS) transgene was shown to exhibit PTGS. Genetic crosses and segregation analyses revealed that a P1/ HC-Pro transgene suppressed PTGS of the GUS sequence. Nuclear transcription assays indicated that the silencing suppression activity of P1/HC-Pro was at the posttranscriptional level. These data reveal that plant viruses can condition enhanced susceptibility within a host through interdiction of a potent defense response.

Formation of plant RNA virus replication complexes on membranes: role of an endoplasmic reticulum‐targeted viral protein

The mechanisms that direct positive‐stranded RNA virus replication complexes to plant and animal cellular membranes are poorly understood. We describe a specific interaction between a replication protein of an RNA plant virus and membranes in vitro and in live cells. The tobacco etch virus (TEV) 6 kDa protein associated with membranes as an integral protein via a central 19 amino acid hydrophobic domain. In the presence or absence of other viral proteins, fluorescent fusion proteins containing the 6 kDa protein associated with large vesicular compartments derived from the endoplasmic reticulum (ER). Infection by TEV was associated with a collapse of the ER network into a series of discrete aggregated structures. Viral RNA replication complexes from infected cells were also associated with ER‐like membranes. Targeting of TEV RNA replication complexes to membranous sites of replication is proposed to involve post‐translational interactions between the 6 kDa protein and the ER.

Arabidopsis RTM2 Gene Is Necessary for Specific Restriction of Tobacco Etch Virus and Encodes an Unusual Small Heat Shock–like Protein

Arabidopsis plants have a system to specifically restrict the long-distance movement of tobacco etch potyvirus (TEV) without involving either hypersensitive cell death or systemic acquired resistance. At least two dominant genes, RTM1 and RTM2, are necessary for this restriction. Through a series of coinfection experiments with heterologous viruses, the RTM1/RTM2–mediated restriction was shown to be highly specific for TEV. The RTM2 gene was isolated by a map-based cloning strategy. Isolation of RTM2 was confirmed by transgenic complementation and sequence analysis of wild-type and mutant alleles. The RTM2 gene product is a multidomain protein containing an N-terminal region with high similarity to plant small heat shock proteins (HSPs). Phylogenetic analysis revealed that the RTM2 small HSP–like domain is evolutionarily distinct from each of the five known classes of plant small HSPs. Unlike most other plant genes encoding small HSPs, expression of the RTM2 gene was not induced by high temperature and did not contribute to thermotolerance of seedlings. The RTM2 gene product was also shown to contain a large C-terminal region with multiple repeating sequences.

Viral invasion and host defense: strategies and counter-strategies

The outcome of infection of plants by viruses is determined by the net effects of compatibility functions and defense responses. Recent advances reveal that viruses have the capacity to modulate host compatibility and defense functions by a variety of mechanisms.

Highlights from the Ninth International Congress on Molecular Plant–Microbe Interactions

The President of the International Society for Molecular Plant–Microbe Interactions (IS-MPMI), Barry Rolfe, wrote most insightfully in a recent online article about the importance of basic scholarship to the cultural integrity of modern civilized society (<http://www.scisoc.org/ismpmi/pubs/99spr.