Plant cells have an amazing ability to adapt and survive the impacts of virus infection. Cellular membranes provide a network for communication with all parts of the cell. Membranes vesicles transport cargo to various locations within and the cell as well as depositing cargo outside of the cell. The contain channels for ion signaling and sensors for activating changes in gene expression. All viruses depend upon the endoplasmic reticulum (ER) and golgi for protein synthesis, maturation and transport. All RNA viruses depend upon cellular membranes as scaffolds for virus replication, encapsidation, and egress. Creating these scaffolds requires significant rearrangement of cellular membranes. Our investigations are to understand the cell adaptive machinery to the changes that viruses bring to cells. We are particularly interested in the cellular adaptive machinery, known as the ER stress machinery, which is well conserved between plants and animals and humans. We study Potato virus X and Potato virus Y in their natural host, potato. We also study turnip mosaic virus and plantago asiatica mosaic virus in Arabidopsis. These four viruses cause significant economic damage to production of potato and leafy green vegetables. Potato virus X is an important model system in biotechnology, having played a central role in the discovery of critical cellular responses to virus infection, including gene silencing, gene mediated resistance, and intercellular transport mechanisms. Potato virus X and plantago asiatica mosaic virus are two species of the same genus, potexvirus. Potato virus Y and turnip mosaic virus are two species of a second genus, potyvirus. We identified two classes of small membrane binding proteins that trigger ER-to-nucleus signaling cascades. These viral proteins are recognized at the ER by different transmembrane stress sensors which enable nuclear translocation of transcription factors to upregulate expression of a wide range of genes. Our results show that a major outcome is to increase the protein folding capacity of the ER by increasing cellular chaperone expression. Other branches of the ER stress machinery control autophagy and cell death regulation. The primary goal of our work is to understand how plant virus proteins at the ER manipulate cellular signaling to maintain a cellular environment that is favorable for infection.
Karen-Beth G. Scholthof
A disease complex of panicum mosaic virus (PMV) and its satellites is being investigated in our laboratory. Panicum mosaic virus is a small RNA virus that infects members of the Gramineae. In the field it is commonly associated with a satellite virus (SPMV) and satellite RNAs (satRNAs). SPMV and the satRNAs are encapsidated by a coat protein expressed from the satellite virus genome, but both are dependent on PMV for replication. Interestingly, the gene regulation of PMV and symptom induction in the host is significantly perturbed by co-infection with SPMV and/or the satRNAs. The interaction of PMV with SPMV results in a serious disease on lawn and pasture grasses in Texas and the southern United States. Preliminary studies suggest that the disease enhancing feature is located on the satellite virus. Maize, pearl millet, and turfgrass are the experimental hosts that we use to analyze and identify the genes associated with replication, protein expression, and movement of each of the RNAs, and to investigate the interactions between PMV, SPMV, and the satRNAs. Our objectives include resolving the determinants for synergism and/or interference, gene regulation, and competitive mechanisms that pathogenic RNAs employ to maintain themselves during multiple infections. These data should provide some perspective on developing disease resistance or tolerance in plants with multiple virus infections.
Herman B. Scholthof
In my Plant Virology laboratory, we study the molecular mechanisms that determine whether a plant is susceptible or resistant to virus infection. Although there are multiple factors that influence the ability of a virus to infect a plant, crucial events are viral movement and the evasion of resistance responses, including RNA silencing or interference (RNAi). To investigate some of these aspects we use Tomato bushy stunt virus (TBSV) as a model system. TBSV has an RNA genome with five genes of which two (encoding for proteins designated P22 and P19) are involved in virus spread and which can also act as elicitors of resistance responses. P22 is required for cell-to-cell movement and P19 performs host-specific activities for virus transport. P19 is also a suppressor of RNAi through sequestration of short interfering RNAs (siRNAs) that are consequently unavailable to program any Argonaute (AGO) in the RNA-induced silencing complex (RISC). We are currently conducting experiments to examine the biochemical properties of P22 and P19 and to examine their interaction with host proteins. We also use P19 mutants and biochemical isolation procedures to characterize a novel virus-induced RISC-like complex from plants, and through genetic approaches aim to identify AGO proteins involved in antiviral silencing. A biotechnologically directed effort deals with exploiting virus proteins (e.g., suppressors) and replicons to increase expression of value-added foreign genes in suspension cells or whole plants.