Push on through to the other side - molecular basis of viral cell-to-cell movement in plants

Lead Research Organisation: University of St Andrews
Department Name: Biology

Abstract

Plant viruses are responsible for many diseases of economically important crops. In the developing world, virally caused yield losses of staple crops can have direct and devastating effects for the local population, but viral diseases also cause substantial financial losses in industrialised agriculture. As the growing world population and climate change put increasing pressure on world-wide agricultural productivity, the ability to protect and defend crop plants from viruses forms an important part of global strategies towards sustainable food security. This project will investigate a crucial step in the life cycle of plant viruses, with the long-term aim of developing crops with improved virus resistance.
Because plant cells are separated by a cellulose cell wall (the source for making paper), the only pathway for viruses to spread their infection into new, uninfected host cells is through extremely small pores called plasmodesmata that connect plant cells across the cell wall. Cell-to-cell movement happens early during the infection as the virus has to stay ahead of host defences, and only a handful of viruses initiate infection in the next cell. Therefore this step is a prime target for interupting the infection cycle to protect the host plant. Plant viruses possess specialised "movement proteins" that mediate this intercellular transport. Because the viruses are large compared to the size of plasmodesmata, movement poteins need to 1) force open the pores and 2) push or pull their viral 'cargo' through them. The former of these two tasks is understood to some degree, but virtually nothing is yet known about the latter.
This project will test a new hypothesis which posits that movement proteins generate the mechanical force required for directional transport through plasmodesmata by assembling into filaments. Polymerisation of individual protein subunits into filaments releases binding energy and can thus generate mechanical force and also provide directionality. Despite overall differences, the movement proteins of most plant viruses form some sort of polymer or aggregates. This circumstancial evidence suggests that the proposed movement mechanism may be shared by the majority of plant viruses, so that resulting antiviral strategies will have a wide impact.
The project will test the proposed movement mechanism experimentally by reconstituting movement protein polymers in a 'test tube', and analysing the filament assembly process in detail. This will involve purifying movement proteins, combining them with lipid vesicles that serve as scaffolds, and identifying biochemical conditions as well as plant proteins that are required to trigger polymerisation. Sophisticated light and electron microscopy techniques will be employed to analyse the kinetics of the assembly and the physical properties of the polymers. If the underlying hypothesis is verified, plant proteins required for polymerisation constitute susceptibility factors that enable viral infection. Knowledge of these susceptibility factors will enable targeted breeding of more virus-resistant crops by screening seed banks and germplasm libraries for variant forms that do not support virus transport.

Technical Summary

Plant viruses cause large-scale economic damage and their control must form an integral part of strategies for sustainable food security. Transport of viral genomes from infected into uninfected cells constitutes an extreme bottleneck and is rate-limiting during early infection. It is thus a prime target for developing new antiviral crop protection strategies. Cell-to-cell movement is mediated by virus-encoded movement proteins (MPs) which target and dilate plasmodesmata (PD), the nanopores which connect all plant cells across the cell wall. MPs also bind viral genomes and are directly responsible for their transport through PD. Whilst considerable progress has been made in understanding how movement proteins target to, and dilate PD, the molecular mechanism by which MPs generate the necessary force to move megadalton-sized viral ribonucleoprotein complexes (vRNPs) through the pores remains unclear.
Whilst MPs of different plant virus groups show little overall similarity, some common properties can be identified in most MP systems: 1) they are membrane-associated; 2) at least one MP binds either viral genomes directly, or the capsid protein; 3) One MP in each system forms ordered polymers or aggregates, or shows strong homotypic interactions. Based on these common properties, it is proposed that different MP systems utilise the same general principle to drive transport through narrow PD channels: Directional polymerisation of MPs along a PD membrane generates a mechanical force which is transmitted onto the vRNP, thereby pushing or pulling it through the nanopore.
The project will test this hypothesis by assembling membrane-anchored MP polymers in vitro and analysing their properties by light and electron microscopy. The role of host factors in triggering MP polymerisation and anchoring of polymers will also be investigated.

Planned Impact

The project will have three types of beneficiaries: Academia, Commercial Private Sector, and General Public.
The research is of a fundamental nature and its most immediate impact will be to Academia. It will address a long-standing problem in plant pathology and increase the wider utilisation of new, cross-disciplinary methodologies amongst plant scientists and protein biochemists (see 'Academic Beneficiaries'), thus contributing to worldwide advancements in these fields, and to strengthening the competitiveness of the UK's knowledge-based economy, which relies heavily on scientific and technological innovation. In particular, it will contribute to maintaining the UK's status as the world's second highest impacting plant science research community. In their 2014 report on current status & future challenges, the UK Plant Sciences Federation identified inspiring & educating the next generation of plant scientists, and more efficient translation of plant science research into applications as top strategic priorities. This project will train a highly skilled PDRA, who will gain both plant science-specific and advanced technical, as well as generic & transferable skills. In the longer term, it will generate economical and societal impacts through two different aspects of the work:
1) Immediate commercial beneficiaries will be plant breeders and seed companies. Understanding the mechanism of plant virus movement will facilitate better antiviral protection of crops and improved food security in a changing climate. Host factors found to be required for movement protein polymerisation constitute susceptibility genes that enable targeted resistance breeding by screening available germplasm and seed banks for polymorphisms that prevent or reduce movement protein anchoring. As plant viruses are in a 'race' against defence responses, even moderate delays in movement will result in significant, durable improvements in resistance. Breeding will also be facilitated by understanding how this resistance works. It is expected that the results obtained with a limited number of model viruses will be applicable to the majority of plant viruses, and thus to the protection of most crops. Ultimately, the General Public will benefit from increased health & wealth through improved food security. In the UK and other industrialised countries, the benefits will mainly be in terms of minimizing economic losses, whereas in developing countries, catastrophic yield losses due to viruses will be reduced.
2) Additionally, the movement protein polymers studied here may be adaptable for nanotechnology or as novel RNA transfection tools; another area with potential for commercial exploitation. Protein tubules produced by the animal-infecting Blue tongue virus have been used to display epitopes for HIV vaccination (J Virol 79:14822 [2005]).It is likely that plant viral movement protein polymers can be employed similarly. They may also prove suitable surfaces for the assembly of nanoparticles. In the future, structural data may allow the engineering of movement proteins to attach to other membranes and deliver RNA into different cell types as a novel transfection tool, e.g. siRNAs into human cultured cells.
The General Public will also benefit through direct scientific education in the form of outreach activities. By communicating and explaining the research topic and outcomes to non-scientists, the project will increase public understanding of plant science and inspire the next generation of researchers.
Outreach and academic impacts will begin in year 1 and continue after project conclusion. Impacts on plant breeders will become possible from ~year 2/3 as susceptibility factors are identified, but dialogue with breeders will be established from year 1. Food security and other impacts will extend beyond the conclusion of the project and will include follow-up funding applications.
 
Description Plant viruses are major crop pathogens with a global impact on agro-economy & food security. They possess so-called movement proteins, which mediate the spread of the virus from infected into naive cells, and throughout the host plant. The molecular mechnism by which these movement proteins fulfil their transport function has remained unclear for decades.
Our work began with the hypothesis that movement proteins form polymers that act like "injection needles" for viral RNA genomes penetrating into uninfected cells. Research on this topic has been held back because these polymer-forming proteins are extremely intractable for experimental analysis. We have established protocols for producing the filaments in vitro, facilitating their functional analysis and potentitally the route to atomic level structure determination. Eventually, the research is expected to lead to the development of new crop protection strategies, e.g. anti-viral agri-chemicals.
Exploitation Route The findings require further/continued research, but will eventually lead to a molecular understanding of a critical step of plant virus infections that can be targeted for antiviral crop protection strategies. A follow-up funding application to take this forward towards more detailed functional and structural analysis is in preparation. Once the proteins under investigation are better understoof, they may also have utility in nanotechnology applications.
Sectors Agriculture, Food and Drink,Manufacturing, including Industrial Biotechology

 
Title New high-sensitivity RNA in vivo imaging system 
Description A new RNA live cell imaging system has been developed that for the first time is sensitive enough to track plant virus RNA genomes during cell-to-cell transport through plasmodesmata. 
Type Of Material Technology assay or reagent 
Provided To Others? No  
Impact The new RNA imaging system has not yet been published. 
 
Description Electron tomography 
Organisation University of Bordeaux
Country France 
Sector Academic/University 
PI Contribution provided plant samples
Collaborator Contribution prepared samples for electron tomography, collected data and reconstructed tomograms
Impact morphologial analyses of protein filaments formed in vivo
Start Year 2016
 
Description cryoEM analysis of protein filaments 
Organisation University of Dundee
Country United Kingdom 
Sector Academic/University 
PI Contribution providing purified protein samples
Collaborator Contribution 200 kV cryoEM imaging to obtain preliminary data to exploit feasibility of cryoEM structural analysis
Impact no outcomes yet
Start Year 2018