Workshop Summary

Plant viruses in tropical regions, particularly Africa, were the focus of the lead-off session of the 4th PVEN workshop co-sponsored by CIRAD and the NSF-funded PVEN. Africa is arguably the most important stage on which emergence of viruses influenced by human activity and, at the same time, serious impacts of viruses on human activity are played out. Two emergences received the spotlight. Darren Martin analyzed the emergence and spread of Maize streak virus (MSV) across Africa from East to West and presented calculations that indicated that the edge of the MSV threat to maize moves at 35 km per year. Claude Fauquet explained the seriousness of viral threats to cassava production in Africa. The synergistic disease interaction caused by co-infection with African and East African cassava mosaic viruses is becoming complicated by the recent emergence of cassava brown streak disease. These and other emergences motivated a passionate plea by Walter Leke Nkeabeng for more investigation of the world of viruses lurking in native plants in Africa.

Three aspects of ecosystem and landscape ecology related to viruses were considered in additional sessions. One aspect, presented by Val H. Smith, was the effect of resource variation on productivity and diversity in ecosystems, and the dynamics between viruses and hosts in regard to nutrients such as carbon and phosphorus. A second aspect, presented by Abbie Schrotenboer, explored the consequences of large scale changes in human use of land, such as the planting of large acreages in biofuel crops, such as switch grass. In another aspect, environmental gradients, such as of moisture and temperature, and their effects on pathogen resistance genes of hosts and on pathogen and vector prevalence were presented by Karen Garrett. A molecular explanation of the effect of virus infection on host fitness was proposed by W. Allen Miller and Carolyn Malmstrom discussed the influence of anthropogenic selection pressures on plant traits and consequences for host-virus interactions.

New viruses continue to be discovered, emerging into the theater of our attention, exemplified by a probably new Vitivirus described by Nancy Robertson. Yet, studies of viruses in weed and crop plants reveal continued occurrence of many known viruses such as in cucurbits of Oklahoma (Akhtar Ali). In possible contrast to studies on non-cultivated plants, deep sequencing studies on cultivated plants and their weeds are revealing the presence of many known viruses (Thierry Candresse, Hui Wang).

Deep sequencing was also used to gain insights into the generation of variants within a plant and the consequent emergence of novel viruses, an interplay of ecology and evolution. Variant genomes may thrive better than old genomes when they encounter new niches. Yet, mutation also leads to lethality. To investigate this balance (Gaël Thébaud), deep sequencing of infected plants revealed accumulation of polymorphisms in about 3% of sites. Some variants are positively selected. Phenotypic effects of some of these mutations were identified by Benoit Moury. Their nature suggested that the mutations are conditionally beneficial to virus fitness. Also important in the emergence of new viruses is how efficiently variants that arise are transmitted to other cells and tissues. In a statistical study of virus variants tagged with differently fluorescing fluorescent protein genes (Serafín Gutiérrez), the multiplicity of infection varied with growth stage of the plant, being lowest in young plants and flowering plants and highest in the rapid growth phase.

On the final day of the workshop, virus-vector relationships were considered. Following a presentation of experimental observations on the relationship of aphid preferences to virus transmission by Nilsa Bosque-Pérez, two presentations on modeling the relevant interactions were made. Samuel Alizon examined the feedback loop between ecology and evolution (trait evolution and selective pressure) during infection of single hosts while Bryan Roossien modeled the spread of viruses as functions of plant disease status and vector behavior.

The workshop concluded with an effort to bring together opinions on a research agenda for plant virus ecology. At the 4th PVEN Workshop in Montpellier 31 May- 2 June, attendees discussed items they would like to see on the agenda for research in plant virus ecology. See the agenda.

Sessions:

Overall theme:

Viruses exist in an ecological context and play important roles in that context.  The context includes hosts, vectors and the physical environment.

Objective:

This workshop brought together ecologists, virologists, vector biologists and scientists in related disciplines to discuss emerging scientific issues, forge collaborations, exchange information and contemplate future directions for plant virus ecology. 

Sessions:

Session 1: Tropical environments(Denis Fargette, Oumar Traore, Darren Martin, Claude Fauquet, Abdessalem Tahiri, Walter Leke Nkeabeng, Fidèle Tendrebeogo, Justin Pita, Cic Urbino)

Tropical and sub-tropical environments are especially important because of their large diversity and complexity. There is little information available on the ecology of plant viruses under tropical and sub-tropical conditions. This session will include presentations by scientists working on the ecology of plants as affected by viruses in tropical and sub-tropical conditions. The session will encourage comparisons of the ecology of plants and viruses in different climatic zones. Such comparisons are important as all continents consider the effects of climate changes on the interactions of viruses and ecosystems.  Studies in tropical and sub-tropical regions provide instructive observations on the ecology of plant viruses at the interface between wild and cultivated ecosystems.

Session 5:  Ecology of virus emergence (Ulrich Melcher Serafin Guttierez, Gaël Thébaud, Benoit Moury, Akhtar Ali, Nancy Robertson, Thierry Candresse, Hui Wang)

Major shifts in ecosystem composition, such as inadvertent or purposeful introduction of new vectors or plants, are often associated with viral emergence.  Parameters that are important in conditioning emergence include densities and biodiversities of insusceptible and susceptible plant species, virus population diversity, the fitness landscape of the virus on different hosts, its mutation rate, the number and size of bottlenecks, vector poulation sizes and diversities, the efficiency of transmission, the capacity for adaptation, epistasis among mutations and other factors.  Some of these parameters will be examined more deeply in this session.

Session 2: Poster Discussion

Solicited poster submissions were clustered according to the topic areas of the five other sessions.  Brainstorming and discussions around the posters were encouraged.

Session 4: Expanding perspectives in plant virus ecology: host fitness (W. Allen Miller, Carolyn Malmström)

Many sources describe viruses as pathogens.  Yet pathogenesis is not an inherent property of viruses.  Indeed, virus-like agents are now know that have no know pathogenicity associated with them.  Some observations suggest that viruses have existed for long periods of time, implying that viruses have evolved as part of complex ecosystems and must therefore play important roles in such ecosystems. They likely serve evolutionarily selected functions in ecosystems.  Depending on host plant species, viruses may have commensal or mutualistic relationships with their hosts in addition to pathogenic ones with other hosts.  The viruses also interact in interesting multitrophic ways with organisms other than plants in ecosystems (vectors, herbivores, etc.).  Some of these multiple, positively selectable interactions will be considered in this section.

Session 3: Expanding perspectives in plant virus ecology: ecosystem and landscape ecology (Carolyn Malmström, Val H. Smith, Abbie Schrotenboer, Karen Garrett)

Understanding large scale interactions affecting plants and their relationships with viruses will be a focus of this session. A variety of environmental gradients varying parameters such as day-length, altitude, average daily temeperature, moisture, landscape fragmentation, etc. , can be exploited to discover such interactions. 

Session 6: Expanding perspective in plant virus ecology: virus-vectorrelationships (Nilsa Bosque Pérez, Samuel Alizon, Bryan Roosien)

Modeling interactions at multiple ecological levels allows testing our understanding and creation of predictive tools.

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