Retaining the Ashes: The potential for ash populations to be restored following the dieback epidemic

Lead Research Organisation: John Innes Centre
Department Name: Crop Genetics

Abstract

Ash dieback has been a destructive disease of European ash since 1992 and was first seen in the UK in 2012. The fungus which causes this disease is native to East Asia and is thought to have arrived in north-east Europe during the 1980s. The dieback epidemic has serious implications for the UK given the ecological role of ash as a keystone species in nature, its economic value as timber, its amenity value in cities and parks, and its service value in landscaping and ecosystem functions, notably reduction of flooding by reducing water run-off from fields and consolidating river and canal banks. While most trees in heavily affected areas are severely damaged, a small minority are clearly less susceptible, raising a significant hope that this genetic variation might lead to the long-term recovery of ash in the UK and Europe generally.

This project aims to understand why this variation in dieback-resistance has evolved in European ash. This tree species is highly variable and the population in the UK has diverged substantially from that in continental Europe. Our hypothesis is that certain chemicals (secondary metabolites) confer resistance to damage by certain insects but increase the susceptibility of ash to dieback. As a resource for this project and for future research, we have collected 328 diverse ash genotypes from the UK, including numerous lines which have had low susceptibility to dieback even under intense disease pressure. This collection, known as JENNIFER, will be used to research the relationship between multiple traits to understand the ecological context of ash dieback.

Ash trees with low susceptibility to dieback may either have partial resistance to dieback, so their leaves are less easily infected by the fungus, or have a form of growth and development which leads to lower exposure to spores of the fungus, thus promoting disease escape. Objective 1 of the project is to characterise the diversity of low susceptibility to dieback in UK ash. We will measure disease severity in replicated field trials exposed to natural infection, investigate components of resistance, estimate contributions of resistance and escape to low susceptibility, and test if genetic resistance to dieback in UK trees has diverged from that studied previously in Denmark. Small-scale methods of studying infection by the dieback fungus, developed by the project partners, greatly increase the capacity to study variation in resistance.

In earlier research, low levels of iridoid glycosides, a class of secondary metabolite, were found to be associated with resistance to dieback but ash also contains many other secondary metabolites. Objective 2 is to test the hypothesis that diverse secondary metabolites in ash are associated with variation in dieback-resistance. We will use untargeted metabolite profiling to identify chemical features which most strongly discriminate infected and uninfected leaves of resistant and susceptible clones, combine metabolite and transcriptome analysis to determine the genetic basis of changes in secondary metabolites in response to infection, characterise temporal variation in levels of discriminant molecular features, and relate them to resistance in the JENNIFER panel.

Objective 3 is to test the hypothesis that secondary metabolites control contrasting responses of ash to dieback and to diverse herbivorous insects. This will be done by testing if variation in dieback-resistance and levels of key metabolites are associated with variation in insect feeding, growth and reproduction, and by assessing the susceptibility of the JENNIFER ash clones to attack by invertebrates in natural conditions.

Technical Summary

Ash dieback, caused by the invasive fungus Hymenoscyphus fraxineus (Hf), has been a destructive disease of European ash (Fraxinus excelsior) since 1992 and was first seen in the UK in 2012. While most trees are very susceptible to this alien pathogen, a small minority are less diseased. As ash in Europe was not previously exposed to Hf, this project aims to understand the evolutionary origin of this polymorphism in terms of the forces of natural selection which act on dieback-susceptibility.

We have assembled 328 diverse ash lines from the UK, including many which have low dieback-susceptibility even at heavily affected sites. In Objective 1, we will estimate the contributions of resistance and disease escape to low susceptibility in this collection in replicated field trials exposed to natural infection. We will use controlled inoculation in lab and greenhouse experiments to investigate components of resistance and assess the divergence of resistance in the UK from that in continental Europe.

In a limited sample of trees from Denmark, dieback-susceptibility was correlated with levels of iridoid glycosides in uninfected leaves. In Objective 2, we will combine untargeted metabolite analysis by LC-QToF-MS with transcriptome analysis to survey a very wide range of secondary metabolites (SM), both constitutive and induced on infection, and identify SM and biosynthetic pathways that discriminate responses of resistant and susceptible ash to Hf. We will then test the association between SM and responses to Hf across our panel of ash lines.

Diverse SM, including iridoid glycosides, reduce attack by invertebrate herbivores by acting as repellants, anti-feedants or toxins. In Objective 3, we will test the hypothesis that certain SM mediate a trade-off between responses of ash to dieback and to certain insects, by testing if dieback-resistance and levels of key SM in ash clones are correlated, positively or negatively, with insect feeding, growth and reproduction.

Planned Impact

Ash dieback has caused severe damage to European ash (Fraxinus excelsior) since it was first observed in north-eastern Europe in 1992. It was first seen in the UK in 2012 but may have been present here since c.2000. Although the fungus kills juvenile shoots quickly, disease progress in mature trees is slow but relentless, causing death in 5-10 years. In the most severely affected counties of the UK, almost all trees are infected and some ashwoods have been devastated. It is striking, however, that a minority of ash trees are much less diseased than others of the same size and age nearby.

Ash is a keystone species in the natural environment which supports diverse wildlife, a source of high-quality timber for furniture, tools and sporting goods and an important tree for landscaping and shade in urban and rural settings including residential areas, car parks and industrial estates. Perhaps the most significant benefit of ash is for ecosystem services, including such as flood prevention by preventing run-off of rainwater from fields, soil erosion and collapse of river and canal banks.

We have assembled a panel of 328 ash genotypes, known as JENNIFER, for use in this project and as a resource for future research. An important impact of this project is that JENNIFER contains a substantial number of lines with strong partial resistance to dieback. Seed from trees such as these has the potential to re-establish ash for all the purposes mentioned above, although further selection will be needed to identify plants with good form for timber production. A previous project discovered genetic markers for resistance among trees from Denmark but they have limited power to predict resistance in ash from the UK. Moreover, reliance on a few markers should be avoided in plant breeding, to minimise the risk of pleiotropy or linkage drag with undesirable traits. Further understanding of the pathology, diversity and ecology of ash dieback in the UK is therefore required.

In a small sample of the Danish study population, certain secondary metabolites (SM), assigned as iridoid glycosides (IG), were associated with susceptibility to dieback. IG are well-known as compounds which deter insect herbivory in diverse plants but it is not yet known why low IG levels are associated with resistance to dieback. It is thus possible that if selection for dieback-resistance were to reduce levels of IG, susceptibility to herbivores such as insects could be enhanced, so replacing one destructive agent by another and thwarting attempts to restore ash populations. An insect of special concern is another invasive alien species, the emerald ash borer, which has been extremely destructive in the USA, Canada and Russia. Fraxinus species contain many diverse SM, however, so it may be possible to select for (or against) different metabolites which confer resistance (or susceptibility) to dieback and to herbivores, or even for metabolites which have positive effects on both types of bio-antagonist. Exciting advances in technology for identifying low levels of metabolites mean it will soon be possible to select F.excelsior breeding stock with a high probability of good resistance to dieback and to herbivores, based on sensing of multiple odours.

The project will also contribute essential information for informed selection and replanting of ash with lower susceptibility to dieback, for the benefit of public bodies, charities and forestry companies concerned with tree-planting. It will reveal the diversity of dieback-resistance in ash from the UK and indicate the risk of trade-offs between dieback-resistance and deterrence of herbivores mediated by secondary metabolites. In particular, it will help greatly to predict the likely damage by emerald ash borer, should that highly destructive beetle ever become established in the UK. It will also identify traits which can be selected in order to promote disease escape.

Publications

10 25 50
 
Description Contribution to developing DEFRA policy for management of ash in the UK.
First Year Of Impact 2018
Sector Agriculture, Food and Drink,Environment,Leisure Activities, including Sports, Recreation and Tourism,Transport
Impact Types Societal,Economic,Policy & public services

 
Description Collaboration on emerald ash borer 
Organisation East Malling Research
Country United Kingdom 
Sector Academic/University 
PI Contribution Identification of suitable Fraxinus excelsior accessions from the JENNIFER collection for emerald ash borer trials. Data analysis. Leading the publication.
Collaborator Contribution RBG Kew: Formation of collaboration and planning experiments. EMR: Provision of plant material for EAB experiments. OSU: Conduct of experiments and data analysis.
Impact A paper has been accepted subject to revision, reporting data on response of UK Fraxinus excelsior to emerald ash borer in controlled trials.
Start Year 2016
 
Description Collaboration on emerald ash borer 
Organisation Ohio State University
Country United States 
Sector Academic/University 
PI Contribution Identification of suitable Fraxinus excelsior accessions from the JENNIFER collection for emerald ash borer trials. Data analysis. Leading the publication.
Collaborator Contribution RBG Kew: Formation of collaboration and planning experiments. EMR: Provision of plant material for EAB experiments. OSU: Conduct of experiments and data analysis.
Impact A paper has been accepted subject to revision, reporting data on response of UK Fraxinus excelsior to emerald ash borer in controlled trials.
Start Year 2016
 
Description Collaboration on emerald ash borer 
Organisation Royal Botanic Gardens, Kew
Country United Kingdom 
Sector Charity/Non Profit 
PI Contribution Identification of suitable Fraxinus excelsior accessions from the JENNIFER collection for emerald ash borer trials. Data analysis. Leading the publication.
Collaborator Contribution RBG Kew: Formation of collaboration and planning experiments. EMR: Provision of plant material for EAB experiments. OSU: Conduct of experiments and data analysis.
Impact A paper has been accepted subject to revision, reporting data on response of UK Fraxinus excelsior to emerald ash borer in controlled trials.
Start Year 2016
 
Description Retaining the Ashes collaboration with Warwick and E.Malling 
Organisation East Malling Research
Country United Kingdom 
Sector Academic/University 
PI Contribution Led collaboration to develop research proposal on the influence of secondary metabolites in European ash (Fraxinus excelsior) on possible trade-offs between resistances to ash dieback and to herbivores. As well as coordinating the project, JIC is working on plant pathology, entomology and transcriptomics.
Collaborator Contribution Contributed to research proposal on the influence of secondary metabolites in European ash (Fraxinus excelsior) on possible trade-offs between resistances to ash dieback and to herbivores. EMR is working on different aspects of plant pathology, complementary to the work at JIC. Warwick is working on metabolomics, coordinated with research at JIC on transcriptomics.
Impact Response-mode grant proposal funded by BBSRC. Multi-disciplinary: pathology, entomology, genetics, population genetics, ecology, forestry, analytical chemistry, plant molecular biology.
Start Year 2017
 
Description Retaining the Ashes collaboration with Warwick and E.Malling 
Organisation University of Warwick
Country United Kingdom 
Sector Academic/University 
PI Contribution Led collaboration to develop research proposal on the influence of secondary metabolites in European ash (Fraxinus excelsior) on possible trade-offs between resistances to ash dieback and to herbivores. As well as coordinating the project, JIC is working on plant pathology, entomology and transcriptomics.
Collaborator Contribution Contributed to research proposal on the influence of secondary metabolites in European ash (Fraxinus excelsior) on possible trade-offs between resistances to ash dieback and to herbivores. EMR is working on different aspects of plant pathology, complementary to the work at JIC. Warwick is working on metabolomics, coordinated with research at JIC on transcriptomics.
Impact Response-mode grant proposal funded by BBSRC. Multi-disciplinary: pathology, entomology, genetics, population genetics, ecology, forestry, analytical chemistry, plant molecular biology.
Start Year 2017
 
Description Defra Ash Research Strategy 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Policymakers/politicians
Results and Impact Workshop on Defra Ash Research Strategy to develop policy on management of Chalara ash dieback in the UK. James Brown (JIC) gave a talk on prospects for evolution of resistance of ash to dieback by natural selection, and on possible trade-offs between resistance to dieback and responses to invertebrates.
Year(s) Of Engagement Activity 2019
 
Description Discussions with DEFRA about ash research requirements 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Policymakers/politicians
Results and Impact Discussion with DEFRA officials about the impact of current research on ash dieback, notably research at JIC, for policy on control of ash dieback and for international trade in live plants, timber and firewood. These conclusions of the discussion fed into a strategy meeting held in January 2019 and will thus feed into a strategy document being written by DEFRA.
Year(s) Of Engagement Activity 2018
 
Description Public involvement in collection of ash accessions 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Collection of seed from 20 ash trees in Norfolk which show clear indications of having resistance to Chalara ash dieback. The collection was made with input from farmers, charitable organisations and members of the general public. A portion of the seed has been deposited at RBG Kew's Millennium Seed Bank. The remainder will be used in research on genetics of ash dieback and in restoration projects (plans are in progress).
Year(s) Of Engagement Activity 2018