Understanding the dynamics of root symbioses to improve the environmental management of an invasive species

Overview

Common gorse (Ulex europaeus L.) is a thorny evergreen shrub in the family Fabaceae, and native to the British Isles and Western Europe. Gorse is an important part of the UK landscape in terms of providing habitat (particularly for invertebrates and birds), stabilising poor or sandy soils and often used as a windbreak hedge on exposed land, and its early flowering is important for pollinators. Although most often associated with heathland or upland vegetation in the UK, gorse commonly invades neglected or disturbed land, and forests, and by forming dense impenetrable thickets can suppress plantation forests, exclude grazing animals from pastures, and increase the risk of fire in native habitats and urban areas. Due to the competitive success of gorse, it has become naturalised in many temperate countries around the world and is now regarded as a serious weed in New Zealand, Australia, Chile, North America, and Hawaii, and listed by the International Union for Conservation of Nature (IUCN) as one of the global top 100 invasive plants. Often found growing alongside gorse is the evergreen shrub common broom (Cytisus scoparius L.), which is also a successful invasive species outside of its natural range.

The competitive success of both gorse and broom is due to their large seed output and persistent soil seedbanks, the ability to modify their surrounding environment to favour their spread, and the short time to maturity and seed production. Importantly, gorse and broom are legumes, which means they can form symbiotic associations with certain soil-dwelling bacteria (called Bradyrhizobia) that form nodules on the plant roots. In these root nodules, Bradyrhizobia fix atmospheric nitrogen (N), which is then transferred to the plant and plays a key role in making gorse and broom dominant within their environment. Both plants can be replaced within plant communities through species succession (after 20-30 years); however, the occurrence of fire or other major disturbances can rejuvenate gorse and broom populations and promote further invasiveness.

It is not known whether Bradyrhizobia are ubiquitous in soils where gorse and broom are found. Importantly however, not all species of Bradyrhizobia are nodule-forming symbionts of gorse and/or broom and it is not clear how widely distributed the conserved gene for this symbiosis is within Bradyrhizobia populations. Therefore, the overarching aim of this project is to, (1) determine the spatial and temporal dynamics of the symbiotic interactions between the gorse and broom roots and natural populations of soil Bradyrhizobia, and (2) test the hypothesis that the competitive ability and invasiveness of these two plants is linked to specific strains of Bradyrhizobia.

Methodology

Using a combined field and laboratory-based experimental approach, this interdisciplinary project will provide the fundamental understanding necessary to refine environmental management of gorse and broom. We anticipate that this project will lead to novel management strategies for sustainably addressing the invasiveness of these legumes in both a UK and a global context and provide a template for the more general management of invasive weedy species. Specifically, the large gaps in our understanding of Bradyrhizobium dynamics with gorse and broom will be directly addressed through this studentship by focusing on the following questions:

1. How ubiquitous are Bradyrhizobia in the soil, and which specific strains produce nodules on the roots of gorse and broom?
2. Is the invasiveness of gorse and broom determined by specific strains of symbiotic Bradyrhizobia being present in the soil, or is Bradyrhizobia directly transmitted from one plant generation to the next through their seeds?
3. What is the effect of fire on the inoculum potential of the soil, and does gorse and broom need a root-nodule forming symbiont in order to colonise fire-disturbed soil?

Project Timeline

Year 1

Following a critical review of the literature (months: 0-4), an intensive field monitoring programme of upland gorse populations in Stirlingshire, Angus and Fife will be undertaken to determine spatial and temporal patterns of associations between Bradyrhizobia, and gorse and broom. This will involve isolation and molecular characterisation/sequencing of specific strains of Bradyrhizobia in soil samples and in root nodules of gorse and broom. This initial phase of the project will provide the strains that will be used in the subsequent phases of the project.

Year 2

A comprehensive series of field-relevant controlled laboratory and glasshouse experiments will investigate physiological and growth parameters of gorse and broom induced by a range of Bradyrhizobia strains isolated during the first year. This will provide an understanding of how specific bacterial symbionts are related to the ‘invasiveness’ of these plants.

Year 3
Field surveys investigating the affect of fire and disturbance will be conducted to determine the role of Bradyrhizobia on gorse re-colonisation dynamics, and the biological nitrogen fixation of gorse will be estimated by the 15N natural abundance technique. This will be complemented by glasshouse trials designed to manipulate a range of environmental variables (e.g., linked to climate change) to understand the effect of Bradyrhizobia in driving the dominance of gorse in plant communities.
Year 3.5

The student will spend the final six months writing up the thesis and papers for publication and presentation, and they will also develop a policy brief for environmental regulators and conservation organisations, e.g., SEPA and NatureScot.

Training
& Skills

By combining molecular, physiological and microscopy methods with ecological and environmental approaches during field-scale trials and manipulative pot-grown glasshouse experiments, this PhD project will provide both applied and empirical data on the symbiotic dynamics of specific strains of Bradyrhizobia.
This studentship will provide a platform to build an interdisciplinary research career in ecology, environmental management, soil science and biological conservation. Extensive skill development at the field-scale will be complemented by glasshouse scale manipulative experiments and include comprehensive training in microbiology and molecular biology, whilst benefiting from developing broad expertise in plant-soil-microbe interactions. The studentship will also broaden the scope of the applicant’s skills base by providing specialist training in microbiological techniques. The student will benefit from extensive co-supervision at the UKCEH and the James Hutton Institute with use of lab and extensive glasshouse facilities at the JHI site in Dundee. Supervision and training will be tailored to the individual needs of the student, who will benefit from internationally transferrable analytical, field-based and project management skills.

References & further reading

Sprent et al. (2017). Biogeography of nodulated legumes and their nitrogen‐fixing symbionts. New Phytologist, 215, 40-56.

Horn et al. (2014). Disparate origins of Bradyrhizobium symbionts for invasive populations of Cytisus scoparius (Leguminosae) in North America. FEMS microbiology ecology, 89, 89-98.

Weir et al. (2004). Unexpectedly diverse Mesorhizobium strains and Rhizobium leguminosarum nodulate native legume genera of New Zealand, while introduced legume weeds are nodulated by Bradyrhizobium species. Applied and Environmental Microbiology, 70, 5980-5987.

Further Information

Professor Richard Quilliam
Email: richard.quilliam@stir.ac.uk
Tel: 01786 467769

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