Developing novel seed treatments for grain legumes: optimising sustainable outcomes in agricultural systems
Increasing food security whilst simultaneously reducing environmental impacts and enhancing resilience to future climate are the key challenges facing agriculture. Agronomic sustainability can be facilitated by incorporating legume crops, such as peas and beans, into crop rotations. Legumes are able to fix atmospheric nitrogen through their symbiotic root-nodule bacteria (e.g. Rhizobia spp.), and thus reduce the need for synthetic fertiliser input. Successful root nodulation relies upon agricultural soils having a sufficiently high inoculum potential. However, intensively farmed soils are often lacking in populations of rhizobia due to the rotation of non-leguminous crops and high application rates of synthetic nitrogenous fertilisers. A strategy to combat this is to directly treat the seed with a concentrated inoculum of rhizobia, which ensures suitably high concentrations of root-nodule bacteria in the rhizosphere of the growing root. Because this technology is suitably advanced, there is now the opportunity to enhance and optimise this process by combining seed treatments that are able to simultaneously increase biological nitrogen fixation and induce disease resistance through the addition of plant growth-promoting rhizobacteria (PGPR) and resistance elicitors.
The mechanistic and physiological basis of such seed treatments however, needs critical evaluation in a sustainable agricultural context. Therefore, the focus of this studentship will be to develop novel legume-microbe seed treatments as either practical liquid, solid or seed coating formulations, and assess subsequent root nodulation, plant development and disease resistance in peas and faba bean, which are important agronomic legume species in both the UK and in the developing world. In addition to the benefits of legume seed treatments for European agricultural systems, this project will also explore how this technology can be transferred and adapted to local situations in developing countries for adoption by resource-poor farmers.
Specifically, the objectives of this project are to:
1. Characterise novel rhizobial isolates for improved inoculum longevity and improved N-fixing efficacy
2. Combine rhizobial isolates with other key symbionts such as PGPR and resistance elicitors (e.g. biomass-derived elicitors)
3. Determine the mechanistic and physiological basis of these novel seed treatments on different legume genotypes in terms of subsequent plant fitness, growth & yield
Study Summary
“Nitrogen (N) is a limiting element for plants; however, the use of synthetic N fertilisers in agriculture has increased crop production and yield. Importantly, a significant proportion of chemical fertilisers applied to soils will not be taken up by the roots of crops, but lost to the environment via run-off into waterways, or denitrification by soil bacteria. Legumes are plants that can transform atmospheric di-N into ammonia through a symbiotic association with rhizobia, a group of N-fixing bacteria, in root organs called nodules.
Natural populations of rhizobia often exhibit below optimal N-fixation or nodulation, although so-called 'elite’ strains with optimal abilities can be applied as inoculants. Although inoculants can be formulated with crop-compatible elite strains of rhizobia, their shelf-life is often compromised by high rates of cell die-off caused mainly by desiccation, which is an environmental stress that rhizobia are not good at withstanding. Therefore, there is a need to identify novel rhizobial strains that are able to tolerate desiccation stress. Recent evidence has suggested that strains isolated from areas with higher water deficit can better tolerate desiccation than those from wetter locations. Therefore, the overarching aim of this project was to isolate and characterise novel rhizobia strains from a semi-arid environment and assess their tolerance to desiccation for their potential use in inoculants for grain legumes. In addition, this project also evaluated the impact of agricultural land management on natural soil populations of rhizobia.
Over 80 strains were isolated from soil from a semi-arid area of Spain using pea as a trapping plant. After a series of glasshouse and growth room experiments two strains were tested in field trials during two consecutive seasons where they showed a similar performance to strains from commercially available inoculants (used as positive control strains). Desiccation tolerance of strains isolated from Spain was tested and compared in vitro against strains from a wetter environment. The strains isolated from the semi-arid region showed 1.55-fold increased tolerance to desiccation.
The genomes of 70 strains were sequenced and characterised, and a genome-wide association study on desiccation tolerance revealed that genes involved with regulating the concentration of solutes in the cytoplasm, and the protection and stabilisation of genetic material, were involved in the tolerance to this environmental stress.
Finally, it was found that a change in land management and the presence of legumes in the crop rotation increased nodulating rhizobia in soil by 15 and 30 % respectively over a period of 4 years. This project has successfully isolated strains with comparable symbiotic performance to standard commercial strains that show improved tolerance to desiccation, which makes them potentially superior for use in commercial inoculants with longer shelf-lives. Furthermore, this project has demonstrated that the reintroduction of a legume host after long absences produces an at least 4-year lasting effect that increases the proportion of nodulating rhizobia in soil year-on-year.”
Sponsors:
University of Stirling, Legume Technology Ltd., Processors and Growers Research Organisation (PGRO), and the James Hutton Institute.
Student:
Francesc Ferrando Molina - University of Stirling, 4 Union St, Stirling FK8 1NZ
Start Date: October 2017
Duration of study: 3 years
