The Genetic and Molecular Basis of Aphid Virulence

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Although aphids are major global crop pests, the mechanisms by which they successfully infest plants remain unknown, and very few aphid resistance genes have been defined in crops. We predict that salivary secretions include effector proteins and other molecules that during probing and feeding will result in suppression of host defences by virulent genotypes, or will trigger R-gene dependent resistance against avirulent genotypes. We will exploit the advantages of the pea aphid-Medicago truncatula model system to attempt to make a step change in the understanding of genetics and genomics of aphid virulence. Specifically, we have demonstrated Mendelian inheritance of at least one major determinant of virulence, and have shown that both the transcriptomes and the proteomes of virulent and avirulent aphids differ substantially in candidate secreted effector proteins. We will develop further segregating F1 and F2 aphid populations for assessment of virulence phenotypes. Whole transcriptome analysis of bulked segregant aphid clones by RNAseq and parallel proteomics will narrow down candidate effector genes, through detection of sequence polymorphisms, and comparison of RNA and protein level between virulent and avirulent groups. The top aphid candidate genes will be evaluated by delivery into Medicago leaf tissue, looking for effects on aphid performance and on plant response phenotypes. We will initiate translational studies to other aphid species of major UK importance, specifically testing the extent of conservation of effector gene functions between the specialist pea aphid and the generalist Myzus persicae. We will advance our understanding of cognate host resistance genes by characterising the function of RAP1, a major aphid resistance locus. Together, the outputs of this project will define the potential for genetic rather than agrochemical routes to crop pest control.

Economic importance of the problem we are addressing:
Aphids infest many crops worldwide, causing extensive direct damage and transmitting many economically important plant viruses. Economic losses due to such pests in the UK alone are in the order of £100 million a year. As highlighted by the Horticultural Development Company (HDC), there is a need for new chemistries with improved environmental profiles, and for non-chemical approaches to protect field crops from pests and diseases.
Global problems:
Insect infestations are expected to increase due to the combined impacts of climate change, pressure to reduce pesticide use, and intensification of agriculture. Up to 50% of crop losses in developing nations result from plant pests and pathogens according CABI. With the pressing need for sustainably increased food production worldwide, there is unprecedented demand to develop new strategies to protect crops from pests and diseases.
Scientific impacts:
Our combined advances in understanding of inheritance of aphid virulence and effector functions open up new opportunities for scientific advances. This potential is greatly augmented by availability of pea aphid and Medicago genomic sequence and gene expression resources, providing opportunities to increase our understanding of the molecular fundamentals of gene-specific defences. Wider benefits from deep understanding of the pea aphid - Medicago system can emanate from our testing of translation into other aphid species.

In addition to the several academic beneficiaries described above, the proposed research is therefore expected to benefit i) agri-food and related industries, ii) research staff, iii) the general public.
The proposed project will benefit agri-food and related industries by enabling development of novel effective control strategies against crop pests. We have had contact with key crop groups through AHDB representatives for horticulture, potato and cereal sectors, and with BBRO, the UK sugarbeet research organisation.
The project aims to identify aphid proteins that are essential for virulence and/or trigger immunity. Such proteins are potential targets for the development of novel control strategies in economically important crops based on, for example, RNAi or novel chemical compounds. In addition, this project will characterise plant proteins involved in resistance.
Career benefits:
Research staff involved in the project will benefit in terms of career development. The research draws on a vast array of molecular biology and biochemistry techniques that represent transferable skills in the biological sciences. With results of the research expected to be of high impact and of great interest to the research community, this project will likely generate multiple publications in peer-reviewed journals as well as lead to invitations to national and international meetings. In addition, opportunities will be provided to young talented undergraduate and postgraduate students to receive training within the research groups involved in this project, aiming to inspire and engage potential future academics in biological sciences research.
Environmental and health benefits:
A reduction in the use of insecticides will benefit the general public. Currently, aphid control strategies, and those of other plant pests, rely on extensive use of such chemicals, some of which pose a threat to human health and the environment. By instead using genetics to work towards robust resistant crops, use of chemicals is potentially reduced. Such benefits are within the framework of sustainable agriculture, contributing to protection of natural environments and reducing the environmental impact of agricultural practices.