Hijacking plant immunity: winners and losers in dual pest and pathogen attacks on a shared host

Plants cannot run away when an attacker comes to attack them. Being unable to flee means that they are under pressure to resist attackers... they do this by switching on defences such as the production of antibiotic plant chemicals. Meanwhile, the attackers have adapted to supress plant defence responses to colonise their host. An interesting and important question is, "what happens when plants are exposed to more than one attacker?"

Here we consider the case of wheat, a globally important crop for global food security, and two of its key attackers: aphids (greenfly) and Fusarium head blight (FHB) disease. FHB is a major disease of wheat caused by Fusarium graminearium, an aggressive fungal pathogen. The disease also produces toxins (called mycotoxins), deoxynivalenol (DON) and nivalenol (NIV), in the grain, which are harmful to humans and animals when consumed. Cereal crop production is constrained by plant pests and diseases, which reduce the yield and quality of harvested grain. They are becoming more difficult for farmers to control because availability of pesticides is going down due to the evolution of pesticide resistance and changes in the law that ban pesticides, for example, neonics. Most previous studies of plant diseases and pests have considered them in isolation and little is known of the interactions between them. Aphids occur in cereal fields at the same time as FHB and interact with the disease and the wheat host plant.

Our project will discover how wheat plants respond to these attackers, not only on their own but also when exposed to dual attack. Our early findings show that FHB disease infection is doubled on plants with aphids when compared to clean plants. Here we will determine how and why this happens. We will investigate the biochemical and molecular basis of this aphid-induced plant defence suppression. We will conduct gene expression analyses of the wheat host and the aphid and will analyse differences in biochemical production. We will define and characterise the modulated host-defence networks in our biological experiments and determine their biological significance. Insects are influenced by the odours that plants release: a diseased plant smells different from a healthy one and may become repellent. We will collect plant odours, identify their chemical structures and expose aphids to them to test how they respond. We will make electrical recordings from insect antennae to determine which chemicals they can smell and do behaviour tests to see if they are attracted or repelled. Finally, the project will carry out experiments to determine if the aphids are able to metabolise the toxins produced by the FHB disease. The results from changes in the gene expression and metabolism of the aphid exposed to the mycotoxins will identify new detoxification pathways in the insect relevant to future targets for insect control. Our project will allow us to define novel molecular and metabolomic targets for making our crops more resilient to the aphid pest and the pathogen causing FHB disease in wheat.

The research will advance scientific understanding of how plants respond to combined attack from a pest and a disease. The information this project will provide is essential because we do not know how the aphids change the host to increase its susceptibility to FHB in wheat. This is a novel approach because most previous studies have overlooked how attackers sharing a host plant influence each other by manipulating the host. Outcomes of our work will create future opportunities to improve crop resilience to these attacking organisms.

It is important to understand how host defences are modulated by prior attack by other species because plants, in the field, are exposed to multiple attacking organisms. Our data show that Sitobion avenae and Fusarium graminearium profoundly affect each other by altering the condition of their shared host plant, wheat. Fusarium head blight disease severity doubles on aphid infested plants but diseased plants are poorer hosts for aphids. Our goal is to determine the mechanisms underpinning this host manipulation by the aphid or pathogen. We will use a multi-omics approach to define the changes in the host transcriptome and metabolome to identify modulated defence. To isolate local and systemically induced defence suppression we will perform experiments with distal and local aphid feeding on the host and explore the effect of duration of exposure to attackers. To separate the effect of aphid saliva and honeydew we will use bioassays with artificially collected saliva/honeydew to quantify effects on pathogen growth. Conversely, we will determine the effect of secondary fungal metabolites (deoxinivalenol (DON) and nivalenol (NIV)), host metabolites and volatiles on aphid behaviour and performance. Studies will be facilitated by available DON/ NIV producers and Tri5 knockout-transformant strains unable to produce the mycotoxins. Chemical ecology studies will use electrophysiological recordings from aphid antennae to identify bioactive disease induced volatiles and olfactometer bioassays to measure aphid behavioural responses. Transcriptomic and metabolomic analyses will define changes in the functional pathways of both the host plant and the aphid, including putative DON and NIV detoxification mechanisms in aphids. The project will thus determine how the presence of another attacking organism alter host defence and change the fitness of the interacting organisms. It will identify the consequences of aphid-pathogen interactions for disease and pest infestation in wheat.

The project will primarily benefit the R&D community seeking to develop novel approaches to crop protection and the farming industry, who face the reality of multiple attacking organisms in their crops. These parties will benefit from a better understanding of wheat plant responses to aphids and Fusarium head blight (FHB) disease. To give an indication of the scale of the challenge, economic losses from FHB in the USA in wheat and barley are £563 million over two years with secondary economic losses of £1.16 billion [1]. Fungicides are applied to 99% of conventionally produced wheat crops in the UK and receive an average of 3 sprays per season. Sustainable disease control via improved disease resistance would save the industry £2.1million per annum in recovered loss and spray costs.

The project will have added value as it will not only provide new information about plant-aphid interactions, particularly how aphids suppress plant defence, but also add to knowledge of plant-pathogen interactions and our understanding of how crops respond to dual attack. Information about plant responses to attack will be of value to industrial plant pathologists, entomologists and crop breeders working in the area of crop protection. Outputs from the project will inform future crop varietal selection for improved resilience against multiple enemies. It will also inform the identification of new bioactive compounds for insect control.

Specifically, the crop protection and farming industries are anticipated to benefit through provision of:

- New transcriptomic and metabolomic resources for wheat interaction with a serious pest (grain aphid) and pathogen (Fusarium graminearum)
- New targets for breeding for aphid and disease resistance in wheat
- New molecules with repellent and/or antibiotic activity against aphids
- Information for agronomists about the increased disease and mycotoxin risk to wheat in the presence of aphids
- New knowledge to support collaborative projects to exploit the research findings

FHB is of huge significance due to the health threat posed by the mycotoxins produced by the fungal pathogen. The food industry must minimise the risk of mycotoxins in foods, through good practice undertaken during growing, harvesting and storage. This project will provide insights into factors increasing mycotoxin risk and information will benefit the food and feed industry seeking to minimise risk. It will also be benefit health professionals and the wider public, by making a contribution to sustainable, safe food supply chains.

Development of sustainable crop protection is crucial for attaining sustainable intensification of agriculture [2] and pressures from pests and diseases are expected to increase with climate change [3]. Cereal crop production is constrained by plant pests and diseases, which reduce the yield and quality of harvested grain. They are becoming more difficult for farmers to control because availability of pesticides is going down due to the evolution of pesticide resistance and changes in the law that ban pesticides, for example, neonics.

(1) Nganje and Johnson (2003) J. Can. Ag. Econ. Soc. 4:16-26
(2) Pretty et al. (2018) Nature Sustainability 1: 441-446
(3) Bebber et al. (2018) Nature Climate Change 3, 985-988