Chemical enhancement of plant resistance to aphids

Aphids (greenfly and blackfly) are among the most important pests of crop plants. From a plant's point of view, being attacked by an aphid is very much like being bitten by a mosquito. Aphids probe the surface with long, sharp, needle-like mouthparts and drink from the 'phloem' tubes (the plant's 'arteries': a network of tiny vessels that transport sugar and other nutrients). Aphids reproduce very rapidly, so a colony quickly builds up on the plant, removing nutrients and reducing plant growth. Like mosquitoes, aphids also transmit disease (in this case plant viruses), and are therefore a major threat to crop production around the word. What do farmers do to protect their crops against aphids? In the vast majority of cases, the answer is: apply chemical insecticides. A problem with this approach is that the chemicals do not only kill the aphids. They also affect other animals in the environment, including those that normally eat aphids and help to keep the pest population in check. Another problem is that aphids are able to quickly evolve new strains that are resistant to insecticides, so the chemicals are not always effective and there is a need to continually search for new, more effective chemicals. This research project will investigate a new approach to protecting plants against aphids. We have recently discovered that we can control aphids by applying a chemical called 'BABA' to plants. BABA doesn't kill the aphids directly, but gives a boost to the plant's natural immune system. As a result, the plant is able to resist attack by aphids much more effectively and the insects grow and reproduce more slowly, and die more quickly. These new results are very exciting because it may be possible to develop new chemical sprays to 'vaccinate' plants against aphids and other pests/diseases. However, we first need to develop a better understanding of how BABA protects plants against aphids. Our research project aims to investigate three types of plant defence that may be boosted by BABA: 1) When plants are attacked by many insects, diseases or other enemies, they activate a range of defences which often make the plant tissue more difficult to penetrate and lead to the build-up of toxic / repellent chemicals. These 'basal' defences are not specifically activated by just one or a few attacking species, but are a common reaction to many plant enemies. The result may be a slowing down of the invasion by the insect or disease. But in many cases these defences are activated too slowly to completely stop the attack, so the attacker eventually wins the battle and successfully eats or infects the plant. We already know that BABA boosts these basal defences after the aphid has punctured a phloem tube, making feeding difficult. We will study in more detail what happens in the plant and how this affects the insect. 2) Another form of defence occurs in crop plants that have been bred for resistance. In many cases, having just one extra resistance gene ('R-gene') will completely protect the plant against the invader. This form of defence can be very effective because it is activated very rapidly (much more quickly than basal defence) and stops the invader at the first point of attack. But the disadvantage is that the resistance is highly specific - it recognises and reacts to only a few species or strains of attacker. We will therefore investigate whether R-gene resistance can be improved by BABA - can it be made even faster, or effective against a wider range of invaders? 3) Plants may also deploy a third, 'indirect' type of resistance to aphids. When plants are attacked by aphids, they often release chemical smells to attract other species of insect. The species being lured to the plant eat or parasitise the aphids, and are therefore beneficial to crops. We will also investigate whether BABA enhances this indirect defence mechanism, enabling the plant to attract beneficial insects more effectively

This proposal builds on our recent discovery that treating plants with a synthetic non-protein amino acid (beta-aminobutyric acid; BABA) induces very effective resistance to aphid pests under laboratory and field conditions. BABA does not affect aphids directly, but enhances plant defence responses, leading to disrupted phloem sap ingestion and poor growth, reproduction and survival by the insects. Comparing aphid-plant interactions on BABA-treated and control plants will improve our understanding of the processes involved in plant defence against aphids. We therefore propose to investigate the mechanisms behind BABA-induced resistance (BABA-IR) and its potential for exploitation as part of pest management strategies. In particular, we will focus on the effects of BABA on three types of active plant resistance to aphids: 1) Plant basal defences are deployed against a broad range of pathogens but often act too slowly to be effective against aphids. We will examine how BABA enhances phloem defences (particularly callose deposition and phloem metabolites) of normally aphid-susceptible plant genotypes and the consequences of these changes for aphid feeding behaviour. 2) Aphid resistance genes (R genes) often confer effective phloem-localised defence against aphids, but may not protect plants against a broad spectrum of aphid genotypes. We will investigate whether BABA-IR and R-gene-mediated resistance share common features, and whether an aphid genotype that is not affected by an R gene (virulent aphid clone) can be constrained effectively by BABA-IR. 3) A third type of resistance to aphids involves biosynthesis of plant volatiles that attract aphid natural enemies such as parasitoids. We will investigate whether BABA also enhances this 'indirect' defence mechanism.