Priming of plant defences against pests and pathogens using seed treatments.

Crop plants are constantly under threat from a variety of insects and other animal pests, as well as numerous diseases caused by viruses, bacteria and fungi. During the last century, agricultural modernisation included the development of chemicals that can be used as pesticides to kill pests and pathogens. Pesticides can be very effective, but have the disadvantages that they can potentially damage the environment via effects on non-target species, which might include beneficial animals such as pollinating or predatory insects. This, along with associated retailer and consumer concerns related to the possible human toxicity of some conventional pesticides, has led to increased pressure to find alternative approaches to pest and disease control that are both environmentally- and consumer-friendly. Control methods based on genetic modification can be effective, but meet increasing consumer resistance in many areas. Another approach is integrated pest management (IPM), which combines cultivation practices with the use of parasites or predators of pests as a means of biological control. However, IPM rarely delivers the completely 'clean crop' that can be achieved using conventional pesticides, and there is a need for additional tools in the IPM armoury. One approach that may contribute is the development of methods to enhance plants' natural defence mechanisms. Our proposal centres around one such method which has recently been pioneered at Lancaster. It is based on the discovery that soaking seeds of plants in natural plant hormones that are normally involved in plant defence responses, leads to a long-lasting increase in pest and disease resistance in plants grown from those seed. We have previously shown that seed treatments using one particular hormone, jasmonic acid (JA), are effective in a range of crop species, including tomato, sweet pepper, cucumber, wheat and maize and provide increased resistance to different types of pest, including aphids, caterpillars and mites. Although the idea of using chemicals such as JA to activate natural plant resistance by application to growing plants is not new, the discovery that the treatment of a seed can provide long-lasting protection without adverse effects on plant growth or seed set is novel, and provides a simple strategy for deployment in agriculture and horticulture. However, we currently do not know how a plant 'remembers' the application of the activator to the seed that leads to an augmented induced defence response when subsequently attacked by insects or pathogens. Thus, the current project will investigate the underpinning science of the seed treatment phenomenon. We will compare the molecular (gene expression and epigenetic imprint) and biochemical (volatile organic compound) changes in seed-treated and untreated plants before and after pest attack in order to begin to understand how the seed treatment leads to greater levels of pest resistance. A second important aspect of the project will be to test the effectiveness of another compound, beta-aminobutyric acid (BABA) that is known to prime both salicylic acid-dependent defences and cell wall-based defences (associated with resistance to pathogenic fungi and oomycetes), when used as a seed treatment in tomato. By understanding how compounds can be used as seed treatments to boost plant resistance and by assessing the range of pests and diseases against which they offer protection, our project will generate an essential knowledge base that can be used in the exploitation of seed treatments in agriculture and horticulture.

Jasmonic acid (JA) is a plant hormone with key roles in defence against pests and pathogens. We have recently found that treating seed with JA prior to germination provides enhanced, long-lasting resistance to herbivores and fungal pathogens for periods of at least 8-10 weeks. Moreover, we have found that a second priming agent, beta-aminobutyric acid (BABA), also functions as a resistance-inducing seed treatment. Here, we will investigate the mechanism behind these effects using a multidisciplinary approach. The project will test hypotheses relating to gene expression, epigenetics-related effects, volatile emission, pathogen resistance and multitrophic interactions with insects to establish the science underpinning the phenomenon. We hypothesise that long-lasting induced resistance after seed treatment is based on priming of defence, which mediates a faster and/or stronger defence induction upon subsequent biotic attack. Experimental approaches, using control and seed-treated plants before and after pest attack, will comprise (i) gene transcript profiling, (ii) chromatin immunoprecipitation assays (iii) DNA methylation assays (iv) analysis of volatile emissions and (v) insect bioassays. We will determine impacts of JA and BABA seed treatments on direct defences against red spider mite (Tetranychus urticae). Impacts on indirect defences against herbivores (tritrophic interactions) will be studied with the predatory mite of T. urticae (Phytoseiulus persimilis). Effects on tomato resistance to the biotrophic fungus powdery mildew (Leveillula taurica) will also be explored. Using these systems, we will test the ability of JA and BABA seed treatments to work individually and in combination to improve biotic stress resistance and plant defence.