Crop vaccination without side effects: optimising the cost-benefit balance of chemically induced immune priming in greenhouse vegetables.

It is possible that over half of the crops cultivated each year are lost to pests and diseases. The use of chemical pesticides helps to reduce these losses, but many pesticides are becoming less efficient as diseases develop resistance, and others are being removed from the market because of fears that they might affect the environment or human health. The objective of our project is to develop a different approach to protecting crops by boosting their own immune system. This strategy, known as chemically induced immune priming, has been used to protect some crops against commercially important diseases, including grey mould and downy mildew. However, the commercialisation of these resistance-inducing chemicals has not been successful, because they often also repress plant growth. A recent breakthrough by our research team showed that it is possible to separate the resistance-inducing effects of the chemical priming agent beta-aminobutyric acid (BABA) from its growth repressing effects. Although BABA protects plants against an exceptionally wide range of commercially relevant diseases, it also stunts plant growth and affects crop yield when applied at higher doses. Recently, we discovered that the receptor for BABA controls disease resistance and the accompanying plant stress response via separate signalling pathways (Luna et al. 2014; Nature Chemical Biology 10: 450 - 456). The current project aims to translate these research findings from the model system (Arabidopsis) to vegetable crops (tomato and lettuce). The international plant breeding company Enza Zaden, one of the world's largest suppliers of vegetable seeds to farmers, are so excited about the opportunity to enhance the benefits of immune priming that they have offered to make their research materials available to us and to co-fund our work. Together, we have designed an entirely novel project to optimize chemically-induced immune priming in vegetable crops, using an integrated approach of genetic and chemical strategies to reduce input of pesticides in the vegetable industry. If successful, our approach can be used effectively to protect vegetable crops in a more sustainable manner from damaging diseases.

Immune priming provides plants with an enhanced defensive capacity against a broad spectrum of diseases. Beta-amino butyric acid (BABA) has emerged as a powerful priming agent in different plant species. However, a disadvantage of BABA is that over-stimulation reduces plant growth. This undesirable side effect has hampered commercialization of BABA as a crop defence activator. Previous work by the Ton lab has identified a key regulatory gene of BABA-induced priming in Arabidopsis, called IMPAIRED IN BABA-INDUCED IMMUNITY1 (IBI1). This gene encodes an aspartyl tRNA synthetase (AspRS), which binds in planta to the active R-enantiomer of BABA. This BABA-IBI1 interaction blocks default AspRS activity of IBI1, leading to a plant stress response that is caused by accumulation of uncharged tRNA and repression of gene translation. Independently of this stress response, BABA-IBI1 binding primes the IBI1 protein for an alternative defence function that becomes active after pathogen attack. Hence, IBI1 controls BABA-induced stress and BABA-induced resistance via separate pathways. Further evidence revealed that the trade-off between resistance and growth repression by BABA can be optimised by targeting IBI1 gene expression and chemical modification of the (R)-BABA molecule. This outcome forms the basis of this project, which follows a combination of genetic and chemical strategies to optimise IBI1-dependent resistance in tomato and lettuce. The project involves genetic alteration of the IBI1 gene, chemical synthesis and characterization of a new BABA analogue, quantification of chemical residues in crop products, and disease phenotyping to identify optimal crop-chemical combinations. The project will be conducted in partnership with ENZA Zaden, who are assisting with the identification of crop orthologs of IBI1. This project will allow Enza to generate cis-genic/non-GM mutant crop varieties that develop optimal levels of immune priming without affecting growth and yield.

Fungal and oomycete diseases pose a serious threat to the production of vegetable crops in greenhouses. At present, these diseases are mostly controlled by repeated pesticide applications. There is growing concern about pesticide resistance and their impacts on health and environment, which is reflected by recent changes in EU legislation to limit pesticide usage (e.g. The European Union Directive 2009/128/EC). The objective of this project is exploit the plant's immune system to protect two vegetable crops (tomato and lettuce) against commercially relevant diseases without compromising crop growth and yield. The project will be conducted in close partnership with Enza Zaden, a world-wide supplier of vegetable seeds, and will follow a combination of genetic and chemical strategies. Supported by solid preliminary evidence, this integrated approach ensures high levels of efficiency and successful deliverables for the industrial partner. Consequently, we are confident that this project will make a significant contribution to a more effective and sustainable strategy of disease protection in greenhouse-cultivated vegetables, thereby reducing the need for repeated applications of pesticides.