Understanding the impact of soil nitrogen on plant disease resistance

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Plants obtain most of the nitrogen they need for growth and metabolism in the form of inorganic nitrogen ions such as ammonium and nitrate. Nitrogen ions are absorbed by roots and used to make amino acids that can be transported throughout the plant. Plant growth and development is regulated by the availability of nitrogen in soil, and nitrogen is frequently a growth limiting nutrient in natural ecosystems. Soil nitrogen and plant growth can be increased by treating soil with nitrogenous fertilisers, but the level and type of nitrogen used must be carefully controlled. High levels of nitrogen, especially ammonium, are toxic to some plants and moderately high levels promote lush vegetative growth that is susceptible to pests and diseases. An additional source of concern is that plants do not take up all of the nitrogen that is applied as fertilisers. Excess fertilisers are costly for farmers and act as environmental pollutants that can promote algal blooms through run-off into lakes and rivers and disturb natural ecosystems. Furthermore, increases in diseases and pests in fertiliser treated plants may require additional applications of pesticides and fungicides, again at an added cost to farmers and the environment. In this project we are particularly concerned with the link between soil nitrogen and increased plant disease. Researchers have observed that high soil nitrogen results in increased levels of inorganic nitrogen ions in plant tissues and alterations to both primary and secondary metabolism. The increased pest and disease susceptibility observed in over-fertilized plants could be due to two processes. Firstly, alterations to plant metabolism may make more nutrients available to pathogens (disease causing organisms such as bacteria and fungi). Secondly, the complex biosynthetic pathways used to synthesise anti-microbial chemicals may be suppressed by high soil nitrogen, making plants less able to defend themselves against infection. Intriguingly, some changes in plant physiology caused by high soil nitrogen resemble those caused by pathogen infection, which suggests that pathogens produce chemicals that inhibit and alter plant nitrogen metabolism in order to promote pathogen growth. We will use the interaction of the bacterial plant pathogen Pseudomonas syringae pv. tomato with tomato and the model plant Arabidopsis thaliana to investigate the role of soil and leaf nitrogen in disease resistance. P. syringae pv. tomato colonises the spaces between plant cells, taking nutrients from the apoplastic fluid that surrounds plant cells. This bacterium uses secreted proteins, toxins and hormones to control plant metabolism and can reach levels of 10 million bacteria/cm2 in the leaves of susceptible plants. We aim to describe the effect of soil nitrogen concentration on disease resistance to P. s. pv. tomato, and to measure the composition of apoplastic fluid in healthy and infected plants. We will specifically examine whether apoplastic fluid from plants treated with high levels of nitrogen supports higher rates of bacterial multiplication, and whether bacteria induce changes in apoplastic fluid that promote bacterial multiplication. We will also examine whether and how soil nitrogen affects the ability of plants to defend themselves against pathogens. The results of these analyses will provide three clear benefits. Firstly, they will clearly describe the mechanistic link between soil nitrogen and disease resistance. Secondly, we will be able to use this information to design experiments that use apoplastic composition analyses to optimise fertiliser composition and application. Finally, we may be able to use pathogen-induced changes in apoplast composition as an early sign of infection, facilitating early intervention and disease prevention.