Understanding the impact of soil nitrogen on plant disease resistance

植物以无机氮离子（如铵和硝态氮）的形式获得生长和代谢所需的大部分氮。氮离子被根部吸收，用来制造氨基酸，这些氨基酸可以在整个植物中运输。植物的生长发育受土壤氮素有效性的调控，而氮素在自然生态系统中往往是一种限制生长的养分。用氮肥处理土壤可以增加土壤氮和植物生长，但必须仔细控制所使用氮的水平和类型。高水平的氮，特别是铵，对某些植物是有毒的，而适度高水平的氮促进繁茂的植物生长，易受病虫害的影响。另一个令人担忧的问题是，植物并不会吸收所有用作肥料的氮。过量的化肥对农民来说代价高昂，而且会成为环境污染物，通过流入湖泊和河流促进藻类大量繁殖，扰乱自然生态系统。此外，经过肥料处理的植物的病虫害增加，可能需要使用更多的杀虫剂和杀菌剂，这又增加了农民和环境的成本。在这个项目中，我们特别关注土壤氮与植物病害增加之间的联系。研究人员观察到，高土壤氮导致植物组织中无机氮离子水平增加，并改变了初级和次级代谢。在过度施肥的植物中观察到的病虫害易感性增加可能是由于两个过程。首先，植物代谢的改变可能会使病原体（引起疾病的生物体，如细菌和真菌）获得更多的营养。其次，用于合成抗微生物化学物质的复杂生物合成途径可能被高土壤氮抑制，使植物抵御感染的能力降低。有趣的是，土壤高氮引起的植物生理变化与病原体感染引起的生理变化相似，这表明病原体产生抑制和改变植物氮代谢的化学物质，以促进病原体的生长。我们将利用植物细菌病原体丁香假单胞菌pv的相互作用。以番茄和模式植物拟南芥为研究对象，探讨土壤和叶片氮在抗病中的作用。丁香属；番茄寄生在植物细胞之间的空隙中，从植物细胞周围的胞外液中获取营养。这种细菌利用分泌的蛋白质、毒素和激素来控制植物的新陈代谢，在易感植物的叶片中可以达到每平方厘米1000万个细菌的水平。研究了土壤氮素浓度对油菜抗病性的影响。并测定了健康植株和感染植株外胞浆液的组成。我们将专门研究来自高水平氮处理的植物的外胞体液是否支持更高的细菌繁殖率，以及细菌是否诱导外胞体液的变化促进细菌繁殖。我们还将研究土壤氮是否以及如何影响植物抵御病原体的能力。这些分析的结果将提供三个明显的好处。首先，他们将清楚地描述土壤氮与抗病性之间的机制联系。其次，我们将能够利用这些信息来设计实验，使用外质体成分分析来优化肥料成分和施用。最后，我们可能能够利用病原体诱导的外质体组成变化作为感染的早期迹象，促进早期干预和疾病预防。

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.