Inorganic ions and plant metabolism: targets signals and responses

大量营养素氮、磷和钾（NPK）的平衡供应对于粮食生产至关重要，但在田间往往无法实现。特别是在发展中国家，由于氮肥的使用而忽视了钾肥的使用，这导致了土壤中钾的严重流失。世界粮食生产也受到日益严重的农业土地次生盐渍化的威胁，这是由于通过灌溉输入钠（Na）。钠胁迫通常导致钾缺乏，因为两种离子竞争进入植物和在植物内的相同运输途径。钾在生长和新陈代谢中发挥重要作用。田间充足的钾供应可以保护作物免受草食动物攻击、真菌病害和非生物胁迫（例如干旱和盐碱），并且有证据表明钾可以提高氮的利用效率。与缺钾有关的问题很难在田间发现，因为可见的缺钾症状直到很晚才出现，在这个阶段通常不可能纠正这种情况。这是由于植物有效地在不同的组织和细胞区室之间重新分配K。钾稳态依赖于植物识别土壤和组织离子状态以及在细胞和组织之间交换生物信号的能力。如果我们能够解开这个隐藏的通讯系统，并确定钾胁迫的生物标志物，我们就可以开发一个早期预警系统。因此，主要应力目标和缺钾信号的知识将是一个宝贵的帮助，在该领域的预测和处理钾相关的问题。在我们的实验室中，我们最近发现了一个大的基因，改变他们的表达，以响应外部钾供应。许多钾调节基因编码代谢酶，例如那些催化糖和氨基酸代谢反应或硫和硝酸盐同化的酶。我们还发现，植物激素茉莉酸，这是最有名的植物防御昆虫和真菌的功能，在控制K诱导的基因表达的变化中起着核心作用。这些研究结果涉及第一次植物无机离子胁迫植物代谢和病原体防御在单个基因和信号化合物的水平。为了进一步研究钾诱导的代谢变化，我们测定了钾饥饿植物中氨基酸的水平。我们观察到谷氨酰胺的增加，这解释了之前观察到的硝酸盐转运蛋白的下调，这反过来可能是缺钾作物氮素使用量减少的原因。在钾饥饿过程中观察到的谷氨酸减少可能表明这种氨基酸的合成受损，因此催化该反应的酶（GOGAT）可能是钾胁迫的早期靶点。因此，我们相信，结合信息K（和Na）胁迫诱导的基因表达和代谢产物的变化可以揭示重要的见解无机离子和植物代谢之间的相互作用。开发新的工具，用于标准化分析植物组织中广泛的代谢物（“代谢组学”），使我们能够进行详细的研究K的可用性对植物代谢的作用。特别是，代谢组学技术的高灵敏度和高通量有助于代谢物在时间和空间上的变化的良好分辨率。这样的数据集将使我们首次有机会在三个不同的水平上识别矿物质缺乏的代谢成分：（1）主要的酶应激目标，（2）代谢应激信号和（3）参与重新编程初级和次级代谢的适应性反应。

A balanced supply of the macronutrients nitrogen, phosphorus and potassium (NPK) to crops is essential for food production but often not achieved in the field. Especially in developing countries K fertilisation has been neglected in favour of N fertilisation, which has led to serious depletion of soils in K. World food production is also threatened by increasing secondary salinisation of agricultural land due to sodium (Na) input through irrigation. Na stress often causes K deficiency as both ions compete for the same transport pathways into and within the plant. K carries out vital functions in growth and metabolism. Sufficient K supply in the field protects crops against herbivore attack, fungal diseases and abiotic stresses (e.g. drought and salinity), and there is evidence that K improves the efficiency of nitrogen usage. Problems related to K deficiency are difficult to spot in the field as visible deficiency symptoms don't appear until very late, at which stage it is often impossible to correct the situation. This is due to the fact that plants efficiently re-distribute K between different tissue and cellular compartments. K homeostasis relies on the ability of the plant to recognise the soil and tissue ion status and to exchange biological signals between cells and tissues. If we can unravel this hidden communication system and identify biological markers of K stress we can develop an early warning system. Thus, knowledge of primary stress targets and signals of K deficiency will be an invaluable help for predicting and treating K related problems in the field. In our laboratory we have recently identified a large set of genes that change their expression in response to the external K supply. Many of the K-regulated genes encode metabolic enzymes, for example those that catalyse reactions in sugar and amino acid metabolism, or sulphur and nitrate assimilation. We also found that the plant hormone jasmonic acid, which is best known for its function in plant defence against insects and fungi, plays a central role in controlling K induced changes in gene expression. These findings relate for the first time plant inorganic ion stress to plant metabolism and pathogen defence at the level of individual genes and signalling compounds. To further characterise K-induced changes in metabolic events we measured levels of amino acids in K starved plants. We observed an increase in glutamine, which explains the previously observed down-regulation of nitrate transporters, which in turn might be the reason for decreased N usage of K deficient crops. An observed decrease in glutamate during K starvation might indicate that the synthesis of this amino acid is impaired and thus the enzyme that catalyses this reaction (GOGAT) might be an early target of K stress. We therefore believe that combined information on K (and Na) stress induced gene expression and metabolite changes can reveal important insights into the interaction between inorganic ions and plant metabolism. The development of novel tools for the standardised analysis of a wide range of metabolites in plant tissues ('metabolomics') allows us to carry out a detailed study of the role of K availability for plant metabolism. In particular, the high sensitivity and the high throughput of metabolomics techniques facilitate a good resolution of metabolite changes in time and space. Such data set will give us for the first time the opportunity to identify metabolic components of mineral deficiency at three distinct levels: (1) primary enzymatic stress targets, (2) metabolic stress signals and (3) adaptive responses involved in re-programming primary and secondary metabolism.