Inorganic ions and plant metabolism: targets signals and responses

向作物平衡供应大量营养素氮、磷和钾(NPK)对粮食生产至关重要，但在田间往往无法实现。特别是在发展中国家，人们忽视了钾肥而偏重于氮肥，这导致了钾肥土壤的严重枯竭，世界粮食生产也受到了农业土地次生盐碱化的威胁，因为通过灌溉输入的钠(Na)。钠胁迫经常导致钾缺乏，因为这两种离子竞争进入植物和植物内部的相同运输途径。钾在生长和代谢中起着至关重要的作用。田间充足的钾素供应可以保护作物免受食草动物攻击、真菌疾病和非生物胁迫(如干旱和盐碱)的侵袭，而且有证据表明，钾肥可以提高氮肥的利用效率。与缺钾有关的问题很难在田间发现，因为明显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。这是因为植物有效地在不同的组织和细胞间重新分配钾。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一隐藏的通讯系统，并找出钾胁迫的生物标记，我们就可以开发出一个早期预警系统。因此，对K缺乏的主要胁迫靶标和信号的了解将对预测和处理与K有关的田间问题有非常重要的帮助。在我们的实验室里，我们最近发现了一大组基因，它们会随着外部钾供应的变化而改变表达。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的基因，或硫磺和硝酸盐同化反应的基因。我们还发现，植物激素茉莉酸在控制钾诱导的基因表达变化中起着核心作用，茉莉酸在植物抵御昆虫和真菌方面的功能最为人所知。这些发现首次在单个基因和信号化合物的水平上涉及植物无机离子胁迫对植物代谢和病原菌防御的影响。为了进一步描述钾引起的代谢事件的变化，我们测量了钾饥饿植物中的氨基酸水平。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，这可能是缺钾作物氮素利用减少的原因。在K饥饿过程中观察到的谷氨酸的减少可能表明这种氨基酸的合成受到损害，因此催化这一反应的酶(GOGAT)可能是K胁迫的早期靶标。因此，我们认为，结合K(和Na)胁迫诱导的基因表达和代谢物变化的信息，可以揭示无机离子与植物代谢之间的相互作用。对植物组织中各种代谢物进行标准化分析的新工具(代谢组学)的发展使我们能够对K有效性在植物代谢中的作用进行详细的研究。特别是，代谢组学技术的高灵敏度和高通量有助于很好地解决代谢物在时间和空间上的变化。这样的数据集将使我们第一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)初级酶应激目标，(2)代谢应激信号和(3)参与重新编程初级和次生代谢的适应性反应。

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.