Factors controlling N2-fixing ability and competitiveness of rhizobia to nodulate legumes

植物根系对于植物吸收矿质营养至关重要。此外，它们与土壤环境以及细菌、真菌、单细胞动物细胞、线虫和其他生物的复杂组合相互作用。细菌是简单的单细胞微生物，缺乏在植物和动物的高等细胞中发现的膜结合结构。然而，虽然细菌的细胞组织可能不那么复杂，但它们可以进行植物和动物中没有的大量化学反应。细菌负责许多营养物质的循环，例如N2（N2也被称为氮气，由两个氮原子通过强三键结合组成），这是一种非常惰性的大气气体。N2占大气的78%，但非常不活泼，不能直接用作氨基酸，蛋白质和DNA合成所需的氮源。然而，少数细菌可以将N2还原（添加氢）并将其转化为氨（NH3），氨很容易被包括细菌和植物在内的各种生物体纳入氨基酸，然后是所有其他生命的组成部分。在世界上许多地方，限制植物生长的因素是以氨或硝酸盐的形式供应氮，而植物反过来又支持动物的生命。在过去，大部分氮是由生物固氮提供的，特别是由一组被称为豆类的植物提供的。豆科植物在其根部形成根瘤，其中含有称为根瘤菌的细菌，这些细菌将N2还原为氨并将其供应给植物，以换取碳和能源。这种豆科植物-根瘤菌共生体负责提供生物圈中高达50-60%的生物可利用氮（即氨），因此对地球上的生命至关重要。然而，尽管豆类的重要性，最近它们的使用已经下降，氮主要通过化学合成肥料提供给作物。这对环境产生了重大的负面影响，因为这些氮中的大部分作为污染物损失到环境中，导致藻类大量繁殖并产生温室气体。根瘤菌的研究已经超过100年，因为这种能力，以增加豆类作物的产量和根瘤菌作为接种剂，作为一种替代经济昂贵的化学肥料常规应用。豆科植物根瘤中产生的生物有效氮有利于轮作或同时种植的非豆科作物。然而，根瘤菌接种剂，具有较高的N2固定率（即有效菌株）接种到豆科植物在实验室条件下，往往无法竞争在土壤中的定殖豆科植物对本地根瘤菌与劣质N2固定能力。这被称为“根瘤菌竞争问题”。因此，接种物选择的圣杯是确定既高效又有竞争力的精英菌株。这种竞争有效性的问题是巨大的实际重要性，豆类的使用，但它也突出了基本的生物学问题，是什么决定了细菌的竞争力，为植物根部的殖民。因此，了解根瘤菌的竞争力是一个具有基础和应用重要性的问题的最好例子。在根瘤菌的研究中，我们第一次能够评估细菌的遗传潜力和根瘤菌在真实的土壤中的竞争力和固氮效率所需的因素。研究是在无菌条件下进行的，并且（或者）只侧重于结核。在这里，我们建议逐步全面评估根瘤菌的生命周期。我们的研究结果将解释为什么有效的N2固定器不一定是好的殖民者。确定必需基因和调控途径将使我们详细了解不同土壤中的菌株行为以及与不同植物品种的共生成功，使我们能够更好地选择最佳接种剂。

Plant roots are critical for the uptake of mineral nutrients by plants. In addition, they interact with the soil environment and a complex assemblage of bacteria, fungi, single celled animal cells, nematodes and other organisms. Bacteria are simple single celled microorganisms that lack the membrane bound structures found in higher cells of plants and animals. However, while bacteria may have a less complex cellular organisation, they carry out a huge range of chemical reactions not found in plants and animals. Bacteria are responsible for the cycling of many nutrients such as N2 (N2 is also known as nitrogen gas and consists of two nitrogen atoms bound by a strong triple bond), which is a very inert atmospheric gas. N2 makes up 78% of the atmosphere but is very unreactive and cannot be used directly as a source of nitrogen, which is needed for amino acid, protein and DNA synthesis. However, a small number of bacteria can reduce (add hydrogen) to N2 and convert it into ammonia (NH3), which is readily incorporated into amino acids and then all the other building blocks of life, by a wide range of organisms including bacteria and plants. In many parts of the world the limitation to growth of plants, which in turn support animal life, is the supply of nitrogen as ammonia or nitrate. In the past, much of the nitrogen was provided by biological nitrogen fixation, particularly by a group of plants known as legumes. The legumes form nodules on their roots which house bacteria, called rhizobia, which reduce N2 to ammonia and supply it to plants in return for a carbon and energy source. This legume-rhizobia symbiosis is responsible for providing up to 50-60% of the biosphere's biologically available nitrogen (i.e. ammonia) and is therefore essential to life on earth. However, in spite of the importance of legumes more recently their use has declined and nitrogen is mainly provided to crops by chemically synthesised fertiliser. This has major negative impacts on the environment as much of this nitrogen is lost to the environment as pollution causing algal blooms and contributing to greenhouse gases. Rhizobia have been studied for more than 100 years because of this ability to increase yields of legumes crops and rhizobia are routinely applied as inoculants as an alternative to economically expensive chemical fertilizers. The bioavailable nitrogen that is generated in nodules of legumes benefits nonlegume crops grown in rotation or at the same time. However, rhizobial inoculants that have high rates of N2-fixation (i.e. effective strains) when inoculated onto legumes under laboratory conditions often fail to compete in soil for colonisation of legumes against native rhizobia with inferior N2 fixing abilities. This is known as the "rhizobial competition problem". The holy grail of inoculant selection has therefore been to identify elite strains that are both highly effective and competitive. This competition-effectivity problem is of enormous practical importance to use of legumes, but it also highlights the fundamental biological question of what determines the competitiveness of bacteria for colonisation of plant roots. Understanding rhizobial competitiveness is a therefore a prime example of a question that is of both fundamental and applied importance. For the first time in rhizobial research, we are able to assess the bacterial genetic potential and factors rhizobia need for their competitiveness and N2-fixation efficiency in real soil. Research has been conducted under sterile conditions and/or focusing solely on the nodules. Here we propose to step-by-step fully assess the rhizobial life cycle. Our findings will explain why efficient N2 fixers are not necessarily good colonisers. Identifying the essential genes and regulatory pathways will give us detailed knowledge about strain behaviour in different soils and symbiotic success with different plant varieties, allowing us to better select the best inoculants.