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

植物根系对植物吸收矿物质养分至关重要。此外，它们还与土壤环境和细菌、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，缺乏植物和动物高等细胞中的膜结合结构。然而，尽管细菌可能没有那么复杂的细胞组织，它们却能进行大量植物和动物所没有的化学反应。细菌负责许多营养物质的循环，如N2 （N2也被称为氮气，由两个氮原子由一个强大的三键结合而成），这是一种非常惰性的大气气体。氮气占大气的78%，但非常不活泼，不能直接用作氨基酸、蛋白质和DNA合成所需的氮的来源。然而，少数细菌可以将N2还原（加入氢）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合成为氨基酸和所有其他构成生命的物质。在世界上许多地方，限制植物生长的是以氨或硝酸盐形式存在的氮的供应，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科植物的植物提供的。豆科植物在根上形成根瘤，根瘤菌可以将N2还原为氨，并将其提供给植物，作为碳和能量的来源。这种豆科植物-根瘤菌共生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，尽管豆科植物很重要，但最近它们的使用减少了，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，因为大量的氮被污染到环境中，导致藻类大量繁殖，产生温室气体。根瘤菌已经被研究了100多年，因为根瘤菌具有提高豆科作物产量的能力，根瘤菌通常被用作接种剂，作为经济上昂贵的化学肥料的替代品。豆科植物根瘤中产生的生物可利用氮有利于轮作或同时种植的非豆科作物。然而，在实验室条件下将固氮率高（即有效菌株）的根瘤菌接种到豆科植物上时，往往无法在土壤中与固氮能力较差的本地根瘤菌竞争豆科植物的定植。这就是所谓的“根瘤菌竞争问题”。因此，接种剂选择的圣杯是识别既高效又有竞争力的精英菌株。这个竞争效率问题对豆科植物的使用具有巨大的实际重要性，但它也突出了一个基本的生物学问题，即是什么决定了细菌在植物根部定植的竞争力。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主要例子。在根瘤菌研究中，我们首次能够评估细菌的遗传潜力和根瘤菌在真实土壤中的竞争和固氮效率所需的因素。研究是在无菌条件下和/或只关注结节进行的。在这里，我们建议一步一步地全面评估根瘤菌的生命周期。我们的发现将解释为什么高效的N2固定物不一定是好的殖民者。确定基本基因和调控途径将使我们详细了解菌株在不同土壤中的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.