Legume-Rhizobium Interactions that Maintain Mutualism

豆科植物对许多自然和农业生态系统氮素收支的重要性在很大程度上取决于它们与根瘤菌的相互作用，根瘤菌是一种土壤细菌，可以生活在根瘤内，并将氮气转化为植物宿主可以利用的固定氮气。根瘤菌对宿主的成本和收益各不相同。为什么许多根瘤菌(但不是所有根瘤菌)使用植物光合作用(碳水化合物)来固定主要有利于宿主的氮，而不是囤积光合作用来支持自己未来的生存和繁殖？这尤其令人费解，因为每种植物通常都有几个相互竞争的根瘤菌菌株。固定氮素的菌株可能会增加宿主的光合作用，从而增加可用光合物库，但如果所有菌株都分享这种集体利益，那么储存光合作用的自私菌株应该会取代那些固定更多氮素的菌株。为什么这种情况还没有发生？初步数据似乎表明，植物可以监测单个根瘤，并切断那些固定很少或没有固定氮素的根瘤的资源(包括氧气)。数学模型表明，这种植物“制裁”将有效地威慑根瘤菌的“欺骗”，但还需要对植物制裁的机制及其对根瘤菌生存和繁殖的影响进行更多的研究。各种实验将把一些根瘤暴露在无氮大气中，同时允许同一植物上的其他根瘤固定氮。对根瘤生理的非侵入性监测和计算根瘤菌的方法工作得很好，我们还计划测量根瘤菌的资源水平。根瘤菌从根瘤中释放出来对土壤种群的重要性，相对于增加土壤中已有根瘤菌的繁殖，将使用表达两种不同荧光基因的根瘤菌来确定。从田间根瘤中采集的根瘤菌的DNA指纹图谱将检验一个关键假设，即混合根瘤(包含两个或更多菌株)是罕见的。尽管细节会有所不同，但我们的研究可能与任何合作可能破裂的系统有关，例如漂白珊瑚排出藻类。

The importance of legume plants to nitrogen budgets of many natural and agricultural ecosystems depends largely on their interactions with rhizobia, soil bacteria that can live inside root nodules and convert nitrogen gas into "fixed" nitrogen that their plant host can use. Rhizobia vary in costs and benefits to their hosts. Why do many, but not all, rhizobia use plant photosynthate (carbohydrates) to fix nitrogen that mainly benefits their hosts, rather than hoarding that photosynthate to support their own future survival and reproduction? This is especially puzzling since each plant typically hosts several competing strains of rhizobia. Strains that fix nitrogen may increase host photosynthesis, thereby increasing the pool of available photosynthate, but if all strains share in this collective benefit then selfish strains that hoard photosynthate should displace those that fix more nitrogen. Why hasn't this happened? Preliminary data appear to show that plants can monitor individual root nodules and cut off resources (including oxygen) to those that fix little or no nitrogen. Mathematical models show that such plant "sanctions" would be an effective deterrent to rhizobial "cheating," but additional research is needed on the mechanisms of plant sanctions and on their effects on survival and reproduction of rhizobia. Various experiments will expose some nodules to a nitrogen-free atmosphere, while allowing other nodules on the same plant to fix nitrogen. Noninvasive monitoring of nodule physiology and methods to count rhizobia are working well, and we also plan to measure resource levels in rhizobia. The importance to soil populations of rhizobial release from nodules, relative to increased reproduction of rhizobia already in the soil, will be determined using rhizobia expressing two different fluorescence genes. DNA fingerprinting of rhizobia collected from nodules in the field will test the key hypothesis that mixed nodules (containing two or more strains) are rare. Although details will differ, our research may be relevant to any system in which cooperation can break down, such as expulsion of algae by bleaching corals.