Legume-Rhizobium Interactions that Maintain Mutualism

豆科植物对许多自然和农业生态系统的氮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，根瘤菌是可以生活在根瘤内并将氮气转化为植物宿主可以使用的“固定”氮的土壤细菌。 根瘤菌对宿主的成本和收益各不相同。 为什么许多根瘤菌使用植物光合产物（碳水化合物）来固定氮，而不是囤积光合产物来支持自己未来的生存和繁殖？ 这是特别令人困惑的，因为每种植物通常都有几种相互竞争的根瘤菌菌株。 固氮的菌株可能会增加宿主的光合作用，从而增加可利用的光合产物库，但如果所有菌株都分享这种集体利益，那么囤积光合产物的自私菌株应该取代那些固氮更多的菌株。 为什么还没有发生这种情况？ 初步数据似乎表明，植物可以监测单个根瘤，并将资源（包括氧气）切断给那些很少或根本不固定氮的根瘤。 数学模型表明，这种植物“制裁”将有效遏制根瘤菌的“欺骗”，但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研究。 各种实验将使一些结核暴露在无氮大气中，同时使同一植物上的其他结核固氮。 对根瘤生理学的非侵入性监测和根瘤菌计数方法运作良好，我们还计划测量根瘤菌的资源水平。 根瘤菌从根瘤中释放对土壤种群的重要性，相对于已经在土壤中的根瘤菌的繁殖增加，将使用表达两种不同荧光基因的根瘤菌来确定。 从田间根瘤中收集的根瘤菌的DNA指纹图谱将检验混合根瘤（含有两种或两种以上菌株）是罕见的这一关键假设。 虽然细节会有所不同，但我们的研究可能与任何合作可能破裂的系统有关，例如通过漂白珊瑚驱逐藻类。

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.