Cooperation and Conflict in the Legume-Rhizobium Symbiosis

豆科植物，如三叶草、苜蓿和大豆，在农业和自然界都很重要。 土壤中的根瘤菌在感染豆类植物的根部时对它们有益，因为它们可以吸收大气中的氮并将其供应给植物。 这些额外的氮增加了植物的光合作用，从而间接增加了根瘤菌的能量供应。 因此，豆科植物和根瘤菌之间的共生似乎对双方都有利。 但是，如果像经常发生的那样，两种或两种以上不同的根瘤菌感染同一种植物，那该怎么办呢？ 一种最初很罕见的菌株，利用植物资源进行自身繁殖，而不是固氮，可能会以植物和更合作的根瘤菌菌株为代价来增加数量。 尽管大多数根瘤菌似乎主要是有益的，但这种“寄生”菌株确实存在。这项研究将确定植物对不能固氮的根瘤菌的反应是否可以选择更有益的根瘤菌。 以前的研究表明，豆类植物可以控制根瘤内根瘤菌的氧气供应。 PI将确定这种氧供应如何随着根瘤菌固氮的变化而变化，以及这种豆科植物的反应如何影响根瘤菌的生存和繁殖。 大多数研究将在受控的实验室条件下进行，但田间试验将确定豆类是否改变土壤中合作根瘤菌与寄生根瘤菌的频率。 除了可能在农业上的实际应用外，这项研究还将有助于全面了解物种之间合作的一般问题。 这种合作对于花卉授粉、珊瑚礁的生存和生长、贫磷土壤中的植物生长以及许多其他自然系统至关重要。

Legumes, such as clover, alfalfa, and soybean, are important in agricultureand in nature. Soil bacteria known as rhizobia can benefit legumes whenthey infect their roots, because they can take up atmospheric nitrogen andsupply it to the plant. This extra nitrogen increases plantphotosynthesis, thereby indirectly increasing the energy supply to therhizobia. Thus, symbiosis between legume and rhizobium seems beneficial toboth. But what if, as often happens, two or more different strains ofrhizobia infect the same plant? An initially rare strain that uses plantresources for its own reproduction, rather than for nitrogen fixation,could increase in numbers at the expense of both the plant and the morecooperative rhizobial strain. Although most rhizobia seem to bepredominantly beneficial, such "parasitic" strains do exist.This research will determine whether plant responses to rhizobia that failto fix nitrogen can select for more beneficial rhizobia. Previous researchshows that legumes can control the oxygen supply to rhizobia inside theirroot nodules. The PIs will determine how this oxygen supply changes withchanges in nitrogen fixation by rhizobia, and how this legume responseaffects the survival and reproduction of the rhizobia. Most of theresearch will be conducted under controlled laboratory conditions, but afield test will determine whether legumes alter the frequency ofcooperative vs. parasitic rhizobia in soil. In addition to possiblepractical applications in agriculture, the research will contribute tooverall understanding of the general problem of cooperation betweenspecies. Such cooperation is critical to the pollination of flowers, thesurvival and growth of coral reefs, plant growth in phosphorus-poor soils,and many other natural systems.