Spatio-temporal control of mutualism in legumes helps spread symbiotic nitrogen fixation.
Spatio-temporal control of mutualism in legumes helps spread symbiotic nitrogen fixation.
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DOI：
10.7554/elife.28683
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发表时间：
2017-10-12
期刊：
影响因子：
7.7
通讯作者：
Capela D
中科院分区：
文献类型：
--
作者：
Daubech B;Remigi P;Doin de Moura G;Marchetti M;Pouzet C;Auriac MC;Gokhale CS;Masson-Boivin C;Capela D
共生在生态系统中具有根本的重要性。哪些因素有助于保持这种关系互惠互利，并在进化上取得成功，这是一个核心问题。我们针对地球上最重要的互惠相互作用之一——豆科植物和数百种固氮细菌——解决了这个问题。本文分析了台湾铜-含羞草共生过程中固结物与非固结物的时空动态。固氮共生体在发生固氮的根瘤内逐渐胜过等基因非固氮共生体，即使它们共享同一个根瘤。在实验验证的基础上，通过数值模拟预测，在连续结瘤周期中，罕见固定菌入侵非固定菌占主导地位的群体的概率与初始接种量、植物群体大小和结瘤周期长度有关。我们的研究结果提供了对可能驱动n2固定互惠性状传播的选择力和生态因素的见解。根瘤菌是一种能够与豆类植物形成共生关系的土壤细菌，豆类植物包括豌豆、黄豆和扁豆。细菌会进入植物根部的细胞，并形成新的器官，称为根瘤。在结核中，细菌繁殖，然后再次释放到土壤中。同样，在根瘤中，细菌从植物中吸收含碳化合物。作为回报，许多细菌将空气中的氮转化（或“固定”）为植物可以用来构建DNA和蛋白质等分子的化合物。然而，有些细菌是“非固定物”，对宿主植物几乎没有或根本没有好处。有证据表明，豆科植物选择对抗非固定点细菌，尽管尚不清楚这种选择过程何时或如何发生。Daubech， Remigi等人现在已经跟踪了一种叫做Cupriavidus taiwanensis的细菌物种的两个变种的数量和生存能力，因为它们与Mimosa pudica（豌豆家族的一员）形成共生相互作用。这两种细菌的区别仅仅在于它们是否能够固定氮。起初，固定者和非固定者进入结节并以相同的速率繁殖。后来，调整者逐渐战胜了非调整者。然后，在细菌进入植物大约20天后，含有非固定物的结节细胞退化。这表明，结节细胞帮助控制细菌增殖的基础上，他们收到的回报利益。进一步的实验和数学模型也表明，在根瘤形成的重复循环中，固氮菌可以侵入以非固氮菌为主的细菌群体。随着其他三个因素的增加，这种入侵成功的可能性也会增加：固定细菌在初始种群中的比例，可用植物的数量，以及细菌在根瘤中停留的时间。这种机制确保了固氮特性在细菌群体中的维持和传播。改进生物固氮过程有助于减少种植作物所需的肥料量。这在未来可能有助于使农业生态系统更具可持续性。Daubech、Remigi等人提出的结果为将来可能被改造的豆类作物或固氮谷物上选择固氮细菌提供了指导。现在需要进一步的工作来更详细地了解导致非固定细菌死亡的分子机制。
Mutualism is of fundamental importance in ecosystems. Which factors help to keep the relationship mutually beneficial and evolutionarily successful is a central question. We addressed this issue for one of the most significant mutualistic interactions on Earth, which associates plants of the leguminosae family and hundreds of nitrogen (N2)-fixing bacterial species. Here we analyze the spatio-temporal dynamics of fixers and non-fixers along the symbiotic process in the Cupriavidus taiwanensis–Mimosa pudica system. N2-fixing symbionts progressively outcompete isogenic non-fixers within root nodules, where N2-fixation occurs, even when they share the same nodule. Numerical simulations, supported by experimental validation, predict that rare fixers will invade a population dominated by non-fixing bacteria during serial nodulation cycles with a probability that is function of initial inoculum, plant population size and nodulation cycle length. Our findings provide insights into the selective forces and ecological factors that may have driven the spread of the N2-fixation mutualistic trait. Rhizobia are soil bacteria that are able to form a symbiotic relationship with legumes – plants that include peas, beans and lentils. The bacteria move into cells in the roots of the plant and cause new organs called nodules to form. Inside the nodules the bacteria multiply before being released to the soil again. Also while in the nodules, the bacteria receive carbon-containing compounds from the plant. In return many of the bacteria convert (or “fix”) nitrogen from the air into compounds that the plant can use to build molecules such as DNA and proteins. Yet, some of the bacteria are “non-fixers” that provide little or no benefit to the host plant. Evidence suggests that legumes select against non-fixer bacteria, though it was not clear when or how this selection process occurs. Daubech, Remigi et al. have now followed the number and viability of two variants of a bacteria species called Cupriavidus taiwanensis as they form a symbiotic interaction with Mimosa pudica, a member of the pea family. The two types of bacteria differed only by whether or not they were able to fix nitrogen. At first fixers and non-fixers entered nodules and multiplied at equal rates. Later, the fixers progressively outcompeted the non-fixers. Then, around 20 days after the bacteria entered the plant, nodule cells that contained non-fixers degenerated. This indicates that the nodule cells help to control bacterial proliferation based on the benefits they receive in return. Further experiments and mathematical modeling also showed that over repeated cycles of root nodule formation, nitrogen fixers can invade a bacterial population dominated by non-fixer bacteria. The likelihood that this invasion will be successful increases as three other factors increase: the proportion of fixer bacteria in the initial population, the number of available plants, and the length of time the bacteria spend in the nodules. This mechanism ensures the maintenance and spread of nitrogen-fixing traits in the bacterial population. Improving the processes of biological nitrogen fixation could help to reduce the amount of fertilizers required to grow crops. This in the future could help make agricultural ecosystems more sustainable. The results presented by Daubech, Remigi et al. provide guidelines that could be used to select nitrogen-fixing bacteria on legume crops or on nitrogen-fixing cereals that may be engineered in the future. Further work is now needed to understand in more detail the molecular mechanisms that lead to the death of non-fixer bacteria.