Schulte Carolin C M, Borah Khushboo, Wheatley Rachel M, Terpolilli Jason J, Saalbach Gerhard, Crang Nick, de Groot Daan H, Ratcliffe R George, Kruger Nicholas J, Papachristodoulou Antonis, Poole Philip S
Department of Plant Sciences, University of Oxford, Oxford, UK.
Department of Engineering Science, University of Oxford, Oxford, UK.
Sci Adv. 2021 Jul 30;7(31). doi: 10.1126/sciadv.abh2433. Print 2021 Jul.
Rhizobia induce nodule formation on legume roots and differentiate into bacteroids, which catabolize plant-derived dicarboxylates to reduce atmospheric N into ammonia. Despite the agricultural importance of this symbiosis, the mechanisms that govern carbon and nitrogen allocation in bacteroids and promote ammonia secretion to the plant are largely unknown. Using a metabolic model derived from genome-scale datasets, we show that carbon polymer synthesis and alanine secretion by bacteroids facilitate redox balance in microaerobic nodules. Catabolism of dicarboxylates induces not only a higher oxygen demand but also a higher NADH/NAD ratio than sugars. Modeling and C metabolic flux analysis indicate that oxygen limitation restricts the decarboxylating arm of the tricarboxylic acid cycle, which limits ammonia assimilation into glutamate. By tightly controlling oxygen supply and providing dicarboxylates as the energy and electron source donors for N fixation, legumes promote ammonia secretion by bacteroids. This is a defining feature of rhizobium-legume symbioses.
根瘤菌诱导豆科植物根上形成根瘤并分化为类菌体，类菌体将植物衍生的二羧酸分解代谢，将大气中的氮还原为氨。尽管这种共生关系在农业上具有重要意义，但控制类菌体中碳和氮分配以及促进向植物分泌氨的机制在很大程度上尚不清楚。利用从基因组规模数据集推导出来的代谢模型，我们表明类菌体的碳聚合物合成和丙氨酸分泌有助于微需氧根瘤中的氧化还原平衡。与糖类相比，二羧酸的分解代谢不仅诱导更高的需氧量，还诱导更高的NADH/NAD比率。建模和碳代谢通量分析表明，氧气限制会限制三羧酸循环的脱羧分支，从而限制氨同化为谷氨酸。通过严格控制氧气供应并提供二羧酸作为固氮的能量和电子源供体，豆科植物促进类菌体分泌氨。这是根瘤菌 - 豆科植物共生关系的一个决定性特征。
