Liu Ailin, Contador Carolina A, Fan Kejing, Lam Hon-Ming
Centre for Soybean Research, State Key Laboratory of Agrobiotechnology, Shatin, Hong Kong.
School of Life Sciences, The Chinese University of Hong Kong, Shatin, Hong Kong.
Front Plant Sci. 2018 Dec 18;9:1860. doi: 10.3389/fpls.2018.01860. eCollection 2018.
Members of the plant family Leguminosae (Fabaceae) are unique in that they have evolved a symbiotic relationship with rhizobia (a group of soil bacteria that can fix atmospheric nitrogen). Rhizobia infect and form root nodules on their specific host plants before differentiating into bacteroids, the symbiotic form of rhizobia. This complex relationship involves the supply of C-dicarboxylate and phosphate by the host plants to the microsymbionts that utilize them in the energy-intensive process of fixing atmospheric nitrogen into ammonium, which is in turn made available to the host plants as a source of nitrogen, a macronutrient for growth. Although nitrogen-fixing bacteroids are no longer growing, they are metabolically active. The symbiotic process is complex and tightly regulated by both the host plants and the bacteroids. The metabolic pathways of carbon, nitrogen, and phosphate are heavily regulated in the host plants, as they need to strike a fine balance between satisfying their own needs as well as those of the microsymbionts. A network of transporters for the various metabolites are responsible for the trafficking of these essential molecules between the two partners through the symbiosome membrane (plant-derived membrane surrounding the bacteroid), and these are in turn regulated by various transcription factors that control their expressions under different environmental conditions. Understanding this complex process of symbiotic nitrogen fixation is vital in promoting sustainable agriculture and enhancing soil fertility.
豆科植物家族的成员很独特，因为它们与根瘤菌（一类能够固定大气氮的土壤细菌）进化出了一种共生关系。根瘤菌在分化成类菌体（根瘤菌的共生形式）之前，会感染其特定的宿主植物并在其上形成根瘤。这种复杂的关系涉及宿主植物向微共生体供应二羧酸和磷酸盐，微共生体在将大气氮固定为铵的能量密集过程中利用这些物质，而铵又作为氮源提供给宿主植物，氮是生长所需的大量营养素。尽管固氮类菌体不再生长，但它们仍具有代谢活性。共生过程很复杂，受到宿主植物和类菌体的严格调控。宿主植物中碳、氮和磷酸盐的代谢途径受到严格调控，因为它们需要在满足自身需求以及微共生体需求之间找到微妙的平衡。各种代谢物转运蛋白网络负责通过共生体膜（围绕类菌体的植物衍生膜）在两个伙伴之间运输这些必需分子，而这些转运蛋白又受到各种转录因子的调控，这些转录因子在不同环境条件下控制它们的表达。了解这种共生固氮的复杂过程对于促进可持续农业和提高土壤肥力至关重要。
