Plant Biology Graduate Program, University of Massachusetts Amherst, MA 01003, USA.
Plant Biology Graduate Program, University of Massachusetts Amherst, MA 01003, USA; Department of Biochemistry and Molecular Biology, University of Massachusetts Amherst, MA 01003, USA.
Curr Opin Plant Biol. 2018 Aug;44:155-163. doi: 10.1016/j.pbi.2018.04.017. Epub 2018 May 17.
The symbiosis formed by nitrogen-fixing bacteria with plant hosts mainly in the legume family involves a very intimate interaction. Within the symbiotic organ (the nodule) the bacteria are fully internalized by the host cell to become an intracellular organelle surrounded by a host-derived membrane. This arrangement is probably necessary for the efficient provision of energy and the sequestration of free oxygen molecules, two conditions required for sustained nitrogen fixation. Recent advances made in model legume species, such as Medicago truncatula, are beginning to uncover the genetic components allowing rhizobia to access the host cytoplasm and establish chronic intracellular infections without overt detrimental effects. It is now known that the rhizobial compartment in M. truncatula cells, the symbiosome, retains some features of the extracellular space as the target for a redirected host protein secretory pathway. A set of vesicle trafficking proteins function specifically in symbiotic cells to ensure the faithful delivery of secretory proteins to the intracellular bacteria, or bacteroid. This system is co-opted from the more ancient association with arbuscular mycorrhizal fungi found in most land plants, highlighting the evolutionary origin of the legume-rhizobia symbiosis. In some legume lineages, this heightened capability to process secretory proteins is needed to deliver a large number of symbiosis-specific antimicrobial peptides to the bacteria. Known as NCR peptides, these molecules transform bacteroids into a state of terminal differentiation, where the microbe loses its ability to proliferate outside their host. Numbering in their hundreds, these peptides manipulate various aspects of rhizobial biology, and affect the outcome of this symbiosis in complex ways. The extreme size of the NCR peptide family seems to be the result of an evolutionary conflict between the two partners to extract maximum benefit from each other.
固氮细菌与植物宿主之间形成的共生关系主要存在于豆科植物中，这种关系涉及到非常密切的相互作用。在共生器官（根瘤）中，细菌被宿主细胞完全内化，成为被宿主衍生膜包围的细胞内细胞器。这种安排可能对于有效提供能量和隔离游离氧分子是必要的，这是持续固氮所必需的两个条件。在模式豆科植物（如紫花苜蓿）中取得的最新进展开始揭示使根瘤菌能够进入宿主细胞质并建立慢性细胞内感染而没有明显不利影响的遗传成分。现在已知，在紫花苜蓿细胞中的根瘤菌区室，即共生体，保留了一些细胞外空间的特征，作为重新定向的宿主蛋白分泌途径的靶标。一组囊泡运输蛋白在共生细胞中特异性发挥作用，以确保将分泌蛋白准确递送到细胞内细菌或类菌体。该系统是从与大多数陆地植物中发现的丛枝菌根真菌的更古老的共生关系中被选择出来的，突出了豆科植物-根瘤菌共生关系的进化起源。在一些豆科植物谱系中，这种增强的分泌蛋白加工能力是将大量共生特异性抗菌肽递送到细菌所必需的。这些分子被称为 NCR 肽，它们将类菌体转化为终末分化状态，其中微生物失去在宿主外增殖的能力。这些肽的数量达到数百个，它们操纵根瘤菌生物学的各个方面，并以复杂的方式影响这种共生关系的结果。NCR 肽家族的极端大小似乎是两个合作伙伴之间为从彼此中获得最大利益而进行的进化冲突的结果。
