新读(2018-4-9)到一篇文献^[1]，发现有个图能很好地呈现根瘤菌，内生菌，根瘤内生菌，根表菌，土壤微生物，根际(rhizosphere)，Bulk soil的关系，特别分享给各位。

其中很好地示意出根瘤内可以分离出根瘤菌(rhizobium)、根瘤内生细菌(nodule endophytes)的原因。许多人只分离到了生长速度较快的回接不能结瘤的根瘤内生细菌，而没有分离到生长速度较慢的可以结瘤固氮的根瘤菌，结果认为生长快的是根瘤菌，这是不对的。有时还存在这种情况，当对生长快的根瘤内生细菌的一些基因测序时，发现生长快的也属于根瘤菌的范畴，也有结瘤固氮基因，但回接原宿主仍然不能结瘤，那么这些菌仍然属于该宿主植物的内生根瘤菌，不是与之真正结瘤固氮的根瘤菌。

The complexity of bacterial root microbiota.

The bacterial community decreases in complexity from the rich soil microbiota to the rhizosphere, the rhizoplane, the endosphere, and the nodule. The rhizosphere is colonized by a subset of the bulk soil community, and the rhizoplane hosts epiphytes that are firmly attached to the root surface. The endosphere (root interior) is inhabited by endophytes. The root nodule is the habitat of symbiotic nitrogen-fixing bacteria, known as rhizobia, and also harbors a diversity of non-fixing bacteria called nodule endophytes. Infection threads can be coinhabited by endophytes and rhizobia.

[1] Clúa J., Roda C., Zanetti M. E., and Blanco F. A. Compatibility between Legumes and Rhizobia for the Establishment of a Successful Nitrogen-Fixing Symbiosis. Genes 2018, 9, 125. 

全文下载：genes-09-00125.pdf