The rhizobial autotransporter determines the symbiotic nitrogen fixation activity of Lotus japonicus in a host-specific manner
The rhizobial autotransporter determines the symbiotic nitrogen fixation activity of Lotus japonicus in a host-specific manner
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DOI：
10.1073/pnas.1913349117
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发表时间：
2020-01-21
影响因子：
11.1
通讯作者：
Hayashi, Makoto
中科院分区：
文献类型：
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作者：
Shimoda, Yoshikazu;Nishigaya, Yuki;Hayashi, Makoto
豆科植物与根瘤菌建立内共生关系并形成根瘤，根瘤菌在根瘤中固定大气氮。宿主植物和胞内根瘤菌严格控制这种共生固氮。我们最近报道了一个莲花固氮（-）突变体，apn 1（天冬氨酸肽酶nodule-induced 1），损害共生固氮。APN 1编码一种根瘤特异性天冬氨酸肽酶，以根瘤菌株特异性方式参与Fix（-）表型。这种宿主-菌株特异性意味着宿主植物APN 1和根瘤菌因子之间的一些分子相互作用是必需的，尽管APN 1在根瘤中的生物学功能和控制相互作用的机制尚不清楚。为了阐明根瘤菌因子如何参与菌株特异性固氮，我们探索了中根瘤菌株TONO的转座子突变体，其通常在apn 1根上形成Fix（-）结节，并鉴定了在apn 1上形成Fix（+）结节的TONO突变体。已鉴定的致病基因编码一种自转运蛋白，这是革兰氏阴性菌蛋白分泌系统的一部分。自转运蛋白基因在M.通常在apn 1根上形成Fix（+）根瘤的loti菌株MAFF 3030399导致了Fix（-）根瘤。TONO的自转运蛋白的功能是将其自身蛋白的一部分（乘客结构域）分泌到细胞外空间，重组APN 1蛋白在体外切割乘客蛋白。分枝Loti自身转运蛋白显示出以剂量依赖性方式诱导参与根瘤衰老的基因的活性。因此，我们得出结论，根瘤特异性天冬氨酸肽酶，APN 1，抑制根瘤菌自转运蛋白的负面影响，以保持有效的共生固氮在根瘤。
Leguminous plants establish endosymbiotic associations with rhizobia and form root nodules in which the rhizobia fix atmospheric nitrogen. The host plant and intracellular rhizobia strictly control this symbiotic nitrogen fixation. We recently reported a Lotus japonicus Fix(-) mutant, apn1 (aspartic peptidase nodule-induced 1), that impairs symbiotic nitrogen fixation. APN1 encodes a nodule-specific aspartic peptidase involved in the Fix(-) phenotype in a rhizobial strain-specific manner. This host-strain specificity implies that some molecular interactions between host plant APN1 and rhizobial factors are required, although the biological function of APN1 in nodules and the mechanisms governing the interactions are unknown. To clarify how rhizobial factors are involved in strain-specific nitrogen fixation, we explored transposon mutants of Mesorhizobium loti strain TONO, which normally form Fix(-) nodules on apn1 roots, and identified TONO mutants that formed Fix(+) nodules on apn1. The identified causal gene encodes an autotransporter, part of a protein secretion system of Gram-negative bacteria. Expression of the autotransporter gene in M. loti strain MAFF3030399, which normally forms Fix(+) nodules on apn1 roots, resulted in Fix(-) nodules. The autotransporter of TONO functions to secrete a part of its own protein (a passenger domain) into extracellular spaces, and the recombinant APN1 protein cleaved the passenger protein in vitro. The M. loti autotransporter showed the activity to induce the genes involved in nodule senescence in a dose-dependent manner. Therefore, we conclude that the nodule-specific aspartic peptidase, APN1, suppresses negative effects of the rhizobial autotransporter in order to maintain effective symbiotic nitrogen fixation in root nodules.