Similar to pathogenic bacteria, rhizobia can inject effector proteins into host cells directly to promote infection via the type III secretion system (T3SS). Nodulation outer protein P (NopP), a specific T3SS effector of rhizobia, plays different roles in the establishment of multiple rhizobia-legume symbiotic systems. Mesorhizobium amorphae CCNWGS0123 (GS0123), which infects Robinia pseudoacacia specifically, secretes several T3SS effectors, including NopP. Here, we demonstrate that NopP is secreted through T3SS-I of GS0123 during the early stages of infection, and its deficiency decreases nodule nitrogenase activity of R. pseudoacacia nodules. A trafficking protein particle complex subunit 13–like protein (TRAPPC13) has been identified as a NopP target protein in R. pseudoacacia roots by screening a yeast two-hybrid library. The physical interaction between NopP and TRAPPC13 is verified by bimolecular fluorescence complementation and coimmunoprecipitation assays. In addition, subcellular localization analysis reveals that both NopP and its target, TRAPPC13, are colocalized on the plasma membrane. Compared with GS0123-inoculated R. pseudoacacia roots, some genes associated with cell wall remodeling and plant innate immunity down-regulated in ΔnopP-inoculated roots at 36 h postinoculation. The results suggest that NopP in M. amorphae CCNWGS0123 acts in multiple processes in R. pseudoacacia during the early stages of infection, and TRAPPC13 could participate in the process as a NopP target.

Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

与病原菌类似，根瘤菌可以直接将效应蛋白注入宿主细胞，通过 III 型分泌系统 (T3SS) 促进感染。Nodulation external protein P (NopP) 是根瘤菌的一种特异性 T3SS 效应物，在多种根瘤菌-豆科植物共生系统的建立中发挥着不同的作用。Mesorhizobium amorphae CCNWGS0123 (GS0123)，专门感染刺槐，分泌几种 T3SS 效应物，包括 NopP。在这里，我们证明 NopP 在感染的早期阶段通过 GS0123 的 T3SS-I 分泌，其缺乏会降低刺槐的根瘤固氮酶活性结节。通过筛选酵母双杂交文库，已将运输蛋白颗粒复合亚基 13 样蛋白 (TRAPPC13) 鉴定为刺槐根中的 NopP 靶蛋白。NopP 和 TRAPPC13 之间的物理相互作用通过双分子荧光互补和共免疫沉淀分析得到验证。此外，亚细胞定位分析表明，NopP 及其靶标 TRAPPC13 都位于质膜上。与接种 GS0123 的刺槐根相比，在接种后 36 小时，在 Δ nopP接种的根中，一些与细胞壁重塑和植物先天免疫相关的基因下调。结果表明M. amorphae 中的 NopPCCNWGS0123 在感染的早期阶段在刺槐中的多个过程中起作用，并且 TRAPPC13 可以作为 NopP 目标参与该过程。

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