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Front. Microbiol. | doi: 10.3389/fmicb.2018.03155

InnB, a novel type III effector of Bradyrhizobium elkanii USDA61, controls symbiosis with Vigna species

  • 1Tokyo University of Agriculture and Technology, Japan
  • 2Technische Universität Dresden, Germany

Bradyrhizobium elkanii USDA61 is incompatible with mung bean (Vigna radiata cv. KPS1) and soybean (Glycine max cv. BARC2) and unable to nodulate either plant. This incompatibility is due to the presence of a functional type III secretion system (T3SS) that translocates effector protein into host cells. We previously identified five genes in B. elkanii that are responsible for its incompatibility with KPS1 plants. Among them, a novel gene designated as innB exhibited some characteristics associated with the T3SS and was found to be responsible for the restriction of nodulation on KPS1. In the present study, we further characterized innB by analysis of gene expression, protein secretion and symbiotic phenotypes. The innB gene was found to encode a hypothetical protein that is highly conserved among T3SS-harboring rhizobia. Similar to other rhizobial T3SS-associated genes, the expression of innB was dependent on plant flavonoids and a transcriptional regulator, TtsI. The InnB protein was secreted via the T3SS and was not essential for secretion of other nodulation outer proteins. In addition, T3SS-dependent translocation of InnB into nodule cells was confirmed by an adenylate cyclase assay. According to inoculation tests using several Vigna species, InnB promoted nodulation of at least one V. mungo cultivar. These results indicate that innB encodes a novel type III effector controlling symbiosis with Vigna species.

Keywords: Symbiosis, Type III secretion system (T3SS), Bradyrhizobium elkanii, Vigna species, nodulation, effector

Received: 15 Sep 2018; Accepted: 05 Dec 2018.

Edited by:

Kiwamu Minamisawa, Tohoku University, Japan

Reviewed by:

Christian Staehelin, Sun Yat-sen University, China
Francisco J. López-Baena, Universidad de Sevilla, Spain  

Copyright: © 2018 Nguyen, Ratu, Yasuda, Göttfert and Okazaki. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Dr. Shin Okazaki, Tokyo University of Agriculture and Technology, Fuchu, Japan, sokazaki@cc.tuat.ac.jp