Variation in bradyrhizobial NopP effector determines symbiotic incompatibility with Rj2-soybeans via effector-triggered immunity

Masayuki Sugawara,Hirohito Tsurumaru,Hisayuki Mitsui,Hiroya Iwano,Yuki Konno,Kiwamu Minamisawa,Haruka Odake,Satoko Takahashi,Yosuke Umehara,Shusei Sato,Takeo Yamakawa,Hitoshi Kondo,Yuta Suzuki

doi:10.1038/s41467-018-05663-x

Abstract

Genotype-specific incompatibility in legume–rhizobium symbiosis has been suggested to be controlled by effector-triggered immunity underlying pathogenic host-bacteria interactions. However, the rhizobial determinant interacting with the host resistance protein (e.g., Rj2) and the molecular mechanism of symbiotic incompatibility remain unclear. Using natural mutants of Bradyrhizobium diazoefficiens USDA 122, we identified a type III-secretory protein NopP as the determinant of symbiotic incompatibility with Rj2-soybean. The analysis of nopP mutations and variants in a culture collection reveal that three amino acid residues (R60, R67, and H173) in NopP are required for Rj2-mediated incompatibility. Complementation of rj2-soybean by the Rj2 allele confers the incompatibility induced by USDA 122-type NopP. In response to incompatible strains, Rj2-soybean plants activate defense marker gene PR-2 and suppress infection thread number at 2 days after inoculation. These results suggest that Rj2-soybeans monitor the specific variants of NopP and reject bradyrhizobial infection via effector-triggered immunity mediated by Rj2 protein.

Highlights

Genotype-specific incompatibility in legume–rhizobium symbiosis has been suggested to be controlled by effector-triggered immunity underlying pathogenic host-bacteria interactions
Consistent with this report, Hardee plants inoculated with 122ΩrhcJ and 122ΩttsI formed significantly more nodules than those inoculated with wild-type USDA 122; the number of nodules induced by USDA 110T was comparable to that of 122ΩrhcJ and 122ΩttsI (Fig. 1a)
We considered two explanations for the sporadic nodulation on Hardee roots: (i) a few cells of wild-type USDA 122 passed through the barrier of Rj2-mediated incompatibility or (ii) the single nodules were formed by spontaneous USDA 122 mutants with defects in genes responsible for Rj2-mediated incompatibility

Summary

Introduction

Genotype-specific incompatibility in legume–rhizobium symbiosis has been suggested to be controlled by effector-triggered immunity underlying pathogenic host-bacteria interactions. In response to incompatible strains, Rj2-soybean plants activate defense marker gene PR-2 and suppress infection thread number at 2 days after inoculation These results suggest that Rj2soybeans monitor the specific variants of NopP and reject bradyrhizobial infection via effector-triggered immunity mediated by Rj2 protein. The type III secretion system (T3SS) is common to bacterial pathogens of plants and animals[1,2] It delivers effector proteins directly into the host cells[1,2] and helps the pathogen to survive and to escape the immune response activated by microbe/pathogen-associated molecular patterns. Positional cloning has revealed that the Rj2 gene encodes a typical R protein of the Toll-interleukin receptor/NBS/LRR (TIR-NBS-LRR) class[23] This finding indicates that symbiotic incompatibility between USDA 122 and Rj2-soybean plants may be controlled by ETI. The T3SS effector and the molecular mechanism of this genotype-specific incompatibility have been unknown

Methods

Results

Conclusion