The rhizobial autotransporter determines the symbiotic nitrogen fixation activity of Lotus japonicus in a host-specific manner

Yoshikazu Shimoda, Yuki Nishigaya, Hiroko Yamaya-Ito, Noritoshi Inagaki, Yosuke Umehara, Hideki Hirakawa, Shusei Sato, Toshimasa Yamazaki, Makoto Hayashi

Research output: Contribution to journalArticlepeer-review

1 Citation (Scopus)

Abstract

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 nodulespecific 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 strainspecific 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.

Original languageEnglish
Pages (from-to)1806-1815
Number of pages10
JournalProceedings of the National Academy of Sciences of the United States of America
Volume117
Issue number3
DOIs
Publication statusPublished - 2020 Jan 21

Keywords

  • Autotransporter
  • Legume-rhizobium symbiosis
  • Nitrogen fixation

ASJC Scopus subject areas

  • General

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