Pseudomonas syringae type III effector HopAF1 suppresses plant immunity by targeting methionine recycling to block ethylene induction

Erica J. Washington, M. Shahid Mukhtar, Omri M. Finkel, Li Wan, Mark J. Banfield, Joseph J. Kieber, Jeffery L. Dangl*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

42 Scopus citations


HopAF1 is a type III effector protein of unknown function encoded in the genomes of several strains of Pseudomonas syringae and other plant pathogens. Structural modeling predicted that HopAF1 is closely related to deamidase proteins. Deamidation is the irreversible substitution of an amide group with a carboxylate group. Several bacterial virulence factors are deamidases that manipulate the activity of specific host protein substrates. We identified Arabidopsis methylthioadenosine nucleosidase proteins MTN1 and MTN2 as putative targets of HopAF1 deamidation. MTNs are enzymes in the Yang cycle, which is essential for the high levels of ethylene biosynthesis in Arabidopsis. We hypothesized that HopAF1 inhibits the host defense response by manipulating MTN activity and consequently ethylene levels. We determined that bacterially delivered HopAF1 inhibits ethylene biosynthesis induced by pathogen-associated molecular patterns and that Arabidopsis mtn1 mtn2 mutant plants phenocopy the effect of HopAF1. Furthermore, we identified two conserved asparagines in MTN1 and MTN2 from Arabidopsis that confer loss of function phenotypes when deamidated via site-specific mutation. These residues are potential targets of HopAF1 deamidation. HopAF1-mediated manipulation of Yang cycle MTN proteins is likely an evolutionarily conserved mechanism whereby HopAF1 orthologs from multiple plant pathogens contribute to disease in a large variety of plant hosts.

Original languageAmerican English
Pages (from-to)E3577-E3586
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number25
StatePublished - 21 Jun 2016
Externally publishedYes

Bibliographical note

Funding Information:
We thank members of the J.L.D. laboratory, especially Drs. David Hubert, Marc Nishimura, and Sarah Grant, for technical advice, helpful suggestions, and critical reading of the manuscript; Prof. Jeff Jones for plasmids and seeds; Prof. Margaret Sauter for antibodies and seeds; Tony Perdue for assistance with confocal microscopy; Gyeong Mee Yoon and Smadar Harpaz-saad for help with gas chromatography; and James Garzoni for greenhouse assistance. This work was funded by NIH Grant 1RO1 GM107444, Gordon and Betty Moore Foundation Grant GBMF3030, and National Science Foundation Grant IOS-1257373 (all to J.L.D.); NIH Training Grants T32 GM008581 (from the National Institute of General Medical Sciences) and T32 AI007273 (from the National Institute of Allergy and Infectious Diseases) (both to E.J.W.); NIH Dr. Ruth L. Kirschstein National Research Service Award Fellowship GM117758 (to O.M.F.); National Science Foundation Grant IOS 1456658 (to J.J.K.); and Biotechnology and Biological Sciences Research Council (UK) Grants BB/J004553/1 and BB/F008732/1 (both to M.J.B.). J.D.L. is an investigator of the Howard Hughes Medical Institute. M.J.B. is a fellow with the John Innes Foundation.

Publisher Copyright:
© 2016, National Academy of Sciences. All rights reserved.


  • Ethylene
  • Plant immune system
  • Pseudomonas syringae
  • Type III effectors
  • Yang cycle


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