Plant pathogens use effector proteins to target host processes involved in pathogen perception, immune signalling, or defence outputs. Unlike foliar pathogens, it is poorly understood how root‐invading pathogens suppress immunity. The Avr2 effector from the tomato root‐ and xylem‐colonizing pathogen
- Award ID(s):
- 1758843
- NSF-PAR ID:
- 10296977
- Date Published:
- Journal Name:
- Plant Physiology
- Volume:
- 185
- Issue:
- 4
- ISSN:
- 0032-0889
- Page Range / eLocation ID:
- 1986 to 2002
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Fusarium oxysporum suppresses immune signalling induced by various pathogen‐associated molecular patterns (PAMPs). It is unknown how Avr2 targets the immune system. TransgenicAVR2 Arabidopsis thaliana phenocopies mutants in which the pattern recognition receptor (PRR) co‐receptor BRI1‐ASSOCIATED RECEPTOR KINASE (BAK1) or its downstream signalling kinase BOTRYTIS‐INDUCED KINASE 1 (BIK1) are knocked out. We therefore tested whether these kinases are Avr2 targets. Flg22‐induced complex formation of the PRR FLAGELLIN SENSITIVE 2 and BAK1 occurred in the presence and absence of Avr2, indicating that Avr2 does not affect BAK1 function or PRR complex formation. Bimolecular fluorescence complementation assays showed that Avr2 and BIK1 co‐localize in planta. Although Avr2 did not affect flg22‐induced BIK1 phosphorylation, mono‐ubiquitination was compromised. Furthermore, Avr2 affected BIK1 abundance and shifted its localization from nucleocytoplasmic to the cell periphery/plasma membrane. Together, these data imply that Avr2 may retain BIK1 at the plasma membrane, thereby suppressing its ability to activate immune signalling. Because mono‐ubiquitination of BIK1 is required for its internalization, interference with this process by Avr2 could provide a mechanistic explanation for the compromised BIK1 mobility upon flg22 treatment. The identification of BIK1 as an effector target of a root‐invading vascular pathogen identifies this kinase as a conserved signalling component for both root and shoot immunity. -
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Abstract Basic helix–loop–helix (bHLH) transcription factors constitute a superfamily in eukaryotes, but their roles in plant immunity remain largely uncharacterized. We found that the transcript abundance in tomato (Solanum lycopersicum) leaves of one bHLH transcription factor-encoding gene, negative regulator of resistance to DC3000 1 (Nrd1), increased significantly after treatment with the immunity-inducing flgII-28 peptide. Plants carrying a loss-of-function mutation in Nrd1 (Δnrd1) showed enhanced resistance to Pseudomonas syringae pv. tomato (Pst) DC3000 although early pattern-triggered immunity responses, such as generation of reactive oxygen species and activation of mitogen-activated protein kinases after treatment with flagellin-derived flg22 and flgII-28 peptides, were unaltered compared to wild-type plants. RNA-sequencing (RNA-seq) analysis identified a gene, Arabinogalactan protein 1 (Agp1), whose expression is strongly suppressed in an Nrd1-dependent manner. Agp1 encodes an arabinogalactan protein, and overexpression of the Agp1 gene in Nicotiana benthamiana led to ∼10-fold less Pst growth compared to the control. These results suggest that the Nrd1 protein promotes tomato susceptibility to Pst by suppressing the defense gene Agp1. RNA-seq also revealed that the loss of Nrd1 function has no effect on the transcript abundance of immunity-associated genes, including AvrPtoB tomato-interacting 9 (Bti9), Cold-shock protein receptor (Core), Flagellin sensing 2 (Fls2), Flagellin sensing (Fls3), and Wall-associated kinase 1 (Wak1) upon Pst inoculation, suggesting that the enhanced immunity observed in the Δnrd1 mutants is due to the activation of key PRR signaling components as well as the loss of Nrd1-regulated suppression of Agp1.
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Summary The catalytic activity of mitogen‐activated protein kinases (
MAPK s) is dynamically modified in plants. SinceMAPK s have been shown to play important roles in a wide range of signaling pathways, the ability to monitorMAPK activity in living plant cells would be valuable. Here, we report the development of a genetically encodedMAPK activity sensor for use inArabidopsis thaliana . The sensor is composed of yellow and blue fluorescent proteins, a phosphopeptide binding domain, aMAPK substrate domain and a flexible linker. Usingin vitro testing, we demonstrated that phosphorylation causes an increase in the Förster resonance energy transfer (FRET ) efficiency of the sensor. TheFRET efficiency can therefore serve as a readout of kinase activity. We also produced transgenic Arabidopsis lines expressing this sensor ofMAPK activity (SOMA ) and performed live‐cell imaging experiments using detached cotyledons. Treatment with NaCl, the synthetic flagellin peptide flg22 and chitin all led to rapid gains inFRET efficiency. Control lines expressing a version ofSOMA in which the phosphosite was mutated to an alanine did not show any substantial changes inFRET . We also expressed the sensor in a conditional loss‐of‐function double‐mutant line for the ArabidopsisMAPK genesMPK 3MPK 6MPK 3/6 are necessary for the NaCl‐inducedFRET gain of the sensor, while otherMAPK s are probably contributing to the chitin and flg22‐induced increases inFRET . Taken together, our results suggest thatSOMA is able to dynamically reportMAPK activity in living plant cells.
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/plphys/kiab024
