A downy mildew effector attenuates salicylic acid–triggered immunity in Arabidopsis by interacting with the host mediator complex

Caillaud, Marie-Cécile, Asai, Shuta, Rallapalli, Ghanasyam, Piquerez, Sophie, Fabro, Georgina and Jones, Jonathan D. G. (2013) A downy mildew effector attenuates salicylic acid–triggered immunity in Arabidopsis by interacting with the host mediator complex. PLoS Biology, 11 (12). ISSN 1545-7885

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Abstract

Plants are continually exposed to pathogen attack but usually remain healthy because they can activate defences upon perception of microbes. However, pathogens have evolved to overcome plant immunity by delivering effectors into the plant cell to attenuate defence, resulting in disease. Recent studies suggest that some effectors may manipulate host transcription, but the specific mechanisms by which such effectors promote susceptibility remain unclear. We study the oomycete downy mildew pathogen of Arabidopsis, Hyaloperonospora arabidopsidis (Hpa), and show here that the nuclear-localized effector HaRxL44 interacts with Mediator subunit 19a (MED19a), resulting in the degradation of MED19a in a proteasome-dependent manner. The Mediator complex of ∼25 proteins is broadly conserved in eukaryotes and mediates the interaction between transcriptional regulators and RNA polymerase II. We found MED19a to be a positive regulator of immunity against Hpa. Expression profiling experiments reveal transcriptional changes resembling jasmonic acid/ethylene (JA/ET) signalling in the presence of HaRxL44, and also 3 d after infection with Hpa. Elevated JA/ET signalling is associated with a decrease in salicylic acid (SA)-triggered immunity (SATI) in Arabidopsis plants expressing HaRxL44 and in med19a loss-of-function mutants, whereas SATI is elevated in plants overexpressing MED19a. Using a PR1::GUS reporter, we discovered that Hpa suppresses PR1 expression specifically in cells containing haustoria, into which RxLR effectors are delivered, but not in nonhaustoriated adjacent cells, which show high PR1::GUS expression levels. Thus, HaRxL44 interferes with Mediator function by degrading MED19, shifting the balance of defence transcription from SA-responsive defence to JA/ET-signalling, and enhancing susceptibility to biotrophs by attenuating SA-dependent gene expression.

Item Type:	Article
Additional Information:	© 2013 Caillaud et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Uncontrolled Keywords:	arabidopsis,arabidopsis proteins,host-pathogen interactions,mediator complex,peronospora,plant diseases,plant growth regulators,plant immunity,salicylic acid
Faculty \ School:	Faculty of Medicine and Health Sciences > Norwich Medical School Faculty of Science > School of Biological Sciences
UEA Research Groups:	Faculty of Science > Research Groups > Plant Sciences
Depositing User:	Pure Connector
Date Deposited:	09 Nov 2016 15:00
Last Modified:	02 Aug 2023 16:30
URI:	https://ueaeprints.uea.ac.uk/id/eprint/61294
DOI:	10.1371/journal.pbio.1001732

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