Abstract
Author SummaryThe highly conserved Mediator complex plays an essential role in transcriptional regulation by providing a molecular bridge between transcription factors and RNA polymerase II. Recent studies in Arabidopsis have revealed that it also performs an essential role in plant defence. However, it remains unknown how pathogens manipulate Mediator function in order to increase a plant's susceptibility to infection. In this article, we show that a secreted effector, HaRxL44, from the Arabidopsis downy mildew pathogen Hyaloperonospora arabidopsidis (Hpa), interacts with and degrades the Mediator subunit MED19a, resulting in the alteration of plant defence gene transcription. This effector-mediated interference with host transcriptional regulation perturbs the balance between jasmonic acid/ethylene (JA/ET) and salicylic acid (SA)–dependent defence. HaRxL44 interaction with MED19a results in reduced SA-regulated gene expression, indicating that this pathogen effector modulates host transcription to promote virulence. The resulting alteration in defence transcription patterns compromises the plant's ability to defend itself against pathogens, such as Hpa, that establish long-term parasitic interactions with living host cells via haustoria (a pathogen structure that creates an expanded host/parasite interface to extract nutrients) but not against necrotrophic pathogens that kill host cells. HaRxL44 is unlikely to be the sole effector that accomplishes this shift in hormonal balance, and other nuclear HaRxL proteins were reported by other researchers to interact with Mediator components, as well as with other regulators of the JA/ET signalling pathway. Functional analyses of these effectors should facilitate the discovery of new components of the plant immune system. These data show that pathogens can target fundamental mechanisms of host regulation in order to tip the balance of signalling pathways to suppress defence and favour parasitism.
Highlights
Plants and microbial pathogens co-evolve; pathogens are selected to evade host defence, and plants are selected to detect and resist pathogens [1,2]
We show that a secreted effector, HaRxL44, from the Arabidopsis downy mildew pathogen Hyaloperonospora arabidopsidis (Hpa), interacts with and degrades the Mediator subunit Mediator subunit 19a (MED19a), resulting in the alteration of plant defence gene transcription
This effector-mediated interference with host transcriptional regulation perturbs the balance between jasmonic acid/ ethylene (JA/ET) and salicylic acid (SA)–dependent defence
Summary
Plants and microbial pathogens co-evolve; pathogens are selected to evade host defence, and plants are selected to detect and resist pathogens [1,2]. An important role in plant defence has been attributed to nuclear processes, since there are many reports that nuclear localisation of pathogen effectors, R proteins, and key host components, including transcription factors and regulators, is essential for plant immunity [6]. This observation suggests that effectors may manipulate host transcription or other nuclear regulatory components for the benefit of the pathogen. Filamentous phytopathogens such as fungal rusts and powdery mildews and oomycete downy mildews and white rusts are more damaging to agriculture than bacteria, their effector functions are more poorly understood. Fungal and oomycete effectors are secreted, and taken up by the host cell via a poorly understood mechanism that for many oomycetes involves
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