Abstract
An important layer of plant innate immunity to host-adapted pathogens is conferred by intracellular nucleotide-binding/oligomerization domain-leucine rich repeat (NB-LRR) receptors recognizing specific microbial effectors. Signaling from activated receptors of the TIR (Toll/Interleukin-1 Receptor)-NB-LRR class converges on the nucleo-cytoplasmic immune regulator EDS1 (Enhanced Disease Susceptibility1). In this report we show that a receptor-stimulated increase in accumulation of nuclear EDS1 precedes or coincides with the EDS1-dependent induction and repression of defense-related genes. EDS1 is capable of nuclear transport receptor-mediated shuttling between the cytoplasm and nucleus. By enhancing EDS1 export from inside nuclei (through attachment of an additional nuclear export sequence (NES)) or conditionally releasing EDS1 to the nucleus (by fusion to a glucocorticoid receptor (GR)) in transgenic Arabidopsis we establish that the EDS1 nuclear pool is essential for resistance to biotrophic and hemi-biotrophic pathogens and for transcriptional reprogramming. Evidence points to post-transcriptional processes regulating receptor-triggered accumulation of EDS1 in nuclei. Changes in nuclear EDS1 levels become equilibrated with the cytoplasmic EDS1 pool and cytoplasmic EDS1 is needed for complete resistance and restriction of host cell death at infection sites. We propose that coordinated nuclear and cytoplasmic activities of EDS1 enable the plant to mount an appropriately balanced immune response to pathogen attack.
Highlights
IntroductionInnate immune responses of individual cells constitute a major barrier to pathogen infection
In animals and plants, innate immune responses of individual cells constitute a major barrier to pathogen infection
These results show that constitutive Toll/Interleukin-1 Receptor (TIR)-nucleotide-binding/ oligomerization domain-leucine rich repeat (NB-leucine rich repeat (LRR)) resistance increases accumulation of EDS1 protein in both cell compartments
Summary
Innate immune responses of individual cells constitute a major barrier to pathogen infection. Recognition of microbe- or damage-associated molecules is mediated by germ line encoded receptors whose activation is transduced by intracellular signaling systems to an anti-microbial response. Membrane pattern recognition receptors (PRRs) with external ligand recognition domains and intracellular kinase domains detect conserved pathogen molecules (Microbe Associated Molecular Patterns or MAMPs) in a similar manner to non-self recognition in animals [2]. Many pathogen effectors are delivered inside host cells and plants possess intracellular recognition systems mediated by nucleotide-binding and oligomerisation domain (NB or NOD)leucine rich repeat (LRR) immune receptors [1]. Related NOD-LRR proteins, known as CATERPILLER, NACHT-LRR or NOD-like receptors (NLRs), serve as pathogen and damage sensors in innate immune responses and cell death control in mammals [3]. Little is known about events between NB-LRR activation and defense induction or the mechanisms which limit resistance signaling to prevent auto-immune reactions
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