Abstract
Following the recognition of pathogen-encoded effectors, plant TIR-NB-LRR immune receptors induce defense signaling by a largely unknown mechanism. We identify a novel and conserved role for the SQUAMOSA PROMOTER BINDING PROTEIN (SBP)-domain transcription factor SPL6 in enabling the activation of the defense transcriptome following its association with a nuclear-localized immune receptor. During an active immune response, the Nicotiana TIR-NB-LRR N immune receptor associates with NbSPL6 within distinct nuclear compartments. NbSPL6 is essential for the N-mediated resistance to Tobacco mosaic virus. Similarly, the presumed Arabidopsis ortholog AtSPL6 is required for the resistance mediated by the TIR-NB-LRR RPS4 against Pseudomonas syringae carrying the avrRps4 effector. Transcriptome analysis indicates that AtSPL6 positively regulates a subset of defense genes. A pathogen-activated nuclear-localized TIR-NB-LRR like N can therefore regulate defense genes through SPL6 in a mechanism analogous to the induction of MHC genes by mammalian immune receptors like CIITA and NLRC5.
Highlights
Plants employ the Nucleotide Binding-Leucine Rich Repeat (NBLRR) family of intracellular receptors to detect pathogens and initiate defense signaling [1,2]
The tobacco N immune receptor that provides immunity against Tobacco mosaic virus (TMV) infection is present in the nucleus and associates with the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 6 (SPL6) transcription factor
We show that Arabidopsis SPL6 is required for the function of TIR-NB-LRR RPS4 but not for CC-NB-LRRs RPS2 and RPM1
Summary
Plants employ the Nucleotide Binding-Leucine Rich Repeat (NBLRR) family of intracellular receptors to detect pathogens and initiate defense signaling [1,2]. NB-LRR association with an effector and subsequent receptor activation leads to a number of cellular responses that includes massive transcriptional reprogramming [3]. These responses often culminate in a specialized form of programmed cell death (PCD) - the hypersensitive response (HR) that restricts pathogen to the infection site thereby protecting the rest of the plant from disease [4]. Arabidopsis TIR-NB-LRR SNC1 associates with the transcriptional repressor TOPLESS-RELATED 1 (TPR1) to negatively regulate expression of known defense suppressors [7]. Arabidopsis RRS1-R is an atypical immune receptor that has the TIR-NB-LRR domains fused to a C-terminal WRKY domain which is characteristic of WRKY-type plant transcription factors [8]. Plant NB-LRR interaction with a positive regulator of defense gene transcription has not been described
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