Abstract
In plants and animals, nucleotide-binding and leucine-rich repeat domain containing (NLR) immune receptors are utilized to detect the presence or activities of pathogen-derived molecules. However, the mechanisms by which NLR proteins induce defense responses remain unclear. Here, we report the characterization of one basic Helix-loop-Helix (bHLH) type transcription factor (TF), bHLH84, identified from a reverse genetic screen. It functions as a transcriptional activator that enhances the autoimmunity of NLR mutant snc1 (suppressor of npr1-1, constitutive 1) and confers enhanced immunity in wild-type backgrounds when overexpressed. Simultaneously knocking out three closely related bHLH paralogs attenuates RPS4-mediated immunity and partially suppresses the autoimmune phenotypes of snc1, while overexpression of the other two close paralogs also renders strong autoimmunity, suggesting functional redundancy in the gene family. Intriguingly, the autoimmunity conferred by bHLH84 overexpression can be largely suppressed by the loss-of-function snc1-r1 mutation, suggesting that SNC1 is required for its proper function. In planta co-immunoprecipitation revealed interactions between not only bHLH84 and SNC1, but also bHLH84 and RPS4, indicating that bHLH84 associates with these NLRs. Together with previous finding that SNC1 associates with repressor TPR1 to repress negative regulators, we hypothesize that nuclear NLR proteins may interact with both transcriptional repressors and activators during immune responses, enabling potentially faster and more robust transcriptional reprogramming upon pathogen recognition.
Highlights
Plants have evolved a sophisticated immune system to fight against invading microbial pathogens that threaten their normal growth and development
Knocking out three closely related basic Helix-loop-Helix (bHLH) paralogs partially suppresses the autoimmunity of snc1 and compromises RPS4-mediated defense, while overexpression of these close paralogs renders strong autoimmunity, suggesting functional redundancy in the gene family
BHLH84 family transcription factors associate with these nucleotide-binding and leucine-rich repeat domain containing (NLR) to activate defense responses, enabling potentially faster and more robust transcriptional reprogramming upon pathogen recognition
Summary
Plants have evolved a sophisticated immune system to fight against invading microbial pathogens that threaten their normal growth and development. The active state of MLA10 can release MYB6 (MYB DOMAIN PROTEIN 6) from WRKY suppression and promote its binding to cis-elements to initiate defense responses [15]. SNC1 associates with transcriptional corepressor TPR1 (TOPLESS RELATED 1) to negatively regulate the expression of known defense suppressors, thereby activating plant immunity [17]. Studies on N in tobacco showed that it is able to associate with the TF SPL6 (SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 6) upon effector recognition [18]. From these data, it has been hypothesized that some NLRs associate with TFs inside the nucleus to directly participate in transcriptional reprogramming to regulate downstream defense responses
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