The Arabidopsis immune adaptor SRFR1 interacts with TCP transcription factors that redundantly contribute to effector‐triggered immunity

Abstract

The plant immune system must be tightly controlled both positively and negatively to maintain normal plant growth and health. We previously identified SUPPRESSOR OF rps4-RLD1 (SRFR1) as a negative regulator specifically of effector-triggered immunity. SRFR1 is localized in both a cytoplasmic microsomal compartment and in the nucleus. Its TPR domain has sequence similarity to TPR domains of transcriptional repressors in other organisms, suggesting that SRFR1 may negatively regulate effector-triggered immunity via transcriptional control. We show here that excluding SRFR1 from the nucleus prevented complementation of the srfr1 phenotype. To identify transcription factors that interact with SRFR1, we screened an Arabidopsis transcription factor prey library by yeast two-hybrid assay and isolated six class I members of the TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) transcription factor family. Specific interactions were verified in planta. Although single or double T-DNA mutant tcp8, tcp14 or tcp15 lines were not more susceptible to bacteria expressing AvrRps4, the triple tcp8 tcp14 tcp15 mutant displayed decreased effector-triggered immunity mediated by the resistance genes RPS2, RPS4, RPS6 and RPM1. In addition, expression of PATHOGENESIS-RELATED PROTEIN2 was attenuated in srfr1-4 tcp8-1 tcp14-5 tcp15-3 plants compared to srfr1-4 plants. To date, TCP transcription factors have been implicated mostly in developmental processes. Our data indicate that one function of a subset of TCP proteins is to regulate defense gene expression in antagonism to SRFR1, and suggest a mechanism for an intimate connection between plant development and immunity.

TheArabidopsis thalianaTCP transcription factors: A broadening horizon beyond development

“…Of these SRFR1-interacting TCP proteins, TCP8, TCP14, and TCP15 interact most strongly with SRFR1, whereas TCP20, TCP22, and TCP23 display weak interactions. 64 The availability of the single, double and triple mutants of TCP8, TCP14, and TCP15 makes it feasible to explore genetic interactions between SRFR1 and TCP genes. 64 In planta bacterial growth experiments reveal that the growth of DC30000 (avrRps4) in single and double mutants does not significantly differ from that observed in the wild type Col-0.…”

“…A second vivid example supporting the direct participation of TCP proteins in the plant immune system arises from the identification of SUPPRESSOR OF rps4-RLD1 (SRFR1) as a specific negative regulator of ETI. 64 SRFR1 exists as an adaptor protein in cytoplasmic microsomal and nuclear protein complexes containing the defense regulator ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) and resistance proteins of the TIR-NB-LRR class. 106,107 The Arabidopsis accession RLD carrying the srfr1-1 allele exhibits enhanced resistance to DC3000 (avrRps4) but not to virulent DC30000, and excluding SRFR1 from the nucleus disables the complementation of the srfr1-1 mutant.…”

“…[98][99][100][101][102][103][104][105] Several recent studies have demonstrated that TCP transcription factors also function as a cellular hub in plant defense signaling. [63][64][65] Based on the systematic analyses of binary protein-protein interactions between Arabidopsis immune proteins, ca. 8000 full-length Arabidopsis proteins and effector proteins of 2 evolutionarily divergent pathogens, including the Gram-negative bacterium Pseudomonas syringae (Psy) and the obligate biotrophic oomycete Hyaloperonospora arabidopsidis (Hpa), a plant-pathogen protein-protein interactome network was created, which contain 1358 interactions among 926 proteins, including 83 pathogen effectors, 170 immune proteins and 673 immune interactors.…”

“…108 This observation suggests that a nuclear activity of SRFR is required for its function in suppressing resistance to DC30000 (avrRps4) in RLD plants. 64 To identify nuclear proteins that associate specifically with SRFR1, yeast 2-hybrid assays are performed, leading to the isolation of 6 class I members of the TCP transcription factor family. Of these SRFR1-interacting TCP proteins, TCP8, TCP14, and TCP15 interact most strongly with SRFR1, whereas TCP20, TCP22, and TCP23 display weak interactions.…”

“…TCP proteins from various species throughout the plant kingdom have been demonstrated to regulate a large variety of biological processes, such as floral asymmetry, 23-34 plant architectures including branching, [35][36][37][38][39] lateral organ development, 40-55 seed germination, 56,57 gametophyte development, 58,59 leaf senescence, 44,60 circadian rhythms, 61,62 and defense responses. [63][64][65] Noteworthily, several aspects of these plant processes involve a regulatory role for TCPs in the biosynthetic pathways of bioactive compounds. 19,60,[66][67][68][69] In this review, developmental functions of TCP proteins are synoptically summarized and their participation in the biosynthetic processes impacting both plant development and defense against leafhoppers are emphasized (Table A1).…”