Abstract
Plant defense responses need to be tightly regulated to prevent auto-immunity, which is detrimental to growth and development. To identify negative regulators of Resistance (R) protein-mediated resistance, we screened for mutants with constitutive defense responses in the npr1-1 background. Map-based cloning revealed that one of the mutant genes encodes a conserved TPR domain-containing protein previously known as SRFR1 (SUPPRESSOR OF rps4-RLD). The constitutive defense responses in the srfr1 mutants in Col-0 background are suppressed by mutations in SNC1, which encodes a TIR-NB-LRR (Toll Interleukin1 Receptor-Nucleotide Binding-Leu-Rich Repeat) R protein. Yeast two-hybrid screens identified SGT1a and SGT1b as interacting proteins of SRFR1. The interactions between SGT1 and SRFR1 were further confirmed by co-immunoprecipitation analysis. In srfr1 mutants, levels of multiple NB-LRR R proteins including SNC1, RPS2 and RPS4 are increased. Increased accumulation of SNC1 is also observed in the sgt1b mutant. Our data suggest that SRFR1 functions together with SGT1 to negatively regulate R protein accumulation, which is required for preventing auto-activation of plant immunity.
Highlights
To protect themselves from infections by microbial pathogens, plants have evolved a large number of immune receptors to sense pathogen-derived molecules and trigger defense responses [1]
The constitutive activation of immune responses in the srfr1 mutants is dependent on the nucleotide-binding domain and LRR-containing (NLR) Resistance (R) protein SNC1
Consistent with previous findings that SGT1b is involved in the negative regulation of protein levels of certain NLR R proteins, increased accumulation of SNC1 is observed in the sgt1b mutant
Summary
To protect themselves from infections by microbial pathogens, plants have evolved a large number of immune receptors to sense pathogen-derived molecules and trigger defense responses [1]. Resistance (R) proteins with nucleotide-binding (NB) and Leucinerich repeat (LRR) domains constitute the main type of intracellular plant immune receptors. Similar nucleotide-binding domain and LRR-containing (NLR) proteins function as intracellular immune receptors [2]. Among the components that are required for R protein triggered immune responses, RAR1, HSP90 and SGT1 are three conserved proteins that function in correct folding and stabilization of NLR R proteins [4]. Accumulation of barley MLA proteins, potato Rx, and Arabidopsis RPM1 and RPS5 was reduced when RAR1 function was compromised [7,10,11]. Compromising the activity of HSP90 caused reduced accumulation of several R proteins including RPM1, RPS5 and Rx [11,12,13]. It was suggested that SGT1b antagonize RAR1 in regulating the accumulation of certain R proteins [11]
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