The Arabidopsis Resistance-Like Gene SNC1 Is Activated by Mutations in SRFR1 and Contributes to Resistance to the Bacterial Effector AvrRps4

Abstract

The SUPPRESSOR OF rps4-RLD1 (SRFR1) gene was identified based on enhanced AvrRps4-triggered resistance in the naturally susceptible Arabidopsis accession RLD. No other phenotypic effects were recorded, and the extent of SRFR1 involvement in regulating effector-triggered immunity was unknown. Here we show that mutations in SRFR1 in the accession Columbia-0 (Col-0) lead to severe stunting and constitutive expression of the defense gene PR1. These phenotypes were temperature-dependent. A cross between srfr1-1 (RLD background) and srfr1-4 (Col-0) showed that stunting was caused by a recessive locus in Col-0. Mapping and targeted crosses identified the Col-0-specific resistance gene SNC1 as the locus that causes stunting. SRFR1 was proposed to function as a transcriptional repressor, and SNC1 is indeed overexpressed in srfr1-4. Interestingly, co-regulated genes in the SNC1 cluster are also upregulated in the srfr1-4 snc1-11 double mutant, indicating that the overexpression of SNC1 is not a secondary effect of constitutive defense activation. In addition, a Col-0 RPS4 mutant showed full susceptibility to bacteria expressing avrRps4 at 24°C but not at 22°C, while RLD susceptibility was not temperature-dependent. The rps4-2 snc1-11 double mutant showed increased, but not full, susceptibility at 22°C, indicating that additional cross-talk between resistance pathways may exist. Intriguingly, when transiently expressed in Nicotiana benthamiana, SRFR1, RPS4 and SNC1 are in a common protein complex in a cytoplasmic microsomal compartment. Our results highlight SRFR1 as a convergence point in at least a subset of TIR-NBS-LRR protein-mediated immunity in Arabidopsis. Based on the cross-talk evident from our results, they also suggest that reports of constitutive resistance phenotypes in Col-0 need to consider the possible involvement of SNC1.

Two linked pairs of Arabidopsis TNL resistance genes independently confer recognition of bacterial effector AvrRps4

“…2d and Supplementary Fig. 1), in contrast to previous reports 15,21,29 , most likely due to different growth conditions. RPS4B RLD premature stop codon causes its loss of function.…”

Stability of plant immune-receptor resistance proteins is controlled by SKP1-CULLIN1-F-box (SCF)-mediated protein degradation

“…Because SRFR1 interacts with SGT1 and SGT1 associates with SKP1 and CUL1 in vivo (7,11,13), SRFR1 and SGT1b may regulate the stability of SNC1 through modulating the activity of SCF CPR1 . In support of this hypothesis, association of SNC1 and SRFR1 was detected from coimmunoprecipitation experiments in transient expression system in tobacco (12).…”

“…Recently, the evolutionarily conserved SRFR1 was shown to interact directly with SGT1 (11). Loss-of-function of SRFR1 results in increased accumulation of SNC1 and activation of SNC1-mediated defense responses (11,12). However, the mechanism on how SGT1 and SRFR1 negatively regulate the accumulation of R proteins is unclear.…”

Natural Variation in Small Molecule–Induced TIR-NB-LRR Signaling Induces Root Growth Arrest via EDS1- and PAD4-Complexed R Protein VICTR inArabidopsis

“…These findings and the DFPM-induced VICTR promoter activity in wild-type plants suggest a role for VICTR protein levels in determining this interesting root meristemlocalized EDS1-and PAD4-dependent response. As described for delocalized or overexpressed R proteins (Zhang et al, 2003b;Yi and Richards, 2007;Kim et al, 2010), ectopic VICTR expression also leads to stunted growth phenotypes.…”

“…HA-tagged VICTR was generated using pENTR-TOPO-VICTR and the destination vector HA-pBA (Kim et al, 2010). Myc-tagged EDS1, PAD4, and GUS have been used before (Bhattacharjee et al, 2011).…”