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. Plants, like humans, have an immune system to defend against disease. This immune system seeks out the presence of disease-causing microbes and other invaders by detecting non-plant molecules and proteins. Plants rely on this surveillance to activate an antimicrobial response of appropriate strength at the right time; as with humans, an overactive immune system can be harmful to plants. We study how plants achieve an appropriate balance, using genetics and the interaction between the reference plant Arabidopsis thaliana and the bacterial plant pathogen Pseudomonas syringae. So-called plant resistance proteins are important activators of immunity that directly or indirectly intercept foreign proteins deployed by pathogens. Resistance proteins are generally thought to be highly specific detectors that only respond to a single pathogen protein. However, while working with a negative regulator of plant immunity called SRFR1, we discovered a surprising level of cross-talk between different resistance proteins that becomes evident only under certain environmental conditions such as low temperature. We also show that SRFR1 and these resistance proteins bind to each other, possibly explaining the observed cross-talk. Our work thus highlights linkages between resistance pathways and provides insight into the molecular architecture of the plant innate immune response.