A comprehensive structure-function analysis of Arabidopsis SNI1 defines essential regions and transcriptional repressor activity

The expression of systemic acquired resistance (SAR) in plants involves the upregulation of many Pathogenesis-Related (PR) genes, which work in concert to confer resistance to a broad spectrum of pathogens. Because SAR is a costly process, SAR-associated transcription must be tightly regulated. Arabidopsis thaliana SNM (for Suppressor of NPR1, Inducible) is a negative regulator of SAR required to dampen the basal expression of PR genes. Whole genome transcriptional profiling showed that in the sni1 mutant, Nonexpresser of PR genes (NPR1)-dependent benzothiadiazole S-methylester-responsive genes were specifically derepressed. Interestingly, SNM also repressed transcription when expressed in yeast, suggesting that it functions as an active transcriptional repressor through a highly conserved mechanism. Chromatin immunoprecipitation indicated that histone modification may be involved in SNI1-mediated repression. Sequence comparison with orthologs in other plant species and a saturating NAAIRS-scanning mutagenesis of SNM identified regions in SNM that are required for its activity. The structural similarity of SNM to Armadillo repeat proteins implies that SNM may form a scaffold for interaction with proteins that modulate transcription.