OTHER REGULATORY COMPONENTS IN SAR SIGNALING
To identify other components of SAR signaling, several genetic screens have been conducted looking for suppressors of npr1. The recessive sni1 (suppressor of npr1 inducible) mutant restores SA-inducible PR gene expression and pathogen resistance in the npr1 background. The sni1 mutant has wild-type levels of SA and only slightly elevated expression of PR genes in the absence of an SAR inducer, indicating that sni1 is likely downstream of npr. The sni1 phenotype suggests that SNI1 is a negative regulator of PR gene expression and SAR, whose repression is relieved by NPR1 after induction. The low basal levels of PR gene expression and restored induction in the sni1 npr1double mutant indicate that, in addition to NPR1-mediated inactivation of SNI1, an SA-dependent but NPR1-independent regulatory step is also required for SAR gene induction. Consistent with the hypothesis that SNI1 is a repressor of PR genes, SNI1-GFP has been observed in the nucleus when bombarded into onion epidermal cells. This result suggests that SNI1 may repress a general transcriptional mechanism that is conserved between yeast and plants.
SNI1 is a novel plant-specific protein with no similarity to proteins of known function. However, putative homologues have been found in many plant species including barley, Medicago truncatula, potato, rice, soybean, and sugar cane, indicating that SNI1 function may be conserved. SNI1 contains no obvious DNA-binding domain, suggesting that it represses transcription through interaction with other factors rather than binding directly to the promoter. Linker scanning mutagenesis of the PR-1 promoter identified the negative regulatory cis-element, LS4, which has a W-box consensus sequence. Mutation of LS4 resulted in elevated basal expression and stronger induction in response to INA treatment, a pattern similar to that of PR-1 in sni1. This suggests that SNI1 might be recruited to the PR-1promoter through interaction with a WRKY factor.
As an alternative approach to investigate the function of SNI1, a screen for suppressors of sni1 (ssn) was performed. Three mutants were identified, ssn1, ssn2, and ssn3, which alleviate both the dwarf morphology and the background PR gene expression of sni1 (W. Durrant & X. Dong, unpublished observations). In the sni1 ssn double mutants, the pattern of PR gene expression is the same as wild-type, whereas in the sni1 npr1 ssntriple mutants induction of PR gene expression by SA is blocked, similar to npr1. These results indicate that the wild-type SSN genes are involved in controlling both basal and SA-inducible PR gene expression observed in sni1 npr1. In other words, the same transcription factor(s) is probably responsible for the background and SA-inducible NPR1-independent PR gene expression.
Other possible SAR regulatory components include DTH9 (DETACHMENT 9). The dth9mutant has increased susceptibility to virulent pathogens, accumulates elevated levels of SA, and fails to develop SAR in response to pathogen infection or SA treatment. These phenotypes are reminiscent of npr1; however, dth9 differs from npr1 in that it’s PR-1 and PR-2 expression in response to infection or SA treatment is unaltered. Since SA treatment did not reverse the disease susceptibility observed in the mutant, DTH9 should be placed downstream of SA in a pathway parallel to NPR1 that contributes to SAR. Interestingly, dth9 is also insensitive to auxin treatment, indicating that auxin signaling may play a role in plant defense.