A novel mutant allele of SSI2 confers a better balance between disease resistance and plant growth inhibition on Arabidopsis thaliana

Background: Resistance and growth are opposing characteristics in plants. SA INSENSITIVITY OF npr1-5 (SSI2) encodes a stearoyl-ACP desaturase (S-ACP DES) that has previously been reported to simultaneously enhance resistance and repress growth. Results: Here, we characterize ssi2-2, a novel mutant allele of SSI2 that has two amino acid substitutions. Compared with wild-type and two other mutants of SSI2, ssi2-2 showed intermediate phenotypes in growth size, punctate necrosis, resistance to the bacterial pathogen Pst DC3000, salicylic acid (SA) content, pathogenesis-related (PR) gene levels and 18: 1 content. These results indicate that ssi2-2 is a weak mutant of SSI2. Additionally, by using ssi2-2 as an intermediate control, a number of differentially expressed genes were identified in transcriptome profiling analysis. These results suggest that constitutive expression of defense-related genes and repression of IAA signaling-associated genes is present in all SSI2 mutants. Conclusions: Taken together, our results suggest that the weak ssi2-2 mutant maintains a better balance between plant immunity and vegetative growth than other mutants, consequently providing a basis to genetically engineer disease resistance in crop plants.