Abstract: The NIM1 gene product is a structural homologue of the mammalian signal transduction factor I.kappa.B subclass .alpha.. The present invention exploits this discovery to provide altered forms of NIM1 that act as dominant-negative regulators of the systemic acquired resistance (SAR) signal transduction pathway. These altered forms of NIM1 confer the opposite phenotype as the nim1 mutant in plants transformed with the altered forms of NIM1; i.e., the transgenic plants exhibit constitutive SAR gene expression and a constitutive immunity (CIM) phenotype. The present invention further concerns DNA molecules encoding altered forms of the NIM1 gene, expression vectors containing such DNA molecules, and plants and plant cells transformed therewith. The invention also concerns methods of activating SAR in plants and conferring to plants a CIM phenotype and broad spectrum disease resistance by transforming the plants with DNA molecules encoding altered forms of the NIM1 gene product.