Positive and negative regulation of salicylic acid-dependent cell death and pathogen resistance in arabidopsis lsd6 and ssi1 mutants

Salicylic acid (SA) is a key defense molecule in higher plants that is requ ired for resistance to diverse pathogens. A number of mutants of Arabidopsi s with elevated resistance to pathogens and constitutive activation of defe nse-related genes and cell death have been shown to require SA for all of t heir phenotypes. These mutants potentially identify interesting regulatory genes that control diverse SA responses. When dominant mutations confer SA- dependent phenotypes, it is important to know the genetic basis of dominanc e in order to draw conclusions on the possible mechanisms of action of the genes identified. Here I characterize the basis of the dominant phenotypes conferred by the ssi1 and lsd6 mutations, I show that ssi1 is haploinsuffic ient, while lsd6 is a gain-of-function mutation. Thus, SA-dependent respons es are under both negative and positive regulation.