Uncoupling salicylic acid-dependent cell death and defense-related responses from disease resistance in the Arabidopsis mutant acd5

Salicylic acid (SA) is required for resistance to many diseases in higher p lants. SA-dependent cell death and defense-related responses have been corr elated with disease resistance. The accelerated cell death 5 mutant of Arab idopsis provides additional genetic evidence that SA regulates cell death a nd defense-related responses. However, in acd5, these events are uncoupled from disease resistance. acd5 plants are more susceptible to Pseudomonas sy ringae early in development and show spontaneous SA accumulation, cell deat h, and defense-related markers later in development. In acd5 plants, cell d eath and defense-related responses are SA dependent but they do not confer disease resistance. Double mutants with acd5 and nonexpressor of PR1, in wh ich SA signaling is partially blocked, show greatly attenuated cell death, indicating a role for NPR1 in controlling cell death. The hormone ethylene potentiates the effects of SA and is important for disease symptom developm ent in Arabidopsis. Double mutants of acd5 and ethylene insensitive 2, in w hich ethylene signaling is blocked, show decreased cell death, supporting a role for ethylene in cell death control. We propose that acd5 plants mimic P. syringae-infected wild-type plants and that both SA and ethylene are no rmally involved in regulating cell death during some susceptible pathogen i nfections.