Generalized Induction of Defense Responses in Bean Is Not Correlated with the Induction of the Hypersensitive Reaction.

Abstract

Transcripts for phenylalanine ammonia-lyase, chalcone synthase, chalcone isomerase, and chitinase accumulated in common bean after infiltration with the Pseudomonas syringae pv tabaci Hrp- mutant Pt11528::Hrp1, even though a hypersensitive reaction did not occur. The temporal pattern of this transcript accumulation was similar to that seen after infiltration with wild-type P. s. tabaci Pt11528, which resulted in a hypersensitive reaction. Escherichia coli DH5[alpha], P. fluorescens Pf101, heat-killed Pt11528 cells, and Pt11528 cells treated with protein synthesis inhibitors also induced accumulation of defense transcripts but not a hypersensitive reaction. In contrast, these transcripts were not detected in plants infiltrated with water or P.s. pv phaseolicola NPS3121, a compatible pathogen that causes halo blight. Phytoalexins were produced in bean after infiltration with Pt11528, Pt11528::Hrp1, Pt11528 cells treated with neomycin, or Pf101, but not in plants infiltrated with NPS3121 or water. These results suggest that there are unique biochemical events associated with the expression of a hypersensitive reaction which are distinct from other plant defense responses such as phytoalexin biosynthesis. In addition, our results support the hypothesis that there is a general, nonspecific mechanism for the induction of defense transcripts and phytoalexins by pathogenic and saprophytic bacteria that is distinct from the more specific mechanism associated with the induction of the hypersensitive reaction.