THE MOLECULAR BIOLOGY OF HYPERSENSITIVITY TO PLANT PATHOGENIC BACTERIA
It has now been nearly 40 years since the first observations by Klement (Klement, 1963) that resistance to bacterial plant pathogens was correlated with a rapid necrotic response induced in artificially inoculated tissue. This response, subsequently termed the hypersensitive response (HR) (Klement et al., 1964), was initially defined as a rapid necrosis observed within 24 h after inoculation of the tissue with a potentially pathogenic, but incompatible, bacterial strain. The response appeared to be part of a syndrome that resulted in localized plant cell death, bacterial localization and bacteriostasis. Two types of incompatible interactions have been identified that are associated with the elicitation of the HR (Klement, 1982). Inoculation of a pathogen of one plant species into another plant species elicited the HR associated with nonhost resistance. The HR could also be elicited when a virulent strain pathogenic to one plant species was introduced into resistant cultivars of that same plant species (race-specific resistance). Although originally controversial, it is now clear that the defense responses associated with hypersensitivity are a primary line of defense to many pathogens, particularly biotrophic pathogens (Goodman and Novacky, 1994; Hutcheson, 1998; Dangl and Jones, 2001; Lam et al., 2001). The response appears to involve protein-mediated recognition of pathogen-derived proteins to initiate several defense responses, including programmed cell death. Cells responding hypersensitively, in turn, signal adjacent cells to accumulate antimicrobial compounds and to initiate the hormonal signaling process associated with systemic acquired resistance. What is most remarkable is that the molecular events occurring during hypersensitivity appear to be very similar to the initiation of innate cellular immunity in mammals and insects (Hutcheson, 1998; Collmer et al., 2000; Staskawicz et al., 2001). Thus, hypersensitivity may represent an ancient form of cellular defense used by eukaryotes in general. For purposes of brevity and simplicity, the discussion below emphasizes the studies on the HR elicited by Pseudomonas syringae strains.