Induction of Plant Resistance Reactions by Pseudomonas syringae

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A primary mechanism through which plants prevent colonization by potentially pathogenic microorganisms is the hypersensitive response (HR), a rapid, local necrosis accompanied by the production of antimicrobial compounds. Previous genetic studies with Pseudomonas syringae pathovars (host range variants of P.syringae) suggest that elicitation of the HR by incompatible bacterial pathogens is controlled by two types of genes: hrp (necessary for any plant reaction) and avr (affecting host range). The nature of the elicitor(s) remains unknown. The interaction of P.syringae with tobacco suspension culture cells provides an ideal system to investigate the elicitation of the HR by bacteria. Cultured tobacco cells rapidly respond to incompatible bacteria by a K+ efflux/H+ influx exchange response (XR) that is physiologically, genetically and developmentally similar to the in planta HR. In studies thus far this lab has: 1) cloned an apparent set of hrp/avr genes from P.syringae pv. syringae (Pss61) that are phenotypically expressed in the saprophytes, Escherichia coli and P. fluorescens, thus simplifying subsequent genetic analyses; 2) observed that Pss61 containing the cloned wild-type hrp/avr genes produce a stronger XR than wild-type Pss61, suggesting that elicitor production can be enhanced by genetic manipulation; and 3) identified in vitro conditions for preinducing the bacterial elicitor of the XR. These findings provide powerful new tools to investigate the bacterial signals governing host range and pathogenicity. The goal of this investigation is to isolate and characterize biochemically the bacterial elicitors involved in the induction of plant responses. The proposed experiments will define the genetic units present in the hrp/avr clone and use reporter gene fusions to elucidate the regulation of functional loci and characterize the protein products.***//