The RsmA^- Mutants of Erwinia carotovora subsp. carotovora Strain Ecc71 Overexpress hrpN_ECC and Elicit a Hypersensitive Reaction-like Response in Tobacco Leaves. Yaya Cui. Department of Plant Pathology, 108 Waters Hall, University of Missouri, Columbia, MO 65211, U.S.A. Lea Madi, Asita Mukherjee, C. Korsi Dumenyo, and Arun K. Chatterjee Department of Plant Pathology, 108 Waters Hall, University of Missouri, Columbia, MO 65211, U.S.A. MPMI 9:565-573. Accepted 24 May 1996. Copy right 1996 The American Phytopathological Society.

Erwinia carotovora subsp. carotovora wild-type strain Ecc71 does not elicit the hypersensitive reaction (HR) in tobacco leaves. By mini-Tn5-Km and chemical mutagene-sis we have isolated RsmA^- mutants of Ecc71 that produce high basal levels of pectate lyases, polygalacturonase, cel-lulase, and protease; they also are hypervirulent. The RsmA^- mutants, but not their parent strains, elicit an HR-like response in tobacco leaves. This reaction is characterized by the rapid appearance of water soaking followed by tissue collapse and necrosis. The affected areas remain limited to the region infiltrated with bacterial cells, and the symptoms closely resemble a typical HR, e.g., the reactions caused by Pseudomonas syringae pv. pisi. Moreover, low concentrations of cells of the mini-Tn5-Km insertion RsmA^- mutant, AC5070, infiltrated into tobacco leaf tissue prevent elicitation of the rapid necrosis by AC5070 or by P. syringae pv. pisi. Elicitation of the HR-like response by the mutants is not affected by the deficiency of N-(3-oxohexanoyl)-L-homoserine lactone, the cell density (quorum) sensing signal. Cloning and sequence analysis have disclosed that E. carotovora subsp. carotovora strain Ecc71 possesses a homolog of E. chrysanthemi hrpN known to encode an elicitor of the HR; the corresponding Ecc71 gene is designated hrpN_Ecc Northern (RNA) blot data show that the level of hrpN_Ecc mRNA is considerably higher in the RsmA^- mutants than in the RsmA⁺ strains. Moreover, a low copy plasmid carrying the rsmA⁺ allele severely reduces the level of the hrpNEcc transcripts in the RsmA- mutants. These constructs, like the RsmA⁺ E. carotovora subsp. carotovora strains, do not elicit the HR-like response. These data taken along with the effects of rsmA on exoenzyme production and pathogenicity (A. Chatterjee et al., 1995, Appl. Environ. Microbiol. 61:1959-1967) demonstrate that this global regulator gene plays a critical role in plant interaction of E. carotovora subsp. carotovora.