THE RSMA(-) MUTANTS OF ERWINIA-CAROTOVORA SUBSP CAROTOVORA STRAIN ECC71 OVEREXPRESS HRPN(ECC) AND ELICIT A HYPERSENSITIVE REACTION-LIKE RESPONSE IN TOBACCO-LEAVES

Erwinia carotovora subsp. carotovora wild-type strain Ecc71 does not e licit the hypersensitive reaction (HR) in tobacco leaves. By mini-Tn5- Km and chemical mutagenesis we have isolated RsmA(-) mutants of Ecc71 that produce high basal levels of pectate lyases, polygalacturonase, c ellulase, and protease; they also are hypervirulent. The RsmA(-) mutan ts, but not their parent strains, elicit an HR-like response in tobacc o leaves. This reaction is characterized by the rapid appearance of wa ter soaking followed by tissue collapse and necrosis. The affected are as 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 cel ls of the mini-Tn5-Km insertion RsmA(-) mutant, AC5070, infiltrated in to tobacco leaf tissue prevent elicitation of the rapid necrosis by AC 5070 or by P. syringae pv. pisi. Elicitation of the HR-like response b y 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. caroto vora strain Ecc71 possesses a homolog of E. chrysanthemi hrpN known to encode an elicitor of the HR; the corresponding Ecc71 gene is designa ted hrpN(Ecc). Northern (RNA) blot data show that the level of hrpN(Ec c) 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 hrpN(Ecc) transcripts in the RsmA(-) mutants. These constructs, like the RsmA(+) E. carotovora subsp. caro tovora strains, do not elicit the HR-like response. These data taken a long with the effects of rsmA on exoenzyme production and pathogenicit y (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.