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Rhizomonas suberifaciens gen. nov., sp. nov., the Causal Agent of Corky Root of Lettuce

Department of Plant Pathology, University of California, Davis, California 95616

^* Corresponding author.

ABSTRACT

Eleven strains of a slow-growing, gram-negative bacterium causing corky root (CR) of lettuce were examined for morphological, physiological, and biochemical traits. Each strain consisted of small, motile rods with one lateral, subpolar, or polar flagellum. All strains were oligotrophic. Typical colonies were nonpigmented, umbonate, firm, and ultimately wrinkled. All strains were aerobic, metabolized glucose oxidatively, were oxidase positive and weakly catalase positive, and reduced nitrate to nitrite and ammonia but not to nitrogen gas. All of the strains tested were nitrogenase and arginine dihydrolase negative. Ethanol was not converted to acetic acid, and none of the 11 CR strains grew at pH 5.1 or below. The CR bacterium did not fluoresce and did accumulate poly-β-hydroxybutyrate granules. The CR strains did not hydrolyze starch or Tween 80, but did hydrolyze Tween 20. Very few carbon sources were utilized. The only isoprenoid quinone detected was ubiquinone Q10. The whole-cell fatty acid profiles of the CR strains consisted of several saturated and unsaturated straight-chain fatty acids, 2-hydroxy fatty acids, one cyclofatty acid, and one methylated fatty acid and resembled the fatty acid profile of Pseudomonas paucimobilis. The guanine-plus-cytosine content of the DNA was 59 mol%, which is below the range for P. paucimobilis. DNA-DNA homology studies indicated that the CR bacterium and P. paucimobilis are related but not the same species. The quinone and fatty acid compositions of the CR bacterium and P. paucimobilis differ substantially from those of other Pseudomonas spp., indicating that these organisms do not belong to the genus Pseudomonas proper. The characteristics of the CR bacterium do not conform to those of any previously described genus, and we propose a new genus, Rhizomonas, with one species, Rhizomonas suberifaciens, for strains of the CR bacterium. Additional tests will be needed to determine whether P. paucimobilis can be transferred to the genus Rhizomonas. Strain CA1 is the type strain of R. suberifaciens and has been deposited in the American Type Culture Collection and the National Collection of Plant Pathogenic Bacteria as strains ATCC 49355 and NCPPB 3629, respectively, together with strains FL1 (= ATCC 49356 = NCPPB 3628), NY11 (= ATCC 49382 = NCPPB 3631), and WI3 (=ATCC 49381 = NCPPB 3630). One additional strain that caused CR of lettuce had physiological and biochemical traits similar to those of R. suberifaciens but produced yellow colonies, had a higher guanine-plus-cytosine content (64 mol%), and exhibited lower DNA homology (54%) with DNA of the type strain. More strains will need to be studied to determine the taxonomic position of this strain.

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