Genetic Relationships of Some Fastidious Xylem-Limited Bacteria

Abstract

Fastidious, gram-negative xylem-limited bacteria were examined for genetic relatedness to each other and to members of the major phytopathogen-containing genera of gram-negative bacteria. All of the xylem-limited bacterial strains studied had deoxyribonucleic acid base compositions of approximately 50.5 mol% guanine plus cytosine and genome molecular weights of 1.4 X lo9 2 0.2 X lo9. The results of our deoxyribonucleic acid-deoxyribonucleic acid hybridization experiments suggested that these strains are all related at the species level, but supported distinction at the varietal level, and possibly at the subspecies level between the bacteria causing Pierce's disease of grapevine and the bacteria causing phony disease of peach, plum leaf scald, and periwinkle wilt. No relatedness was detected between the xylem-limited bacteria and Erwinia chrysanthemi, Pseudomonas syringae pv. syringae, Xanthomonas campestris pv. campestris, or Agrobacterium tumefaciens.The significance of the recently discovered fastidious gram-negative xylem-limited bacteria (XLB) in plant diseases has become increasingly evident (8,15). XLB, also referred to as rickettsia-like bacteria, have been isolated from diseased plants, insect vectors, and apparently healthy reservoir plants (8,15,26), indicating a complexity of XLB-host-vector interactions. Studies on etiology (14, 15, 191, culture requirements (6, 8, 25), serology (7,12,20,22,26), and certain biochemical characteristics (15, 24) have indicated that the XLB are not related to the major phytopathogen-containing genera of gram-negative bacteria and can be subdivided into at least two closely related subgroups of unknown taxonomic distinction or affiliation.In this study, the relatedness of several XLB to each other and to other gram-negative phytopathogens was examined and defined by the following three criteria: deoxyribonucleic acid (DNA) base composition, genome molecular weight, and DNA homology.(The results were taken from a thesis presented by S.M.K. to Florida State University, Tallahassee.) MATERIALS AND METHODSBacterial strains and culture conditions. Two strains of the plum leaf scald bacterium, strains PLSB-40 and PLSB-53, and one strain of the phony peach disease bacterium, strain PPDB-2.1, were isolated by W. J. French at Monticello, Fla. (strains PLSB-40 and PPDB-2.1) or Cascata, Rio Grande do Sul, Brazil (strain PLSB-53). One strain of the Pierce's disease bacterium, strain PDB-1, was provided by N. W. Schaad, University of Idaho, Moscow, and one strain of periwinkle wilt bacterium, strain PWB-1, was provided by M. J. Davis, University of Florida, Ft. Lauderdale. Stationary cultures of strains PLSB-40, PLSB-53, PPDB-2.1, and PWB-1 in PW broth (8) and shaker cultures of strain PDB-1 in PD2 broth (10) were grown at 28°C for 5 to 7 days. Erwinia chrysanthemi A-17, Xanthomonas campestris B-24, Pseudomonas syringae C-7, and Agrobacterium tumefaciens 84G-102 were provided by N. W. Schaad, Escherichia coli K-12 C600 was provided by H. J. Taylor, Florida State * Corresponding aut...

Phylogenetic relationships of Xylella fastidiosa strains from different hosts, based on 16S rDNA and 16S-23S intergenic spacer sequences.

“…The distinction between plum\peach strains and grapevine strains on the basis of 16S rDNA and 16S-23S sequences seems consistent with other studies using growth characteristics (Wells et al, 1987) and pathogenicity (Hopkins, 1989). In addition, Kamper et al (1985), using DNA hybridization studies, considered that the level of homology (75 %) found between Pierce's disease and plum leaf scald (PLS) strains was low enough to support distinction at the varietal and, possibly, subspecies level. The relatedness of the plum strain to citrus and coffee strains found in both phylogenetic trees was, however, unknown.…”

“…The present findings confirm the homology levels (above 85 %) obtained by Wells et al (1987) in the analysis of Xylella fastidiosa strains isolated from different hosts (grapevine, peach, periwinkle, almond, plum, elm, sycamore, oak, ragweed and mulberry). Using the same approach, Kamper et al (1985) analysed five strains of Xylella fastidiosa from plum, peach, periwinkle (Vinca sp.) and grapevine, and the level of DNA homology ranged from 75 to 100 %.…”

A Multigene Phylogenetic Study of Clonal Diversity and Divergence in North American Strains of the Plant Pathogen Xylella fastidiosa

“…fastidiosa is considered a single species (24), but one that is subdivided into distinct groups (8). It has recently been suggested that it can be divided into three subspecies (35), one in South America and two in North America.…”

“…Early DNA-DNA hybridization studies of several X. fastidiosa isolates indicated at least 70% similarity (24) which, combined with studies of nutritional fastidiousness and reciprocal transmission tests, placed the different host strains into groups (23). The greater than 70% similarity indicated that X. fastidiosa was a single species (11); however, no meaningful phylogenetic relationships could be defined.…”

Comparative Analyses of the Complete Genome Sequences of Pierce's Disease and Citrus Variegated Chlorosis Strains of Xylella fastidiosa

“…The grapevine-infecting variants responsible for PD are found in one group, while the citrus-infecting variants responsible for CVC are found in another (6,7,16). Initial expectations, based on geographical distributions, host diversity, differential disease symptoms, and molecular analyses, were that organisms from the three groups would be sufficiently different to support taxonomic separation at the subspecies or pathovar level (17,19,24).…”