Metabolic Diversity of Xanthomonas axonopodis pv. vignicola, Causal Agent of Cowpea Bacterial Blight and Pustule

Fifty-five strains of Xanthomonas axonopodis pv. vignicola, isolated from blight and pustule symptoms of cowpea leaves, originating from 11 countries, were characterized for their carbon-source metabolization pattern using the Biolog GN microplate system. Great variation was found between strains according to origin. Dextrin, glycogen and succinamic acid were not used by strains from Benin, Uganda or Thailand, but by all the other strains (excluding two strains from Mozambique), whereas N-acetyl-D-glucosamine and malonic acid were used by the strains from Benin, Uganda and Thailand, but generally not by the other strains. The strains from Benin, Uganda and Thailand, as well as strains from Venezuela, Brazil and Mozambique, clustered separately from the others in multivariate analysis. Nineteen substrates were used by all the strains, 47 not by any strain and 29 only by some strains. No considerable differences were found between strains isolated from blight symptoms and from pustules. Virulence of strains was not related to the metabolic pattern. The Biolog database was not representative of the diversity of X. axonopodis pv. vignicola, since all strains were identified as Xanthomonas campestris, although belonging to eight pathovars, while only eight of nine strains from Benin and both strains from Thailand were identified as X. campestris pv. vignicola. The Biolog system appeared to be useful for characterizing the diversity of X. axonopodis pv. vignicola strains. A set of representative strains based on metabolic and molecular diversity, virulence and geographic origin is suggested for screening for resistant cowpea cultivars.     

Isolation and Sequence of a Gene, celD Xo Involved in Cellobiose Utilization in Xanthomonas oryzae pv. oryzae

A genomic library of Xanthomonas oryzae pv. oryzae (X. o. pv. oryzae) T7174 was screened for 4-methylumbelliferyl β-D-glucoside-hydrolyzing (MUGase) activity. In subcloning of one of the MUGase-positive clones, an approximately 4.2-kb SacI-SphI fragment conferred not only MUGase activity but also 4-methylumbelliferyl β-D-cellobioside-hydrolyzing (MUCase) activity. Sequence analysis showed that the fragment contained an ORF of 2951 bp. The conceptual ORF product was significantly homologous with 1,4-β-D-glucan glucohydrolase D (CELD) from Pseudomonas fluorescens subsp. cellulosa, and was named CELD_Xo. Cell fractionation experiments suggested that CELD_Xo is localized in the cell-envelope fraction. We constructed a CELD_Xo-deficient mutant (74ΔCELD) from X. o. pv. oryzae. Little MUCase activity was detected in the cell-envelope fraction prepared from the mutant. The mutant 74ΔCELD did not grow in synthetic medium containing cellobiose as the sole sugar source. On the other hand, growth in rice leaves and pathogenicity of the mutant and the parental strain did not differ. These results suggested that CELD_Xo is involved in cellobiose utilization of X. o. pv. oryzae but that the gene is not required for bacterial growth in rice leaves. Received 16 February 2001/ Accepted in revised form 11 April 2001     

Bacterial leaf blight of strawberry (Fragaria (x) ananassa) caused by a pathovar of Xanthomonas arboricola, not similar to Xanthomonas fragariae Kennedy & King. Description of the causal organism as Xanthomonas arboricola pv. fragariae (pv. nov., comb. nov.)

A new bacterial disease of strawberry is described. This disease, called bacterial leaf blight of strawberry, is characterized by dry, brown necrotic leaf spots and large brown V-shaped lesions along the leaf margin, midrib and major veins. Symptoms are different from angular leaf spot of strawberry caused by the bacterium Xanthomonas fragariae . Strains of the bacterial leaf blight pathogen were characterized in a polyphasic approach by biochemical tests, fatty acid analysis, protein electrophoresis, serology, PCR, pigment analysis, ice-nucleation activity, AFLP analysis, DNA:DNA hybridization, pathogenicity and host range tests, and compared with a number of reference strains of X. fragariae and other Xanthomonas species. Bacterial leaf blight strains formed a homogeneous group in all tests, completely different from X. fragariae . They were the only strains causing leaf blight of strawberry upon artificial inoculation into strawberry. Fatty acid and protein electrophoretic analysis showed that the strains belong to the phenon X. campestris ( sensu latu , including pathovars now classified as belonging to X. arboricola ). AFLP analysis and DNA:DNA hybridization further clarified their taxonomic position as belonging to X. arboricola. The name X. arboricola pv. fragariae is proposed for the bacterium causing leaf blight of strawberry with strain PD2780 (LMG 19145) as pathovar type strain. Criteria for routine identification are given and the taxonomic status is discussed.