3-Methylarginine from Pseudomonas syringae pv. syringae 22d/93 suppresses the bacterial blight caused by its close relative Pseudomonas syringae pv. glycinea

The epiphyte Pseudomonas syringae pv. syringae 22d/93 (Pss22d) produces a toxin that strongly inhibits the growth of its relative, the plant pathogen P. syringae pv. glycinea. The inhibition can be overcome by supplementing the growth medium with the essential amino acid, L-arginine; this suggests that the toxin acts as an inhibitor of the arginine biosynthesis. The highly polar toxin was purified by bioassay-guided fractionation using ion-exchange chromatography and subsequent RP-HPLC fractionation. The structure of the natural product was identified by HR-ESI-MS, HR-ESI-MS/MS, and NMR spectroscopy experiments as 3-methylarginine. This amino acid has previously only been known in nature as a constituent of the peptide lavendomycin from Streptomyces lavendulae. Results of experiments in which labeled methionine was fed to Pss22d indicated that the key step in the biosynthesis of 3-methylarginine is the introduction of the methyl group by a S-adenosylmethionine (SAM)-dependent methyltransferase. Transposon mutagenesis of Pss22d allowed the responsible SAM-dependent methyltransferase of the 3-methylarginine biosynthesis to be identified.

Biological Control of Pseudomonas syringae pv. glycinea by Epiphytic Bacteria under Field Conditions

The efficacy of a bacterial strain as a biocontrol agent in the field may be related to the ecological similarity between the biocontrol agent and the target pathogen. Therefore, a number of different Pseudomonas syringae strains were evaluated for their antagonistic activities in vitro (agar-diffusion assay) and in planta (greenhouse assay) against the target pathogen, Pseudomonas syringae pv. glycinea. Six strains of five different pathovars were found to be antagonistic in vitro as well as in planta. The epiphytic fitness of the antagonistic Pseudomonas syringae strain 22d/93 and its two antibiotic-resistant mutants were examined on soybean plants in the fields. After adaptation the parental strain and its mutants had the ability to establish and maintain large epiphytic populations (about 106 cfu/g FW) over the whole growing season after a single spray inoculation. The epiphytic behaviors of the mutants and the parent were not significantly different. The introduced bacteria did not influence the total bacterial population size. When the antagonist was coinoculated with the pathogen, the development of the pathogen was significantly reduced during the whole growing season. When the antagonistic strain was inoculated 4 weeks in advance of the pathogen, this antagonistic effect could be markedly enhanced. The final population size of the pathogen reached just 104 cfu/g FW and was significantly reduced to 0.12% compared to the pathogen alone. This study demonstrates that biological control of foliar pathogens through colonization of the host plants with near isogenic or ecologically similar antagonistical strains seems to be a realistic goal.

Characterization of two epiphytic bacteria from soybean leaves with antagonistic activities against Pseudomonas syringae pv. glycinea

The strains 48b/90 and 22d/93 are naturally occurring ephiphytes which were isolated from soybean leaves. On the basis of pheno- and genotypic characteristics 48b/90 was identified as Erwinia herbicola and 22d/93 as Pseudomonas syringae. These two isolates produced biological active substances against different indicator organisms. The E. herbicola strain showed clear antagonistic properties against Escherichia coli and Pseudomonas syringae pv. glycinea, but not against Geotrichum candidum. 22d/93 was active against P. glycinea and G. candidum, but not against E. coli. Strain 48b/90 produced at least two different inhibitors: an antibiotic substance and an inhibitor of the alginate synthesis. Strain 22d/93 produced at least three different compounds inhibitory to P. glycinea and one to G. candidum. Their activities against the bacterial blight pathogen, P. glycinea, can be observed in planta, too. Under the influences of the antagonists the pathogen multiplied at lower rates and to lower stationary phase population levels. The development of bacterial blight symptoms was suppressed.

Phaseolotoxin production by Pseudomonas syringae pv. phaseolicola: the influence of temperature

Phaseolotoxin (N-sulphodiaminophosphinyl-ornithyl-alanyl-homoarginine) is a phytotoxic secondary metabolite produced by Pseudomonas syringae pv. phaseolicola. The production of the phytotoxin is strongly regulated by temperature. The optimal temperature for phaseolotoxin production is 18 degrees C. Temperatures in the range between 18 degrees C and 30 degrees C inhibit the production of phaseolotoxin in an increasing manner. By temperature shift experiments and the inhibition of protein synthesis by chloramphenicol it was demonstrated, that the synthesis of enzymes related to toxin synthesis and not the regulation of the activity of the responsible enzyme system is affected by higher temperatures.