Milton N. Schroth

Pseudomonas siderophores: A mechanism explaining disease-suppressive soils

The addition of either fluorescentPseudomonas strain B10, isolated from a take-all suppressive soil, or its siderophore, pseudobactin, to bothFusarium-wilt and take-all conducive soils inoculated withFusarium oxysporum f. sp.lini orGaeumannomyces graminis var.tritici, respectively, rendered them disease suppressive. Our findings suggest that disease suppressiveness is caused in part by microbial siderophores which efficiently complex iron(III) in soils, making it unavailable to pathogens, thus inhibiting their growth. Amendment of exogenous iron(III) to disease-suppressive soils converted them to conductive soils presumably by repressing siderophore production.

Disease-suppressive soil and root-colonizing bacteria.

Soils in many areas suppress certain plant diseases. Understanding the basis for this disease suppressiveness could lead to improved plant health in less favorable areas. Some forms of disease suppression may be caused by bacteria in the genus Pseudomonas which aggressively colonize root surfaces. Increased plant growth and yield are closely associated with the capacity of some of these bacteria to produce iron-binding compounds called siderophores. This article addresses the biological characteristics of these soil-bome root epiphytes, their contribution to plant health, and their potential use in biotechnology.

Genetic Analysis of Fluorescent Pigment Production in Pseudomonas syringae pv. syringae

Genes involved in the biosynthesis of a fluorescent pigment by Pseudomonas syringae pv. syringae (P. s. syringae) JL2000 were investigated. A genomic library of this strain was constructed using the broad host range cosmid vector, pLAFRl. Nonfluorescent (Flu-) mutants of JL2000, defective in the biosynthesis of a fluorescent pigment, were obtained after ethyl methanesulphonate mutagenesis. Individual recombinant plasmids from the genomic library were introduced into Flu- mutants. Of a total of 146 Flu- mutants, 36 were restored to fluorescence following matings with individual recombinant colonies in the genomic library. Four separate fluorescence restoration groups, each comprised of 5 to 11 Flu- mutants restored to fluorescence by one of four structurally distinct recombinant plasmids, were identified. Whereas the 36 Flu- mutant strains differed in their abilities to grow on an iron-deficient medium, wild-type P. s. syringae strain JL2000 and all Flu+ transconjugants from these 36 crosses grew on an iron-deficient medium. These results indicate that at least four genes or gene clusters are involved in the production of a fluorescent pigment of P. s. syringae strain JL2000. These genes, with few exceptions, also control the ability of strain JL 2000 to grow under iron-limiting conditions.

Epidemiology of Pseudomonas aeruginosa in agricultural areas

&NA; A prevailing opinion is that the strains of Pseudomonas aeruginosa that infects both plants and humans are two separate species. This study strongly disputes that notion until the modern molecular technology proves otherwise. This paper examines a spectrum of strains occurring in nature, their habitats, dissemination, their relationship to clinical strains, and the environmental conditions that favor their colonization of plants. The isolates were obtained from clinical specimens, plants, soil, and water. The identity of these strains was confirmed using pyocin typing and biochemical assays. The data reveal that agricultural soils, potted ornamental plants, hoses, fountains, and faucets frequently harbored P. aeruginosa. However, it was not commonly found in semi‐arid areas, suggesting that moisture and high humidity is necessary for colonization and survival. Though found in soil, P. aeruginosa was seldom isolated on edible plant parts. The pathogenicity of various strains on plants was tested by inoculating vegetables, lettuce slices (Lactuca sativa L. “Great Lakes”), celery stalks (Apium graveolens L. var. Dulce], potato tuber slices (Solanum tuberosum L. “Whiterose”), tomato (Lycopersicon esculentum L. Mill), cucumber (Cucumis sativus L.), rutabaga (Brassica campestris L.), and carrot (Daucus carota L. var sativa). There was considerable variation in the strains’ ability to cause rot, but no difference was observed between clinical isolates and others from agricultural fields, water, and soil. Two of the clinical isolates from burn patients, P. aeruginosa PA13 and PA14, exhibited the greatest virulence in causing rot in all the plants that were tested, especially on cucumber, lettuce, potato, and tomato. The study discusses how closely the epidemiology of P. aeruginosa relates to many plant pathogens, and the ability of human isolates to colonize plants and food material under favorable conditions. The biochemical and phenotypic similarity among strains from the clinical and agricultural material is strongly indicative that they are the same species and that plants and soil are natural reservoirs for P. aeruginosa.

Introduced microbes enhance root health and plant growth

Abstract Growth stimulation and root health, dynamics of root colonization by microorganisms, altered root physiology as the result of infection, rootlet turnover and root health, impact of introduced microbes on plant growth, and future research needs in the root health field are the main topics mentioned and discussed in the paper

Localization of Arbutin in Pyrvs by Means of Alkaline P-Phenylenediamine

A reagent composed of 0.2% p-phenylenediamine in 2 N NH4OH was used for the cytochemical demonstration of arbutin in plant tissue. Sections of fresh tissue were cut at 25-50 μ, mounted in a drop of the reagent, and allowed to stand uncovered 15-20 min before applying a coverslip. Arbutin stained dark blue to dark purple and was easily distinguished from other constituents of the cell, such as chlorogenic acid, isochlorogenic acid, caffeic acid, and quinic acid, which stained yellow, yellow-green, red or brown in color. The limit of sensitivity of the p-phenylenediamine-arbutin reaction was 1:100,000, as determined by spot-plate tests.