Quantification of 2,4-Diacetylphloroglucinol Produced by Fluorescent Pseudomonas spp. In Vitro and in the Rhizosphere of Wheat.

The broad-spectrum antibiotic 2,4-diacetylphloroglucinol (Phl) is a major determinant in the biological control of a wide range of plant diseases by fluorescent Pseudomonas spp. A protocol was developed to readily isolate and quantify Phl from broth and agar cultures and from the rhizosphere environment of plants. Extraction with ethyl acetate at an acidic pH was suitable for both in vitro and in situ sources of Phl. For soil samples, the addition of an initial extraction step with 80% acetone at an acidic pH was highly effective in eliminating polar organic soil components, such as humic and fulvic acids, which can interfere with Phl detection by high-performance liquid chromotography. The efficiency of Phl recovery from soil by a single extraction averaged 54.6%, and a second extraction added another 6.1%. These yields were substantially greater than those achieved by several standard protocols commonly used to extract polar phenolic compounds from soil. For the first time Phl was isolated from the rhizosphere environment in raw soil. Following application of Pseudomonas fluorescens Q2-87 and the Phl-overproducing strain Q2-87(pPHL5122) to the seeds of wheat, 2.1 and 2.4 (mu)g of Phl/g of root plus rhizosphere soil, respectively, were isolated from wheat grown in a Ritzville silt loam; 0.47 and 1.3 (mu)g of Phl/g of root plus rhizosphere soil, respectively, were isolated from wheat grown in a Shano silt loam. However, when the amount of Phl was calculated on the basis of cell density, Q2-87(pPHL5122) produced seven and six times more antibiotic than Q2-87 in Ritzville silt loam, and Shano silt loam, respectively.