1USDA-Agricultural Research Service, Washington State University, Pullman, WA 991 64-6430, USA; 2Department of Plant Pathology, Wageningen University, 6700 EE Wageningen, The Netherlands

Background and objectives
Antibiotics are a chemically heterogeneous group of small organic molecules of microbial origin which, at low concentrations, inhibit the growth or metabolic activity of other microorganisms. Microorganisms capable of antibiotic production reside in the bulk soil and the rhizosphere of plants. During the past decade, genetic and molecular studies coupled with sensitive molecular approaches have demonstrated unequivocally that biocontrol agents produce antibiotics and antibiosis is a major mechanism of biological control of soil-borne plant pathogens [1]. In this paper, we review the topic of the role of antibiosis in the biocontrol activity of introduced and indigenous microorganisms in soil.

Results and conclusions
In the last decade, a variety of antibiotics produced by fungal and bacterial biocontrol agents (e.g. 2,4-diacetylphloroglucinol (Phi), phenazine-l-carboxylic acid, pyrrolnitrin, pyoluteorin, 2-hexyl-5-propyiresorcinol, herbicolin A, iturin A, chaetomin and gliotoxin) have been shown to be involved in biocontrol, and have been isolated from in situ sources. Sites of production include seeds, roots, tubers and rhizosphere soil colonized by biocontrol agents, and bulk soil amended with agents. Thin-layer chromatography (TLC) is widely used to fractionate antibiotics recovered from natural materials and is popular because it does not require expensive instrumentation. However, the versatility, resolving capability, and quantitative accuracy of high-performance liquid chromatography (HPLC) make it the method of choice for most analyses of antibiotics from in situ sources [1].

The polyketide antibiotic Phi inhibits a wide variety of fungi and bacteria, and is especially interesting because it is involved in the control of many root and seedling diseases by fluorescent Pseudomonas spp. These include suppression of black root rot of tobacco (Thielaviopsis basicola) and take-all of wheat (Gaeumannomyces graminis var. tritici) by P. ;fluorescens CHAO, suppression of pre-emergence damping-off of sugar beet (Pythium ultimum) by P. ;fluorescens Fl13 and suppression of take-all by P. ;fluorescens Q2-87. Following application of strain Q2-87 to seeds of wheat, 2.1 and 0.47 ;pg Phi/g of root plus rhizosphere soil were isolated from wheat grown in Ritzville and Shano silt loams, respectively [2].

Take-all decline (TAD) is a natural biological control of take-all that occurs worldwide and is defined as a spontaneous reduction in disease and increase in yield with extended monoculture of wheat or barley. Root-associated, Phi-producing fluorescent Pseudomonas spp. build up during wheat monoculture to a threshold population size of approximately 105 ;cfu/g of root and have been shown to be the key component of the natural biological control that operates in TAD soil in Washington State, USA. The specific suppression that operates in TAD soils was lost when Phi producers were eliminated, and conducive soils gained suppressiveness when Phi producers were introduced by mixing in small amounts of TAD soil [3].

1. Thomashow LS, Bonsall RF, Weller DM, 1997. Manual of Environmental Microbiology, pp. 493-499.
2. Bonsall RF, Weller DM, Thomashow LS, 1997. Applied and Environmental Microbiology 63, 951-955.
3. Raaijmakers JM, Weller DM, 1998. Molecular Plant-Microbe Interactions 11, 144-152.