Wageningen Agricultural University, Binnenhaven 9, PO Box 8025, 6700 EE Wageningen, Tthe Netherlands

Biological control of plant pathogens by application of specific microorganisms to seeds or planting material has been studied intensively over the past three decades. Notable among the group of rhizosphere bacteria are the fluorescent Pseudomonas spp. Various mechanisms may account for the ability of specific Pseudomonas strains to control plant pathogens including the production of antibiotics. Antibiotics encompass a chemically heterogeneous group of metabolites that can inhibit the growth or metabolic activities of other microorganisms. Numerous strains of antibiotic-producing Pseudomonas spp. have been isolated from roots of various plants grown in soils from diverse geographical regions [1]. Moreover, antibiotic producers were readily isolated from soils that are naturally suppressive to diseases such as take-all of wheat, black root rot of tobacco, or fusarium wilt of tomato, indicating that they may play an important role in the natural biological control that occurs in these soils. The antibiotics pyoluteorin, pyrrolnitrin, phenazines, and 2,4-diacetylphloroglucinol are currently a major focus of research in biological control of soil-borne plant pathogens. The biosynthetic loci for these antibiotics have been cloned and most of them have been fully sequenced. Genetic studies, modeled after Koch's postulates, have demonstrated that these antibiotics play an important role in the suppression of various plant pathogens. Many of these antibiotics appear to inhibit a wide range of plant pathogens [2]. For example, 2,4-diacetylphloroglucinol is a phenolic antibiotic with antibacterial, antiviral, anthelminthic, and antifungal properties. Among the soilborne fungi, Gaeumannomyces graminis var. tritici, Pythium ultimum, and Thielaviopsis basicola can be controlled by Pseudomonas strains producing 2,4-diacetylphloroglucinol.

Despite the importance of antibiosis in biological control, most antibiotic-producing Pseudomonas strains are still too variable in their performance to be successfully used as a common practice in agriculture. Multiple factors could acount for this inconsistency given the complex interactions between the bacterial strain, the pathogen, the plant, and the environment. The major factors reported in literature are related to rhizosphere colonization and the expression of genes involved in disease suppression. Antibiotic production and its effect on pathogens depends on a variety of biotic and abiotic conditions. The quantity and quality of nutrients available, and the ability of the introduced strains to successfully compete for them, appear to be major determinants of the population size and antibiotic production in soil and rhizosphere enviromnents. Also, temperature and waterpotential influence antibiotic production. In this context, hydroponic systems seem to be ideal for the application of antibiotic-producing bacteria because of the more uniform environmental conditions, the ease of introducing the biocontrol agent in high doses, and the relatively low microbial content of the substrate allowing the introduced strain to establish rapidly at densities sufficient to preempt or limit infection by the target pathogen. Also, the nature of the cube matrix makes it conducive to the addition of specific nutrients to induce and/or enhance antibiotic production. In this paper, methods to select antibiotic-producing bacteria and to measure in situ antibiotic production will be evaluated, and the potential role of these rhizosphere bacteria in disease suppression in hydroponic systems will be discussed.

1. Keel C, Weller DM, Natsch A, Ddfago G, Cook RJ, Thomashow LS, 1996. Applied and Environmental Microbiology 62, 552-563.
2. Thomashow LS, Weller DM, 1996. In: Stacey G, Keen NT (ed), Plant-Microbe Interactions, Chapman & Hall, London, UK, pp. 187-236.