Metabolites of Pseudomonas involved in the biocontrol of plant disease

The ability of some soils to suppress plant disease has been well documented 1. An important example is the development of soils suppressive to Gaeumannomyces graminis var. tritici (Ggt), the causative agent of 'takeall' of wheat 2. Such natural examples of biological control have been attributed to the indigenous beneficial rhizosphere microflora (see Glossary), especially fluorescent pseudomonads. Numerous mechanisms may account for these biocontrol properties (see Glossary), including the production of inhibitory compounds or metabolites 3-8. Microbial metabolites such as siderophores (see Glossary) and secondary metabolites with antimicrobial properties are considered to play a major role in disease suppression (see Table 1). Metabolites with biocontrol properties have been reported from diverse members of the beneficial rhizosphere flora; however, those produced by the fluorescent pseudomonads have received the most attention. This is probably due to the abundance of this diverse group of bacteria and their obvious importance in the rhizosphere, coupled with the relative ease with which they can be genetically manipulated. Fluorescent pseudomonads produce a variety of metabolites 22 (see Fig. 1), many of which are inhibitory to other microorganisms and some of which are implicated in the biological control of plant