Role of Phenazine-1-Carboxamide Produced by Pseudomonas chlororaphis PCL1391 in the Control of Tomato Foot and Root Rot

Tomato foot and root rot (TFRR) is a tomato root disease caused by the fungus Fusarium oxysporum f. sp. radicis-lycopersici (Forl). No chemicals are available which efficiently suppress TFRR. In this chapter we show that the bacterium Pseudomonas chlororaphis strain PCL1391 is able to suppress the disease. To this end it uses antibiosis as its (major) disease-suppressing mechanism. The produced antibiotic was identified as phenazine-1-carboxamide (PCN). In contrast to the PCN-producing bacterium, pseudomonads which produce the PCN biosynthetic precursor phenazine-1-carboxylic acid (PCA) as their major phenazine were not active in disease suppression. However, when PCA was converted to PCN by complementing these strains with the phzH gene, which encodes an amidotransferase, the complemented strains produced PCN and controled TFRR. In order to be effective in disease control, strain PCL1391 should be able to produce PCN under a variety of environmental conditions. We therefore studied the regulation of PCN production under various environmental factors, by regulatory genes, by the plant, and by the pathogenic fungus. Special attention was paid to the secondary metabolite fusaric acid secreted by the fungus. Fusaric acid is detected by the bacterium as a chemo-attractant to reach the fungus, to colonize its surface and to finally use it as a food source. Conversely, fusaric acid is used by the fungus to inhibit the production of PCN and to reduce the bacterial growth rate. It is clear that during disease control the PCN-producing bacterium wins this battle. The result of the evaluation of the described studies is that we can understand in quite some detail how P. chlororaphis strain PCL1391 acts as a disease control agent and also why it is not active under all environmental conditions.