Influence of fusaric acid on phenazine-1-carboxamide synthesis and gene expression of Pseudomonas chlororaphis strain PCL1391

Production of the antifungal metabolite phenazine-1-carboxamide (PCN) by Pseudomonas chlororaphis strain PCL1391 is essential for the suppression of tomato foot and root rot caused by the soil-borne fungus F oxysporum f. sp. radicis-lycopersici, The authors have shown previously that fusaric acid (FA), a phytotoxin produced by Fusarium oxysporum, represses the production of PCN and of the quorum-sensing signal N-hexanoyl-L-homoserine lactone (C-6-HSL). Here they report that PCN repression by FA is maintained even during PCN-stimulating environmental conditions such as additional phenylalanine, additional ferric iron and a low Mg2+ concentration. Constitutive expression of phzl or phzR increases the production of C-6-HSL and abolishes the repression of PCN production by FA. Transcriptome analysis using P. chlororaphis PCL1391 microarrays showed that FA represses expression of the phenazine biosynthetic operon (phzABCDEFGH) and of the quorum-sensing regulatory genes phzl and phzR. FA does not alter expression of the PCN regulators gacS, rpoS and psrA. In conclusion, reduction of PCN levels by FA is due to direct or indirect repression of phzR and phzl. Microarray analyses identified genes of which the expression is strongly influenced by FA. Genes highly upregulated by FA are also upregulated by iron starvation in Pseudomonas aeruginosa. This remarkable overlap in the expression profile suggests an overlapping stress response to FA and iron starvation.