Seventy bacterial isolates from the rhizosphere of tomato were screened for antagonistic activity against the tomato foot and root rot-causing fungal pathogen Fusarium oxysporum f. sp. radicis-lycopersici. One isolate, strain PCL1391, appeared to be an efficient colonizer of tomato roots and an excellent biocontrol strain in an F. oxysporum/tomato test system. Strain PCL1391 was identified as Pseudomonas chlororaphis and further characterization showed that it produces a broad spectrum of antifungal factors (AFFs), including a hydrophobic compound, hydrogen cyanide, chitinase(s), and protease(s). Through mass spectrometry and nuclear magnetic resonance, the hydrophobic compound was identified as phenazine-1-carboxamide (PCN). We have studied the production and action of this AFF both in vitro and in vivo. Using a PCL1391 transposon mutant, with a lux reporter gene inserted in the phenazine biosynthetic operon (phz), we showed that this phenazine biosynthetic mutant was substantially decreased in both in vitro antifungal activity and biocontrol activity. Moreover, with the same mutant it was shown that the phz biosynthetic operon is expressed in the tomato rhizosphere. Comparison of the biocontrol activity of the PCN-producing strain PCL1391 with those of phenazine-1-carboxylic acid (PCA)-producing strains P. fluorescens 2-79 and P. aureofaciens 30-84 showed that the PCN-producing strain is able to suppress disease in the tomato/F. oxysporum system, whereas the PCA-producing strains are not. Comparison of in vitro antifungal activity of PCN and PCA showed that the antifungal activity of PCN was at least 10 times higher at neutral pH, suggesting that this may contribute to the superior biocontrol performance of strain PCL1391 in the tomato/F. oxysporum system.

Tomato plants, susceptible toFusarium oxysporum f. sp. lycopersici, were inoculated by immersing the roots in a conidial suspension ofF.

The interaction between Fusarium oxysporum f, sp, lycopersici race 2 and near-isogenic resistant and susceptible tomato lines of the cultivar Moneymaker and callus cultures derived from these lines was studied. Resistance was expressed m callus tissue for a short period, durmg 4 days after inoculation. This was shown by a delay in fungal growth on callus tissue derived from resistant plants. Accumulation of the phytoalexin rishitin was similar in callus tissue and whole plants. Tomato plants resistant to race 2 did not accumulate more rishitin than susceptible plants during the first 4 days after inoculation, in contrast to plants resistant to race 1 and inoculated with race 1. The different response of tomato plants and calli towards race 1 and 2 of F. oxysporum f. sp, lycopersici suggests that the mechanism of resistance controlled by the I-l and the 1-2 gene is different. Callus tissue might be useful to study these differences in resistance. ZusammenfassungWechselwirkungen zwischen Fusarium oxysporum f. sp. lycopersici Rasse 2 und nah-isogenen resistenten und anfalligen Linien intakter Tomatenpflanzen oder Kallusgewebe Untersucht wurde die Wechselwirkung zwischen Fusarium oxysporum f, sp. lycopersici Rasse 2 und nah-isogenen resistenten und anfalligen Tomatenpflanzenlinien der Sorte "Moneymaker" und deren Kallusgewebekulturen, Bei den Kallusgewebekulturen wurde eine Resistenz liber eine kurze Zeit, bis zu 4 Tage nach der Inokulation, erkennbar. Die Resistenz aufierte sich durch verzogertes Pilzwachstum auf dem Kallusgewebe, das von resistenten Pflanzen stammte. Die Akkumulation des Phytoalexins Rishitin war sowohl im Kallusgewebe wie auch in intakten Pflanzen gleich, Tomatenpflanzen mit einer Resistenz gegenuber der Rasse 2 akkumulierten genau so viel Rishitin wie anfallige U,S,

Callus cultures derived from isogenic lines of the tomato cultivars Moneymaker and Craigella, resistant or susceptible to F. oxysporum f. sp. lycopersici, were inoculated with Fusarium oxysporum f. sp. lycopersici race 1. Fungal growth was restricted on callus derived from resistant plants, after inoculation with a conidial suspension, whereas callus derived from susceptible plants was totally overgrown by the fungus within 7 days. The concentration of the phytoalexin rishitin was significantly higher in the callus culture derived from a resistant tomato line compared with the callus culture from a susceptible line, 2 and 3 days after inoculation with mycelium. The results of the experiments were compared with experiments with whole plants. Rishitin production as well as growth of the fungus was comparable with responses in plant‐fungus interaction. Therefore callus culture may be useful in studying the interaction between tomato plants and race 1 of F. oxysporum f. sp. lycopersici.

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