Zinc Improves Biocontrol of Fusarium Crown and Root Rot of Tomato by Pseudomonas fluorescens and Represses the Production of Pathogen Metabolites Inhibitory to Bacterial Antibiotic Biosynthesis

Abstract

Crown and root rot of tomato caused by Fusarium oxysporum f. sp. radicis-lycopersici is an increasing problem in Europe, Israel, Japan, and North America. The biocontrol agent Pseudomonas fluorescens strain CHA0 provides only moderate control of this disease. A one-time amendment of zinc EDTA at 33 mug of Zn(2+)/ml to hydroponic nutrient solution in soilless rockwool culture did not reduce disease when used alone, but did reduce disease by 25% in the presence of CHA0. In in vitro studies with the pathogen, zinc at concentrations as low as 10 mug/ml abolished production of the phytotoxin fusaric acid, a Fusarium pathogenicity factor, and increased production of microconidia over 100-fold, but reduced total biomass. Copper EDTA at 33 mug of Cu(2+)/ml had a similar effect as zinc on the pathogen in vitro; it reduced disease when used alone, and increased the biocontrol activity of CHA0 in soilless culture. Ammonium-molybdate neither improved the biocontrol activity of CHA0 nor affected production of fusaric acid or microconidia. Strain CHA0 did not degrade fusaric acid. Fusaric acid at concentrations as low as 0.12 mug/ml repressed production by CHA0 of the antibiotic 2,4-diacetylphloroglucinol, a key factor in the biocontrol activity of this strain. Production of pyoluteorin by CHA0 was also reduced, but production of hydrogen cyanide and protease was not affected, suggesting that fusaric acid affects biosynthesis at a regulatory level downstream of gacA and apdA genes. Fusaric acid did not affect the recovery of preformed antibiotics nor did it affect bacterial growth even at concentrations as high as 200 mug/ml. When microbial meta-bolite production was measured in the rockwool bioassay, zinc amendments reduced fusaric acid production and enhanced 2,4-diacetylphloro-glucinol production. We suggest that zinc, which did not alleviate the repression of antibiotic biosynthesis by fusaric acid, improved biocontrol activity by reducing fusaric acid production by the pathogen, which resulted in increased antibiotic production by the biocontrol agent. This demonstrates that pathogens can have a direct negative impact on the mechanism(s) of biocontrol agents.

The rhizosphere: a playground and battlefield for soilborne pathogens and beneficial microorganisms

“…The first example of interference of a soil-borne pathogenic fungus with the biosynthesis of an antimicrobial compound in a beneficial bacterium was described by Duffy and Défago (1997). In their study, DAPG biosynthesis in fluorescent Pseudomonas strain CHA0 was repressed by F. oxysporum f.sp.…”

“…radicis-lycopersici. Fusaric acid produced by Fusarium was shown to be the fungal metabolite that specifically repressed DAPG biosynthesis (Duffy and Défago 1997;Notz et al 2002): blocking fusaric acid production in Fusarium by addition of zinc relieved repression of the phlA gene and improved the activity of strain CHA0. Subsequent studies showed that among a collection of genotypically different DAPG-producing Pseudomonas strains, several were relatively insensitive to fusaric acid-mediated repression of DAPG biosynthesis (Duffy et al 2004).…”

Identification and Manipulation of Soil Properties To Improve the Biological Control Performance of Phenazine-ProducingPseudomonas fluorescens

“…In the present study, Mn was negatively correlated with disease suppression by 2-79; this may be related to the ability of 2-79 to either oxidize or reduce Mn under different soil conditions. The interaction between a plant pathogen and a biocontrol agent (8) can be influenced by soil properties. Fusaric acid produced by the phytopathogenic fungus Fusarium oxysporum f. sp.…”

“…However, commercial application of these bacteria for biocontrol has been hampered in large part because of their inconsistent performance between field locations (45). Such variability has been attributed to different causes, including host plant genotype (37,44), agricultural practices (28), mutation of the biocontrol strain (7), pathogen resistance to biocontrol mechanisms (23), and vulnerability of the biocontrol strain to pathogen defense mechanisms (8). For biocontrol strains of Pseudomonas fluorescens, antibiotic production often plays a central role in disease control (40).…”

Mechanism of Tomato Bacterial Wilt Suppression in Soil Amended with Lysine

“…The suppression of Fusarium wilt of tomato was enhanced when P. putida AP-1 was applied together with methionine than AP-1 alone (50). The use of zinc also improved the biological control of Fusarium root and crown rot of tomato by P. fluorescens (7). Thereby, the amendment of soil with organic matter or a substrate in the appropriate amount likely improves the rhizosphere competence and the survival of introduced or naturally occurring antagonists in the soil, resulting in effective and sustainable disease control.…”