|Van Pelt, Johannes|
|Van Loon, Leendert|
Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2011
Publication Date: 4/1/2012
Citation: Weller, D.M., Mavrodi, D., Van Pelt, J.A., Pieterse, C.M., Van Loon, L.C., Bakker, P.A. 2012. Induced systemic resistance (ISR) in Arabidopsis thaliana against Pseudomonas syringae pv. tomato by 2,4-diacetylphloroglucinol-producing Pseudomonas fluorescens. Phytopathology. 102(4): 403-412. Interpretive Summary: 2,4-diacetylphloroglucinol (2,4-DAPG) is an antibiotic produced by certain strains of beneficial bacteria that live on the roots of plants. The antibiotic is active against a wide range of organisms including plant-pathogenic bacteria, fungi, and nematodes, and producer bacteria often provide biological control of soilborne pathogens. In this paper, 2,4-DAPG-producing pseudomonads and pure DAPG applied to the soil induced resistance in the model plant Arabidopsis against a foliar disease. These results demonstrate that besides inhibiting pathogens directly, 2,4-DAPG turns on natural defense mechanisms in plants. Because 2,4-DAPG producers occur in many agroecosystems, it is possible that 2,4-DAPG production on roots results induces protection against foliar pathogens in crops colonized by 2,4-DAPG-producing pseudomonads.
Technical Abstract: Pseudomonas fluorescens strains that produce the polyketide antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) are among the most effective rhizobacteria that suppress root and crown rots, wilts and damping-off diseases of a variety of crops, and they play a key role in the natural suppressiveness of some soils to certain soilborne pathogens. Root colonization by 2,4-DAPG-producing P. fluorescens strains Pf-5 (genotype A), Q2-87 (genotype B), Q8r1-96 (genotype D) and HT5-1 (genotype N) induced systemic resistance (ISR) in Arabidopsis thaliana accession Col-0 against bacterial speck, caused by P. syringae pv. tomato. The ISR-eliciting activity of the four bacterial genotypes was similar and all were equivalent in activity to the well-characterized strain P. fluorescens WCS417r. The 2,4-DAPG biosynthetic locus consists of the genes phlF and phlACBDE. phlD or phlBC mutants of Q2-87 (2,4-DAPG minus) were significantly reduced in ISR activity, and genetic complementation of the mutants restored ISR activity back to wild-type levels. A phlF regulatory mutant (over-producer of 2,4-DAPG) had ISR activity equivalent to Q2-87. Introduction of DAPG into soil at concentrations of 10 to 250 µM four days before challenge inoculation induced resistance equivalent to or better than the bacteria. Strain Q2-87 induced resistance on transgenic NahG plants but not on npr1-1, jar1 and etr1 Arabidopsis mutants. These results indicate that the antibiotic 2,4-DAPG is a major determinant of ISR in 2,4-DAPG-producing P. fluorescens; that the genotype of the strain does not affect its ISR activity; and that the activity induced by these bacteria operates through the ethylene- and jasmonic acid-dependent signal transduction pathway.