|De La Fuente, Leonardo|
Submitted to: Plant Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/20/2004
Publication Date: 1/20/2007
Citation: Weller, D.M., Landa, B.B., Mavrodi, O.V., Schroeder, K.L., De La Fuente, L., Bankhead, S.B., Molar, R.A., Bonsall, R.F., Mavrodi, D.M., Thomashow, L.S. Role of 2,4-diacetylphloroglucinol-producing fluorescent pseudomonas spp. in plant defense. Plant Biology 9 (1): 4-20 Jan, 2007. Interpretive Summary: Plants stimulate and enrich populations of antagonistic microorganisms in the rhizosphere environment through the release of nutrients as a first-line of defense against diseases caused by soilborne plant pathogens. Genetic resistance to some of the most common and widespread soilborne pathogens is lacking in most crop species. Take-all is the most important root disease of wheat worldwide, causing over $1 billion in losses in the U.S. Take-all is difficult to control in modern cereal-based cropping systems because growers often must sow several crops of wheat before a break for economic reasons and practice reduced tillage to control soil erosion. However, wheat field soils become suppressive to take-all when wheat or barley is continuously grown in a field for 4 to 6 years following a severe outbreak of take-all, a phenomenon known as take-all decline (TAD). TAD is the best known example of natural plant defense by antagonistic microorganisms. In fields in Washington State and The Netherlands, TAD results from the buildup of populations of Pseudomonas fluorescens that produce the antibiotic 2,4-diacetylphloroglucinol (DAPG). Understanding the fundamental mechanism of TAD has allowed wheat growers to make better use of this natural phenomenon and allowed predictions of how long a wheat field must sustain damage from take-all before the natural suppressiveness of the DAPG producers arrests the disease. DAPG producers can also be introduced into a field that has not undergone TAD to accelerate the process of take-all suppression, thus reducing the number of years a field must sustain severe damage before the onset of natural disease suppression.
Technical Abstract: Plants have evolved strategies of stimulating and supporting groups of antagonistic microorganisms in the rhizosphere as a defense against diseases caused by soilborne plant pathogens owing to the lack of genetic resistance to some of the most common and widespread soilborne pathogens. Some of the best examples of natural microbial defense of plant roots occur in disease suppressive soils, and suppressiveness against many different diseases has been described. Take-all is the most important root disease of wheat worldwide and soils become suppressive to the disease when wheat or barley is continuously grown in a field following an outbreak of take-all, a phenomenon known as take-all decline (TAD). In Washington State, U.S.A. and in some fields in The Netherlands, TAD results from the build up of populations of fluorescent Pseudomonas spp. that produce the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG). Take all is suppressed when the population of 2,4-DAPG producers reaches a threshold density of 105 CFU/g of root. 2,4-DAPG-producing fluorescent Pseudomonas spp. are enriched by monoculture of other crops and evidence is accumulating that these bacteria have a role in natural plant defense in many different agroecosystems. Seventeen different genotypes of 2,4-DAPG producers (designated A Q) have been defined by whole-cell repetitive sequence-based (rep)-PCR analysis. The genotype of an isolate is predictive of its rhizosphere competence on certain crop species. For example D-genotype isolates, which are the dominant genotype in the rhizosphere of wheat grown in Washington State TAD soils, have an affinity for the roots of wheat and pea and out-compete other genotypes when present together on these crops. 2,4-DAPG producers are highly effective biocontrol agents against a variety of plant diseases and ideally suited for serving as vectors for expressing other biocontrol traits in the rhizosphere.