Submitted to: Agrociencia Magazine
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 16, 2005
Publication Date: N/A
Interpretive Summary: Plants stimulate and enrich populations of antagonistic rhizobacteia 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, a phenomenon known as take-all decline (TAD). 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. 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. DAPG producers also can be genetically engineered to produce other antibiotics besides DAPG, and thus broaden the range of root diseases that they can control.
Technical Abstract: Genetic resistance to root diseases of plants is rare, and agriculture controls these diseased by practices such as crop rotation and soil fumigation. However, plants have evolved a strategy of stimulating and supporting specific groups of antagonistic rhizosphere microorganisms as a defense against diseases caused by soilborne pathogens. Antibiotic production plays a significant role in plant defense by many of these bacteria, and detailed information is now available about the genetics, biochemistry, and regulation of synthesis of several commonly-produced antibiotics. Similarly, many genes that contribute to the ability of these strains to colonize roots have been identified, and studies of naturally suppressive soils have provide evidence of preferential interactions between plant hosts and biocontrol bacteria, revealing the existence of functional diversity in protective populations of very closely related strains. Here, we consider how this knowledge can be applied to better manage the indigenous rhizosphere microflora, aid in the selection of more effective microbial amendments, and guide the tailoring of microflora through directed genetic manipulations to enhance crop heatlth and productivity.