Submitted to: Proceedings of Workshop on Global Int Org Biocontrol (IOBC) Working Group
Publication Type: Proceedings
Publication Acceptance Date: 9/1/2005
Publication Date: N/A
Interpretive Summary: Root diseases caused by soilborne pathogens are major yield-limiting factors in the production of food, fiber and ornamental crops. As agricultural practices become more sustainable and less dependent on chemical inputs, there is an increasing need for ecologically sound methods of disease control. Biological control, which exploits the natural antagonistic activity of certain root-colonizing bacteria against fungal pathogens, is one such approach. The activity of many biocontrol agents depends on production of the antifungal metabolite 2,4-diacetylphloroglucinol(DAPG). However, DAPG-producing strains differ in the efficiency with which they colonize roots, and only the most competitive ones have an important role in take-all decline, a natural biocontrol phenomenon in which symptoms or the wheat root disease take-all subside spontaneously after several consecutive wheat crops. Because efficient root colonization is often a bottleneck to effective biological control and more widespread grower and industry acceptance of the technology, the traits that contribute to the exceptional competitiveness of these unique strains are of great interest. This study focused on the contribution of bacteriocins (protein-containing compounds produced by certain strains of bacteria and which kill closely related strains or species) to the competitiveness of 47 DAPG-producing strains of P. fluorescens. Over 70% of the strains inhibited the growth of other isolates in laboratory assays. In studies on the roots of wheat grown in soil in the greenhouse, populations of sensitive strains declined more rapidly in the presence of antagonistic bacteriocin producers than when the sensitive strains were introduced alone. The results indicate that antagonism associated with the production of bacteriocins can influence the ability of biocontrol agents to establish and maintain effective population densities in situ.
Technical Abstract: Certain strains of Pseudomonas fluorescens that produce the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) can colonize roots and suppress soilborne pathogens more effectively than other strains that otherwise are almost identical. We recovered DNA fragments from the superior root colonizer P. fluorescens Q8r1-96 by suppression subtractive hybridization with the moderate colonizer strain Q2-87 in order to identify genetic differences that might account for superior colonization activity. One clone from Q8r1-96 exhibited similarity to a pore-forming bacteriocin and resides in a 23-kb pyocin-like gene cluster that includes a functional two-gene lysis module and a bacteriophage tail assembly. Treatment of strain Q8r1-96 with mitomycin C caused production of phage tail-like particles. Subsequent screening in vitro revealed that bacteriocin activity was common among 2,4-DAPG-producing isolates representing 17 distinct genotypes. Such activity may contribute to strain competitiveness and persistence in vitro and in the rhizosphere.