Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/17/2008
Publication Date: 3/29/2008
Citation: Okubara, P.A., Bonsall, R.F. 2008. Accumulation of pseudomonas-derived 2,4-diacetylphloroglucinol on wheat seedling roots is influenced by host cultivar-bacterial strain interaction. Biological Control. 46(2008)322-331. Interpretive Summary: This study reports the use of a soil-free Petri plate system to grow wheat roots after inoculation with biocontrol isolates of the bacterium Pseudomonas fluorescens, which suppresses root pathogens, partly by production of the antifungal metabolite 2,4-diacetylphloroglucinol (2,4-DAPG). We used this system to explore the relationship between root colonization, host cultivar, bacterial isolate, and 2,4-DAPG accumulation on wheat roots. After four days, P. fluorescens Q8r1-96, an aggressive root colonizer, produced significantly more 2,4-DAPG than Q2-87 (a less aggressive colonizer) on the roots of two wheat cultivars. However, both bacterial isolates produced similar levels of the metabolite on a third host. Populations of the bacteria on the roots were at maximum levels in all treatments, indicating that the differences in metabolite accumulation were not due to amounts of colonizing bacteria. To our knowledge, this is the first report of cultivar-dependent 2,4-DAPG accumulation on roots of wheat. Our findings have implications for the interplay between host and bacterial factors that regulate 2,4-DAPG levels in the rhizosphere.
Technical Abstract: Disease suppression of soilborne pathogens by fluorescent pseudomonads (rhizobacteria) is a sustainable means of controlling root diseases, but the efficacy of rhizobacteria is subject to host and environmental factors. As part of a broad study to determine how host factors impact rhizosphere persistence of biocontrol pseudomonads, we quantified steady-state levels of 2,4-DAPG produced by two isolates of <start ital>Pseudomonas fluorescens<end ital> on the roots of three <start ital>Triticum aestivum<end ital> L. (hexaploid wheat) cultivars four days after seed inoculation and growth in Petri plates. Both <start ital>P. fluorescens<end ital> isolates aggressively colonized wheat roots, and produced the polyketide antifungal metabolite 2,4-diacetylphloroglucinol (2,4-DAPG). However, isolate Q8r1-96 produced more metabolite in culture, and persisted in the wheat rhizosphere longer than Q2-87. As expected, accumulation of 2,4-DAPG on roots of all three cultivars occurred at higher levels following colonization with Q8r1-96, compared to Q2-87. However, there was a striking difference in 2,4-DAPG production by Q8r1-96 and Q2-87 on cv. Tara, whereas no such difference was observed on cv. Buchanan. The accumulation patterns could not be attributed to differences in rhizoplane population densities; both isolates reached an average of 2.0 x 10<start super>9<end super> colony-forming units g<start super>-1<end super> after four days, regardless of cultivar. Likewise, the greater root surface area of Tara only partially explained the high levels of Q8r1-96-derived 2,4-DAPG on this cultivar. Previous studies have shown that steady-state levels of rhizosphere 2,4-DAPG are correlated to the extent of root colonization in a host species-dependent manner. Our findings indicate that 2,4-DAPG accumulation in the wheat rhizoplane also is cultivar-dependent. Progress in identifying the determinants of differential accumulation will be presented.