Succession of Indigenous Pseudomonas spp. and Actinomycetes on Barley Roots Affected by the Antagonistic Strain Pseudomonas fluorescens DR54 and the Fungicide Imazalil

Abstract

In recent years, the interest in the use of bacteria for biological control of plant-pathogenic fungi has increased. We studied the possible side effects of coating barley seeds with the antagonistic strain Pseudomonas fluorescens DR54 or a commercial fungicide, imazalil. This was done by monitoring the number of indigenous Pseudomonas organisms and actinomycetes on barley roots during growth in soil, harvest after 50 days, and subsequent decomposition. Bacteria were enumerated by traditional plate spreading on Gould's S1 agar (Pseudomonas) and as filamentous colonies on Winogradsky agar (actinomycetes) and by two quantitative competitive PCR assays. For this we developed an assay targeting Streptomyces and closely related genera. DR54 constituted more than 75% of the Pseudomonas population at the root base during the first 21 days but decreased to less than 10% at day 50. DR54 was not successful in colonizing root tips. Initially, DR54 affected the number of indigenous Pseudomonas organisms negatively, whereas imazalil affected Pseudomonas numbers positively, but the effects were transient. Although plate counts were considerably lower than the number of DNA copies, the two methods correlated well for Pseudomonas during plant growth, but after plant harvest Pseudomonas-specific DNA copy numbers decreased while plate counts were in the same magnitude as before. Hence, Pseudomonas was 10-fold more culturable in a decomposition environment than in the rhizosphere. The abundance of actinomycetes was unaffected by DR54 or imazalil amendments, and CFU and quantitative PCR results correlated throughout the experiment. The abundance of actinomycetes increased gradually, mostly in numbers of DNA copies, confirming their role in colonizing old roots.

Impact of 2,4-Diacetylphloroglucinol-Producing Biocontrol Strain Pseudomonas fluorescens F113 on Intraspecific Diversity of Resident Culturable Fluorescent Pseudomonads Associated with the Roots of Field-Grown Sugar Beet Seedlings

“…At 19 days after sowing, the number of total culturable fluorescent pseudomonads at the rhizoplane was not affected, but that of RCFP was reduced (Table 1), similar to the results of other studies (10,36,47). This suggests that sampling took place after this transient situation and that the seed inoculant interacted with RCFP while colonizing the roots of sugar beet seedlings.…”

“…S1 agar is a selective medium for fluorescent pseudomonads (17), and colony counts on S1 agar were shown to be in agreement with 16S ribosomal DNA (rDNA) quantitative direct PCR data for enumeration of root-associated pseudomonads (20). In addition, Pseudomonas diversity is higher on S1 agar than on King's B agar (19), which is the medium traditionally used to recover fluorescent pseudomonads, and S1 has become the medium of choice for these bacteria (19,36,47,49). All plates were incubated at room temperature for 3 to 7 days prior to counting of colonies.…”

“…Often, the impact of bacterial inoculants on nontarget populations has been assessed quantitatively, i.e., with respect to the population sizes of particular microbial groups defined on the basis of taxonomy or physiological function (4,11,16,33,37,47). However, this approach is limited by the fact that important ecological impacts may take place in terms of the composition of a particular microbial group without modification of its population size.…”

An Altered Pseudomonas Diversity Is Recovered from Soil by Using Nutrient-Poor Pseudomonas -Selective Soil Extract Media

“…Identification by Pseudomonas-specific immunoassay and Pseudomonas-specific PCR. The isolates were verified as belonging to the genus Pseudomonas by using a Pseudomonas-specific antibody (17) and Pseudomonas-specific PCR (14,31). A Pseudomonas-specific antibody directed against the OprF surface protein was used in a colony blotting assay as described by Kragelund et al (17).…”

Characterization of streptomycetes with potential to promote plant growth and biocontrol

“…Besides acting as organic matter decomposers, these microorganisms have great potential as agents for control of plant pathogens (Hoster et al, 2005;Thirup et al, 2001) and/or for plant growth promotion (Nassar et al, 2003). This is due to their capacity to produce antibiotics, siderophores, enzymes that have antimicrobial activity, substances that promote plant growth, solubilization of phosphates and competition with plant pathogens for substratum and nutrients (Cattelan & Hartel, 2000;Crawford et al, 1993).…”