Repeated Introduction of Genetically Modified Pseudomonas putida WCS358r without Intensified Effects on the Indigenous Microflora of Field-Grown Wheat

Abstract

To investigate the impact of genetically modified, antibiotic-producing rhizobacteria on the indigenous microbial community, Pseudomonas putida WCS358r and two transgenic derivatives were introduced as a seed coating into the rhizosphere of wheat in two consecutive years (1999 and 2000) in the same field plots. The two genetically modified microorganisms (GMMs), WCS358r::phz and WCS358r::phl, constitutively produced phenazine-1-carboxylic acid (PCA) and 2,4-diacetylphloroglucinol (DAPG), respectively. The level of introduced bacteria in all treatments decreased from 10 7 CFU per g of roots soon after sowing to less than 10 2 CFU per g after harvest 132 days after sowing. The phz and phl genes remained stable in the chromosome of WCS358r. The amount of PCA produced in the wheat rhizosphere by WCS358r::phz was about 40 ng/g of roots after the first application in 1999. The DAPG-producing GMMs caused a transient shift in the indigenous bacterial and fungal microflora in 1999, as determined by amplified ribosomal DNA restriction analysis. However, after the second application of the GMMs in 2000, no shifts in the bacterial or fungal microflora were detected. To evaluate the importance of the effects induced by the GMMs, these effects were compared with those induced by crop rotation by planting wheat in 1999 followed by potatoes in 2000. No effect of rotation on the microbial community structure was detected. In 2000 all bacteria had a positive effect on plant growth, supposedly due to suppression of deleterious microorganisms. Our research suggests that the natural variability of microbial communities can surpass the effects of GMMs.

Impact of root exudates and plant defense signaling on bacterial communities in the rhizosphere. A review

“…Whereas only a limited number of studies have investigated effects of plant defense on the non-pathogenic and beneficial microflora, most of these studies suggest that effects on the indigenous microflora are negligible. Development of methodologies to study shifts in the rhizosphere microflora have been stimulated by studies that have focused on the impact of introducing genetically modified bacteria that produce broad spectrum antibiotics (Glandorf et al 2001;Bakker et al 2002;Blouin-Bankhead et al 2004;Timms-Wilson et al 2004;Viebahn et al 2003Viebahn et al , 2006. Most of those studies used fingerprinting techniques of group specific PCR amplified 16S (bacteria) or 18S (fungi) rDNA, enabling detection of shifts but lacking power to identify the organisms affected.…”

Effects of Jasmonic Acid, Ethylene, and Salicylic Acid Signaling on the Rhizosphere Bacterial Community of Arabidopsis thaliana

“…After two weeks, germinated seedlings were transferred individually to 60-ml pots containing either a potting soil-sand mixture (12:5 vol/vol) (Pieterse et al 1996) or a clay soil (Glandorf et al 2001;Viebahn et al 2003). Both soils were either nonautoclaved or autoclaved (twice for 20 min at 121°C with a 24-h interval).…”

Impact of Plant Species and Site on Rhizosphere-Associated Fungi Antagonistic to Verticillium dahliae Kleb

“…Although in recent years molecular tools have been developed to analyze the structures of the rhizosphere-associated fungal communities of several crops (13,19,31,36,37), little is known about the function and possible role of the observed fungal diversity associated with plant roots, especially their antagonistic potential (18,36).…”