Transfer of plasmid RP4 between pseudomonads after introduction into soil; influence of spatial and temporal aspects of inoculation

Abstract

Transfer of plasmid RP4 between introduced strains of Pseudomonas fluorescens was studied in 2 soils, Ede loamy sand and Guelph loam, in non‐rhizosphere and rhizosphere soil using soil chambers and microcosm systems. Short‐term organism survival was generally at high levels (> 106/g dry soil), in both soils, whereas long‐term survival was poorer, particularly in the loamy sand. Amendment of this soil with bentonite clay improved bacterial survival. Plasmid transfer between donor and recipient strains freshly introduced into separate portions of Ede loamy sand, which were subsequently mixed, was only detected in the vicinity of growing wheat roots, suggesting roots stimulate bacterial migration and/or growth. However, no transfer was detected between resident donor and recipient cell populations (introduced 48 days previously), due to poor organism survival. Plasmid transfer was detected in the rhizosphere between established, resident donor cell populations, and newly‐introduced recipients, and vice‐versa, in both soils. These data suggested that plant roots enhance the frequency of bacterial matings not only between organisms present in the same niches, but also between organisms from different niches, or in different conditions of stress, probably by stimulating bacterial migration and/or growth, or by providing additional surfaces for cell‐to‐cell contact.

Survival of genetically engineered Pseudomonas fluorescens in soil in competition with the parent strain

“…In general, both abiotic and biotic factors are known to affect bacterial population sizes in soil. Abiotic soil factors known to affect survival of introduced cells include the (initial) soil moisture content at the time of inoculation [7,19], soil temperature [17], soil texture [5,17,18,20] and the soil nutrient status [5,17]. However, biotic factors such as antagonism, competition or predation are probably mainly responsible for the decline observed in this paper, as may be inferred from a comparison between the trends in population dynamics of the introduced populations in non-sterile versus sterile soils.…”

Horizontal gene transfer of atrazine‐degrading genes (atz) from Agrobacterium tumefaciens St96‐4 pADP1::Tn5 to bacteria of maize‐cultivated soil

“…The low number of isolated transconjugants did not allow us to draw conclusions about atrazine treatment and rhizosphere effects on the transfer frequency of pADP1::Tn5. However, it has already been shown for other catabolic genes that (1) the selective pressure exerted by the xenobiotic can enhance the survival and growth of transconjugants in soil,38, 39 and that (2) the rhizosphere can act as a hotspot for plasmid transfer 40–44. In addition, no effects of either atrazine treatment or rhizosphere were observed on the nature of the isolated transconjugants.…”

Rhizosphere competence of Pseudomonas sp. NBRI9926 and Rhizobium sp. NBRI9513 involved in the suppression of chickpea (Cicerarietinum L.) pathogenic fungi

“…The rationale for the use of antibiotic resistance to mark bacteria in ecological studies is based on the unique advantages of recovery and enumeration, without interfering with organism fitness. Antibiotic‐resistant strains of bacteria have been used to assess the persistence of indigenous and non‐indigenous microorganisms inoculated into soil [22, 23]. The time period of 48 h was sufficient for the first organism to establish itself well on the rhizosphere of the chickpea roots.…”