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Title: Survival of a Genetically Modified Root-Colonizing Pseudomonad and Rhizobium Strain in an Acidic Soil

item Staley, Thomas
item Brauer, David

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2006
Publication Date: 8/2/2006
Citation: Staley, T., Brauer, D.K. 2006. Survival of a Genetically Modified Root-Colonizing Pseudomonad and Rhizobium Strain in an Acidic Soil. Soil Science Society of America Journal. 70(6):1906-1913.

Interpretive Summary: The lack of beneficial microbial populations in the rhizosphere often limits forage production in the acidic, infertile and shallow soils of the Appalachian Region. Understanding what soil acidity-related chemical factors control populations, especially at crop establishment, is critical to taking advantage of their beneficial activities. Marked strains of pseudomonas (plant growth-promoter) and rhizobia (nitrogen-fixer) were used to investigate the survival of these bacteria in nonsterile soils. Survival of both bacteria were directly related to soil acidity and calcium, and inversely related to aluminum concentrations. These results were confirmed in soil solutions derived from the soils, suggesting the absence of detrimental biological and physical effects on survival. The use of fluorescent probes indicated cellular membrane damage as the cause of reduced survival in the more acidic soils. These results suggest that improving the association of these beneficial bacteria with forage roots will necessarily involve the selection of both acidic soil tolerant microorganisms and plant cultivars.

Technical Abstract: Maintaining threshold populations of inoculum microorganisms in the soil environment is important for such practical applications as biocontrol, plant growth-promotion, bioremediation, and nodulation. However, because of both technical and effort constraints in monitoring bacterial viability in nonsterile soils, few studies have reported on survival kinetics, particularly in relation to subtle alterations in soil acidity-related factors. A genetically-modified strain of pseudomonas (P. putida R20/lacZY) or rhizobia (R. trifolii 162S7a/gusA) was used to inoculate a nonsterile Gilpin silt loam soil, limed at 4 low levels (pHw 4.71, 4.81, 4.92 and 4.99) or soil solutions derived from them. Immediate declines in viability of both mutants were found in all soils, reaching 0.1- to 10% initial CFU g-1 soil in 35 h for P. putida and 68 h for R. trifolii. Death rate constants (kd) for both mutants were inversely, and consistently, related to lime level (soil pH), although differences were not significant (P > 0.05) for the rhizobium mutant. Use of soil solutions gave similar viability responses for both mutants, but over much shorter incubation times. As with soils, kd values for both mutants in soil solutions were inversely, and consistently, related to lime level (solution pH). Survival (kd) was negatively correlated (R2 > -0.914) with pH and basic cations (Ca and Mg), and positively correlated (R2 > 0.933) with Al, in both soil and soil solution experiments. This relationship of survival kinetics to soil solution chemistry was confirmed for the pseudomonas mutant, albeit broadly, by use of fluorescent probes, suggesting increased cell membrane damage at lower pHs. These results demonstrate not only the improved utility of using soil solutions, rather than nonsterile soils, for bacterial viability assessments, but also the beneficial effect of low-level liming (~0.28 pH unit increase) on survival of root-colonizing bacteria in acidic soils.