This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Ramirez, C. Articles by Alexander, M. Search for Related Content PubMed PubMed Citation Articles by Ramirez, C. Articles by Alexander, M. Agricola Articles by Ramirez, C. Articles by Alexander, M.

 Previous Article  |  Next Article 

Appl Environ Microbiol. 1980 September; 40(3): 492-499

Evidence Suggesting Protozoan Predation on Rhizobium Associated with Germinating Seeds and in the Rhizosphere of Beans (Phaseolus vulgaris L.)

Carlos Ramirez and Martin Alexander

¹ Laboratory of Soil Microbiology, Department of Agronomy, Cornell University, Ithaca, New York 14853

ABSTRACT

Changes in populations of microorganisms around germinating bean (Phaseolus vulgaris L.) seeds, in the rhizosphere of bean, and in a model rhizosphere were studied. Strains of Rhizobium phaseoli that were resistant to streptomycin and thiram were used, and as few as 300 R. phaseoli cells per g of soil could be enumerated with a selective medium that was devised. A direct role was not evident for bacterial competitors, lytic bacteria, antibiotic-producing microorganisms, bacteriophages, and Bdellovibrio in the suppression of R. phaseoli around germinating seeds and in the rhizosphere. Protozoa increased in numbers in the soil upon planting of the seeds. The extent of colonization of soil by R. phaseoli was inversely related to the presence of large numbers of bacteria and protozoa. Colonization of R. phaseoli was improved upon suppression of protozoa with thiram and also when the soil was amended with other protozoan inhibitors and mannitol to simulate seed and root exudation. The data support the view that the decrease in numbers of R. phaseoli is caused by an increase in protozoan predation, the protozoa increasing in number because they prey on bacteria that proliferate by using seed and root exudates as nutrients.

Present address: Escuela de Fitotecnia, Facultad de Agronomia, Universidad de Costa Rica, Costa Rica.