The relationship between traits of Bacillus megaterium, soybean root colonization, and suppression of rhizoctonia root rot
Zheng, Xiangyang
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Permalink
https://hdl.handle.net/2142/21954
Description
Title
The relationship between traits of Bacillus megaterium, soybean root colonization, and suppression of rhizoctonia root rot
Author(s)
Zheng, Xiangyang
Issue Date
1995
Doctoral Committee Chair(s)
Sinclair, James B.
Department of Study
Biology, Microbiology
Agriculture, Plant Pathology
Discipline
Biology, Microbiology
Agriculture, Plant Pathology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Microbiology
Agriculture, Plant Pathology
Language
eng
Abstract
The significance and importance of biological control of soilborne plant pathogens by introduced bacteria in sustainable agriculture and current problems and challenges have been reviewed. A biocontrol system using Bacillus megaterium de Bary strain B153-2-2 to suppress soybean (Glycine max (L.) Merrill) root rot caused by (Rhizoctonia solani (Kuhn) isolate 2B12 was studied. First, the chemotactic response (CR) of B153-2-2 to soybean root or seed exudates was measured by capillary assay. A exudate at 0.03 mg/ml initiated a significant (P = 0.01) CR. A significant CR occurred at pH 5.5 to 9 and 5$\sp\circ$ to 40$\sp\circ{\rm C}$. The highly sensitive CR of B153-2-2 to the exudates over ranges of pH and temperature may be one of the traits that contribute to its successful colonization of soybean roots and seeds. Second, using this HPLC analysis, six chemoattractants in the exudates were identified: alanine, asparagine, glutamine, malate, serine, and threonine. None of the seven sugars tested was a chemoattractant. Third, B153-2-2 cells were mutagenized by ethylmethane sulfonate. Sixty-eight mutants with altered chemotaxis and/or motility were screened and isolated based on the swarm morphology. Nine of the mutants were found to have the same growth rate as B153-2-2 but differ in chemotaxis, motility, sporulation, and/or antagonism to R. solani. It seemed that mutants with increased chemotaxis and motility always had decreased sporulation and/or antagonism. Finally, B153-2-2 and the nine mutants were introduced either by a seed coating or soil application into different soil mixtures ranging from sandy to clay soil without or with R. solani. Results showed that: (i) there was a significant (P = 0.05) positive correlation between chemotaxis, sporulation, or antagonism and root or seed colonization by B153-2-2 and its mutants; (ii) seedlings grown in sandy soil had a greater root colonization by B153-2-2 or its mutants and a lower disease index than those in clay soil; and (iii) there was a significant correlation between bacterial root colonization and suppression of Rhizoctonia root rot. It was concluded that successful suppression of Rhizoctonia root rot by strain B153-2-2 was based on effective soybean root and seed colonization and antagonism to R. solani. Chemotaxis play an important role for effective root and seed colonization by B153-2-2. Niche exclusion may play a role in successful suppression of R. solani.
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