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Title:Characteristics of biological control agents and their antagonism against rice pathogens and pests
Author(s):Bui, Hung Xuan
Director of Research:Schroeder, Nathan E.
Doctoral Committee Chair(s):Schroeder, Nathan E.
Doctoral Committee Member(s):Spencer, Joseph L.; Eastburn, Darin M.; Zhao, Youfu
Department / Program:Crop Sciences
Discipline:Crop Sciences
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Rice pathogens, beneficial microbes
Abstract:Rice is an important staple food for over half of the world’s population. Rice production routinely faces the risk of pathogen and pest outbreaks which may cause significant yield loss such as rice root-knot nematodes, bacterial leaf blight, and brown planthopper. Currently, pesticide use is the dominant practice for pest and disease management in Asian rice farming. The use of beneficial microbial agents for the control of pathogens and pests is a potential tool to sustain rice productivity and reduce ecological and environmental impacts due to pesticide overuse. Initially, 15 putatively beneficial bacteria, previously shown to be antagonistic to rice fungal pathogens (Rhizoctonia solani, Pyricularia oryzae, Fusarium moniliforme) were revived and cultured. I tested their potential activity against Xanthomonas oryzae pv. oryzae, Meloidogyne graminicola and Nilaparvata lugens. The results of these experiments (Chapter 2) showed that some beneficial bacterial strains demonstrated either their adverse effects on the abovementioned biotic stressors or promoted plant growth. Because of their adverse effects on bacterial leaf blight and plant growth promoting effects, I selected In-b-17, In-b-590 and G266 (a beneficial Xanthomonas sp.) for experiments to explore the mechanisms of their effects against bacterial leaf blight and rice root-knot nematodes. Since In-b-17 and In-590 were not identified, I sequenced their 16S rDNA and identified them as Bacillus sp. and Paenibacillus sp. respectively. Microbial volatile compounds have received attention as tools to promote plant growth and suppress pathogens, nematodes and insects. I hypothesized that the chosen bacterial strains (Bacillus sp., Paenibacillus sp. and Xanthomonas sp.) might produce volatile compounds toxic to rice root-knot nematodes and Xanthomonas oryzae pv. oryzae (bacterial blight pathogen). Rice root-knot nematodes and Xanthomonas oryzae pv. oryzae were exposed to bacterial volatiles in vitro and in planta. The results of these experiments (Chapter 3) suggested that bacterial volatiles were rapidly toxic to the second-stage juveniles of rice root-knot nematodes in vitro and reduced nematode infection in planta. Also, bacterial volatiles inhibited Xanthomonas oryzae pv. oryzae growth in vitro. However, there were no adverse effects of bacterial volatiles on Xanthomonas oryzae pv. oryzae observed in planta. Interestingly, bacterial volatiles from Bacillus sp. and Xanthomonas sp. were toxic to rice germination and seedlings in vitro but not in planta. The results suggested that bacterial volatiles may be a potential tool for the control of rice root-knot nematodes and other soil-borne pathogens but the high concentration of volatiles may be toxic to rice. In Chapter 4, the mechanistic effects of beneficial bacterial strains (Bacillus sp. and Paenibacillus sp.) were investigated in vitro and in planta. Bacillus amyloliquefaciens and Paneibacillus macerans strains, previously shown to have no effect on Xanthomonas oryzae pv. oryzae, were included as positive controls. Results from these experiments suggested that beneficial bacterial strains might use different strategies to control above- and below-ground pathogens (Xanthomonas oryzae pv. oryzae and rice root-knot nematodes respectively). The strategies were likely direct and indirect effects against above- and below-ground pathogens. For instance, Bacillus sp. and Paenibacillus sp. antagonized above- and below-ground pathogens in vitro while B. amyloliquefaciens and P. macerans did not show antagonism against Xanthomonas oryzae pv. oryzae. Putative antimicrobial peptide-encoding genes were amplified from Bacillus sp., but not B. amyloliquefaciens. However, more studies needs to be conducted to confirm the identity and functional importance of these genes. These antimicrobial peptide genes might likely contribute to the antagonism against Xanthomonas oryzae pv. oryzae. The putative antimicrobial peptide genes in Paenibacillus spp. could not be identified. The beneficial bacterial strains also emitted volatile compounds that were toxic to the second-stage juveniles of rice root-knot nematodes and the growth of Xanthomonas oryzae pv. oryzae in vitro. Xanthomonas oryzae pv. oryzae and rice root-knot nematode infection was reduced when rice was treated with the Bacillus sp. and Paenibacillus sp. suspensions in planta. Also, selected beneficial bacterial strains might indirectly induce plant responses against Xanthomonas oryzae pv. oryzae and rice root-knot nematode infection. The RNA-Seq analysis might suggest that the plant cell wall was reinforced against pathogen attacks when treated with selected beneficial bacteria. I conducted a study on postembryonic ventral nerve cord (VNC) development and gonad migration of Steinernema carpocapsae, an entomopathogenic nematode (Chapter 5). The result showed that the development of the VNC of S. carpocapsae was similar to Caenorhabditis elegans. However, the gonad migration pattern of S. carpocapsae was distinct from both C. elegans and the Diplogaster nematode Pristionchus pacificus. This study provides evidence for a possible use of beneficial microbes as biocontrol tools against rice pathogens and pests. The mechanisms of beneficial bacteria against above- and below-ground rice pathogens such as direct, indirect and volatile effects were studied. It will be interesting to explore further the effect of bacterial volatiles for the control of rice root-knot nematodes and other soil-borne pathogens as a form of fumigant. Also, it would be interesting to investigate the effects of cell-wall related genes in resistance against pathogens and transfer the knowledge to a breeding program to create resistant varieties against both above- and below-ground pathogens.
Issue Date:2019-02-20
Rights Information:Copy right 2019 Hung Xuan Bui
Date Available in IDEALS:2019-08-23
Date Deposited:2019-05

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