Submitted to: Archives Of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2006
Publication Date: 9/15/2006
Citation: Roberts, D.P., Mckenna, L.F., Hu, X., Lohrke, S.M., Kong, H.N., De Souza, J.T., Baker, C.J., Lydon, J. 2006. Mutation in cyaa in enterobacter cloacae decreases cucumber root colonization. Archives Of Microbiology. 187:101-115. Interpretive Summary: Soilborne plant pathogenic fungi cause diseases that result in major economic losses to farmers in the United States. Biological control measures for these diseases need to be developed due to environmental problems associated with existing chemical controls. However, little is known regarding the genetic and biochemical means by which biological control agents grow on seeds and roots of plants and suppress pathogens that infect these seeds and roots. Colonization of roots and other plant parts is thought to be important for biological control. In this study we demonstrated that the cyaA gene of the biological control bacterium Enterobacter cloacae was important for colonization of cucumber roots by this bacterium. This gene encodes the enzyme adenyl cyclase, which catalyzes the synthesis of cyclic AMP. cyaA and cyclic AMP were also important for the migration of E. cloacae toward, and growth on, compounds in cucumber root exudate. This information will be useful to scientists devising strategies to improve biological control through enhancement of growth of biological control agents on cucumber roots.
Technical Abstract: Strains of Enterobacter cloacae show promise as biological control agents for Pythium ultimum -induced damping-off on cucumber and other crops. E. cloacae M59 is a mini-Tn5 Km transposon mutant of strain 501R3. Populations of M59 were significantly lower on cucumber roots and decreased much more rapidly than those of strain 501R3 with increasing distance from the soil line. Strain M59 was decreased or deficient in growth and swarm response on most compounds detected in cucumber root exudate and on a synthetic cucumber root exudate medium. Molecular characterization of strain M59 demonstrated that mini-Tn5 Km was inserted in cyaA, which encodes adenyl cyclase. Adenyl cyclase catalyzes the formation of cAMP. cAMP levels in cell lysates from strain M59 were approximately 2 % those of strain 501R3. Addition of exogenous cAMP to strain M59 restored growth and swarm response on the synthetic cucumber root exudate medium and increased cucumber seedling colonization by this strain without serving as a source of reduced carbon, nitrogen, or phosphorous. These results clearly demonstrate a role for cyaA in colonization of cucumber roots possibly due to the requirement of cAMP for chemotaxis toward, and growth on, reduced carbon compounds in cucumber root exudate.