Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/13/2009
Publication Date: 2/21/2009
Publication URL: http://hdl.handle.net/10113/28143
Citation: Roberts, D.P., Baker, C.J., Mckenna, L.F., Liu, S., Buyer, J.S., Kobayashi, D.Y. 2009. Influence of host seed on metabolic activity by Enterobacter cloacae in the spermosphere. Soil Biology and Biochemistry. 41:754-761. 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 how different environments influence the ability of biological control agents to suppress soilborne plant pathogens. In this study we determined that the host seed has significant impact on the overall metabolic activity of bacterial biological control agents and may influence the ability of the biological control agent to suppress disease. Metabolic activity of the plant-beneficial bacterium Enterobacter cloacae was significantly greater when associated with pea seeds than when associated with cucumber seeds. Impact was positively correlated with the release of nutrients from the seed in the form of seed exudate. This information will be useful to scientists devising strategies to improve biological control through enhancement of growth on seeds or of disease suppression.
Technical Abstract: Little is known regarding the influences of nutrients released from plants on the metabolic activity of colonizing microbes. To gain a better understanding of these influences, we used bioluminescence- and oxygen consumption-based methods to compare bacterial metabolic activity expressed during colonization of two different seed types. Metabolic activity expressed by Enterobacter cloacae during colonization of pea seeds, which exude high levels of reduced carbon nutrients, was compared with that during colonization of cucumber seeds, which exude orders of magnitude less reduced carbon nutrients. Metabolic activity levels expressed by E. cloacae populations were much higher throughout a 72 h colonization period on pea seed compared with those observed on cucumber seed, directly correlating metabolic activity level with amounts of nutrients released by seeds. In vitro studies indicated E. cloacae cells expressed different levels of metabolic activity when incubated with different individual carbohydrates commonly found in cucumber and pea seed exudate. The addition of exogenous carbohydrate to cucumber seed increased metabolic activity expressed by colonizing E. cloacae; with the level of increase depended on both quantity and type of carbohydrate supplement. Subtraction of carbohydrate available to E. cloacae on cucumber and pea seeds through mutation in pfkA decreased metabolic activity by this bacterium. Results presented here provide strong evidence that metabolic activity of spermosphere-colonizing bacteria is host dependent, and that levels of activity are based largely on both quantitative and qualitative composition of host exudate released during seed germination.