Influence of host seed on metabolic activity of Enterobacter cloacae in the spermosphere D.P. Roberts a, * , C.J. Baker b , L. McKenna a , S. Liu c , J.S. Buyer a , D.Y. Kobayashi d a Sustainable Agricultural Systems Laboratory, Henry A. Wallace Beltsville Agricultural Research Center, Beltsville, MD 20705, USA b Molecular Plant Pathology Laboratory, Henry A. Wallace Beltsville Agricultural Research Center, Beltsville, MD 20705, USA c Oil Crops Research Institute, Chinese Academy of Agricultural Science, Wuhan 430062, People’s Republic of China d Department of Plant Biology and Pathology, Rutgers University, New Brunswick, NJ 08901, USA article info Article history: Received 23 July 2008 Received in revised form 11 October 2008 Accepted 13 January 2009 Available online 7 February 2009 Keywords: Plant exudate Microbial metabolism Plant growth-promoting rhizobacteria pfkA mutant 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 coloni- zation of two different seed types. Metabolic activity expressed by Enterobacter cloacae during coloni- zation 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 carbo- hydrates commonly found in cucumber and pea seed exudates. The addition of exogenous carbohydrate to cucumber seed increased metabolic activity expressed by colonizing E. cloacae; with the level of increase dependent 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 spermo- sphere-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. Published by Elsevier Ltd. 1. Introduction Plant-associated bacteria have long been recognized to perform a variety of beneficial functions including promotion of plant growth, suppression of plant diseases, and inactivation of toxic pollutants (Davison, 1988; Kloepper et al., 1989; Lugtenberg et al., 1991; Crowley et al., 1997; Larkin et al.,1998). Many strains have been intensively studied and developed for these biotechnological purposes; however, inconsistent performance has been an issue (Larkin et al., 1998; Compant et al., 2005). The inconsistent perfor- mance by microbial products has been attributed to a number of causes, such as failure to establish effective population sizes, failure to colonize appropriate locations in association with plants, or failure to express necessary traits at appropriate times (Compant et al., 2005; Sørensen et al., 2001). Many of these causes are linked, either directly or indirectly, to the metabolic activity of the micro- bial inoculant, as metabolic energy is ultimately responsible for expression of traits required for the beneficial activities of these bacteria (Dowling and O’Gara,1994; Crowley et al., 1996; Sørensen et al., 2001). It is the metabolically active portions of beneficial microbial populations that are thought to be most important (Heijnen et al., 1995; Unge et al., 1999; Ramos et al., 2000). Despite the importance of metabolic activity, there is a limited under- standing of the metabolic behavior of bacterial inoculants in the spermosphere and rhizosphere (Sørensen et al., 2001). Previous studies have identified environmental factors that influence metabolic activity of plant-associated microbes. Microbes are known to have greater activity in association with plants than in bulk soil (Rattray et al., 1995; Kroer et al., 1998). However, only a small portion of inoculant populations maintains high levels of metabolic activity in the rhizosphere; the majority of microbial cells have metabolic characteristics similar to starved cells * Corresponding author. USDA-ARS, Sustainable Agricultural Systems Laboratory, Building 001, Rm. 140, Henry A. Wallace Beltsville Agricultural Research Center, 10300 Baltimore Avenue, Beltsville, MD 20705-2350, USA. Tel.: þ1 301 504 5680; fax: þ1 301 504 8370. E-mail address: dan.roberts@ars.usda.gov (D.P. Roberts). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ – see front matter Published by Elsevier Ltd. doi:10.1016/j.soilbio.2009.01.010 Soil Biology & Biochemistry 41 (2009) 754–761