Amino acids as reduced carbon sources for Enterobacter cloacae during colonization of the spermospheres of crop plants

Enterobacter cloacae 501R3 grew in vitro and in natural soil on seven amino acids, commonly found in seed exudates, when these compounds were supplied as sources of reduced C. A collection of transposon mutants of E. cloacae strain 501R3, each deficient in some aspect of amino acid utilization, was used to determine the nutritional role of these amino acids during growth in spermosphere soil. Mutant strains M2 and M59 were reduced, or deficient in, the ability to use all seven amino acids when these compounds were supplied as sources of reduced C in vitro or in natural soil. However, these two strains were similar to strain 501R3 in growth on amino acids supplied as N sources and in other nutritional tests. Populations of strains M2 and M59 were significantly less (P ≤ 0.05) than populations of strain 501R3 after 45 h in growth assays in corn and cucumber spermosphere soils. Growth by strain M2 was also significantly less than strain 501R3 in pea spermosphere soil (P ≤ 0.05). However, growth by strains M2 and 501R3 was similar in bean, cowpea, radish and sunflower spermosphere soils while growth by strains M59 and 501R3 was similar in pea and radish spermosphere soils. The data are consistent with the hypothesis that amino acids are not required as sources of reduced C for E. cloacae during growth in spermosphere soil. The data suggest, however, that amino acids are important as sources of reduced C for growth in some, but not all, spermosphere environments.