Characterization of bacteria isolated from maize roots: emphasis on Serratia and infestation with corn rootworms (Chrysomelidae: Diabrotica)

Authors: Prischmann, Deirdre; Lehman, R - Michael; Christie, Amy; Dashiell, Kenton

Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/26/2008
Publication Date: 8/6/2008
Citation: Prischmann, D.A., Lehman, R.M., Christie, A.A., Dashiell, K.E. 2008. Characterization of bacteria isolated from maize roots: emphasis on Serratia and infestation with corn rootworms (Chrysomelidae: Diabrotica). Applied Soil Ecology. 40:417-431. Available at www.elsevier.com/locate/apsoil.

Interpretive Summary: Larval corn rootworms (Coleoptera: Chrysomelidae) are soil-dwelling insect pests that frequently cause economic damage to maize (Zea mays L.) by consuming root tissues, thus lowering grain yield. Little is known about interactions between rootworms and soil bacteria, including potential impacts of maize rhizobacteria, such as entomopathogenic Serratia spp., on subterranean rootworm pests. We used selective growth medium (caprylate-thallous agar, CT) to quantify and isolate Serratia spp. from: 1) bulk soil, 2) roots of four field-grown maize genotypes, half of which were infested with corn rootworm eggs, and 3) non-diseased, larval corn rootworms from a reared colony. Phenotypic testing and 16S rRNA gene sequencing were used to identify all bacteria, including non-Serratia spp., that were successfully isolated on the CT solid medium. We also isolated and identified Serratia spp. associated with non-diseased and diseased Diabrotica adults. Serratia spp. associated with maize roots were more abundant and diverse than those associated with bulk soil, where they were undetectable with our methods. There was no impact of plant genotype on densities of bacteria isolated from maize roots. Serratia grimesii was frequently associated with maize roots, regardless of infestation with rootworm eggs. Serratia marcescens biotype A4, S. plymuthica and several other Enterobacteriacae genera were also associated with maize roots. Compared to non-infested maize roots, roots grown in soil infested with rootworm eggs had higher densities of two strains of the S. marcescens biotype A1b, and densities of these orange and pink strains were comparable within infested roots. Both strains were associated with larval corn rootworms, although the orange strain was more abundant, and was also associated with diseased rootworm adults. Our studies identified specific Serratia strains associated with diseased rootworms that may have potential as biological control agents and additional Serratia biotypes associated with the maize rhizosphere that may function as plant growth promoting agents.

Technical Abstract: Larval corn rootworms (Coleoptera: Chrysomelidae) are soil-dwelling insect pests that frequently cause economic damage to maize (Zea mays L.) by consuming root tissues, thus lowering grain yield. Little is known about interactions between rootworms and soil bacteria, including potential impacts of maize rhizobacteria, such as entomopathogenic Serratia spp., on subterranean rootworm pests. We used selective growth medium (caprylate-thallous agar, CT) to quantify and isolate Serratia spp. from: 1) bulk soil, 2) roots of four field-grown maize genotypes, half of which were infested with corn rootworm eggs, and 3) non-diseased, larval corn rootworms from a reared colony. Phenotypic testing and 16S rRNA gene sequencing were used to identify all bacteria, including non-Serratia spp., that were successfully isolated on the CT solid medium. We also isolated and identified Serratia spp. associated with non-diseased and diseased Diabrotica adults. Serratia spp. associated with maize roots were more abundant and diverse than those associated with bulk soil, where they were undetectable with our methods. There was no impact of plant genotype on densities of bacteria isolated from maize roots. Serratia grimesii was frequently associated with maize roots, regardless of infestation with rootworm eggs. Serratia marcescens biotype A4, S. plymuthica and several other Enterobacteriacae genera were also associated with maize roots. Compared to non-infested maize roots, roots grown in soil infested with rootworm eggs had higher densities of two strains of the S. marcescens biotype A1b, and densities of these orange and pink strains were comparable within infested roots. Both strains were associated with larval corn rootworms, although the orange strain was more abundant, and was also associated with diseased rootworm adults. Our studies identified specific Serratia strains associated with diseased rootworms that may have potential as biological control agents and additional Serratia biotypes associated with the maize rhizosphere that may function as plant growth promoting agents.