Endophytic diazotrophs of maize: Diversity, colonization pattern, and NifH production

The objective of this research was to assess the diversity of endophytic diazotrophs of maize with an eventual goal of shifting the emphasis of agronomic practices from reliance on chemical fertilizer to a more sustainable approach to include the use of associative biological fixation. We investigated the diversity of diazotrophs in association with green-house and field grown Zea mays and described the activity of nitrogen-fixing bacteria when growing in association with maize. Colonization events of diazotrophs have been studied in several grass species however, this was the first time the common maize isolate, Klebsiella pneumoniae, has been described as an endophyte of Zea mays. Additionally, the production of dinitrogenase reductase by K. pneumonia when associating with maize was demonstrated using immuno-localization. The expression of nitrogenase by this species when associating with maize suggests that this might be a naturally-occurring phenomenon with field grown crops if the plant cultivation conditions are appropriate.; During a study on the diversity of bacteria that associate with maize, we isolated a new non-nitrogen fixing genus, Dyadobacter sp. This species was isolated from duplicate treatments of maize that were watered with no nitrogen fertilizer. Dyadobacter sp. was recovered from surface sterilized stem and was an efficient utilizer of N, as indicated by growth on a nitrogen-limited medium. Dyadobacter sp. groups with the Cytophaga-Flexibacter-Bacteroides, and is most closely related to Runella slithyformis, a freshwater inhabitant. Another isolate, Runella sp. NS12, was co-isolated from plants receiving the same treatments from which Dyadobacter sp. was isolated. Although the evolutionary distance between Runella sp. NS12 and Dyadobacter sp. is slightly greater than the distance between Dyadobacter sp. and R. slithyformis, Runella sp. NS12 appears to phenotypically "bridge" Dyadobacter and R. slithyformis, as it borrows phenotypic traits from both genera.