| Bacteria interact with plants in four ways. They may be pathogens, symbionts, epiphytes, or endophytes. Of these four types of bacterial-plant interactions, the endophytic interactions are the least studied and least understood. Endophytes are defined here as bacteria that enter the interior of plants without causing disease or the formation of symbiotic structures. Endophytic bacteria are worthy of study, because they can increase plant growth and improve the plant nutrition of non-legumes through nitrogen fixation. However, in some cases, they are human pathogens that can cause disease in humans if the contaminated plant tissues are consumed.;The studies reported here illustrate a systematic approach to understand the biology of endophytic colonization, including the plant's ability to regulate this process. First, the host range and strain specificity for endophytic colonization was determined for the model endophyte Klebsiella pneumoniae 342 (Kp342) on five host plants. The results showed that endophytic colonization is an active process controlled by genetic determinants from both partners. Second, a collection of enteric bacteria associated with alfalfa (Medicago sativa L.) sprout contaminations along with Kp342, E. coli K12, and Salmonella enterica serotype Typhimurium (14028) were compared for their ability to colonize the rhizosphere and the interior of alfalfa and Medicago truncatula. This work revealed the kinetics and strain specificity of endophytic colonization by enteric bacteria. This work also showed that the mechanisms of endophytic colonization differ from symbiotic infections by Sinorhizobium meliloti or mycorrhizal fungi. Third, Kp342 and 14028 inoculation of the model legume M. truncatula, and model plant Arabidopsis thaliana, rendered several lines of evidence showing that endophytic bacteria induce plant defense response pathways. These observations suggest approaches to reduce contamination of raw produce by human enteric pathogens and to increase the number of growth-promoting bacteria in plants.;The last chapter shows definitive evidence that Kp342 inoculation enhances N nutrition of wheat, the world's most important crop. Unlike other diazotrophic endophytes known to fix N in non-legumes, this work presents evidence that the fixed N was incorporated into a plant product. |
