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Characterization and Application of Almond-Derived Bacterial Endophytes


The overall aim of this work was to identify and characterize the endophytic bacteria of almond trees. In order to do this, I first established efficient protocols to remove surface microbes from leaves. My study shows that complete removal of leaf cuticle by the sterilization technique assures loss of epiphytic microbes. Additionally, using electron microscopy, I found that established tests to evaluate surface sterility, PCR and leaf imprints, are unreliable methods to demonstrate plant surface sterility. Applying the inhere established surface sterilization protocols allowed me to analyze the endophytic bacterial composition of three genetically different almond cultivars. My results show that two of these three cultivars were dominated by Pseudomonadaceae, while the bacterial composition of the third cultivar consisted mainly of Streptococcaceae. Multiple commensal Streptococcaceae species are able to suppress growth of pathogenic Pseudomonadaceae strains in animals. Therefore, my findings suggest that Streptococcaceae endophytes might be useful in the development of strategies for reducing pathogenic impacts of Pseudomonadaceae on Prunus trees. Additionally, I isolated endophytic bacteria and tested the isolates for plant growth promoting effects. One of the isolates was determined to be a novel Erwinia gerundensis strain that I named A4. Application of this bacteria to the roots of the model plant Arabidopsis thaliana increases overall Arabidopsis biomass and increased root surface. Moreover, inoculation of roots with a transgenic A4 strain expressing reporter genes allowed me to visualize colonization of internal root tissues and subsequently above ground organs. I then sequenced the A4 genome in order to understand its plant growth promoting effects. This revealed that A4’s genome encodes for enzymes that synthesize organic acids and siderophores that can provide plants with the essential nutrients phosphorus and iron. Furthermore, A4 has the enzymatic pathway to produce the polyamine spermidine that help plants cope with biotic and abiotic stresses. Altogether, this work not only strongly enhances our knowledge of endophytic bacteria in almonds, but could also serve as a foundation for the development of novel agricultural applications using endophytic microorganisms to improve crop productivity.

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