UC Berkeley

The phyllosphere epiphyte Pantoea agglomerans 299R synthesizes indole-3-acetic acid (IAA), an important plant hormone. IAA production was previously shown to confer a small but significant fitness advantage to Pa299R cells inoculated onto bean (Phaseolus vulgaris) leaves, but the mechanism by which bacterial IAA exerts this effect is unknown. In this work, we investigated several hypotheses regarding how bacterial IAA enhances the growth and survival of leaf epiphytic microbes such as Pa299R.

We first tested the hypothesis that bacterial IAA enhances the availability of plant sugars to phyllosphere bacteria, thereby relieving carbon limitation of bacterial growth on the leaf surface (Chapter Two). We inoculated sucrose- and fructose-inducible biosensor bacteria onto bean leaves, and investigated the effects of auxin availability on sugar sensing. We manipulated auxin availability in two ways: first, we compared sugar sensing by wild-type Pa299R (IAA+) biosensors to that of the isogenic, IAA- mutant PaMX149; and second, we compared sugar sensing by PaMX149 bacteria inoculated with exogenous NAA to bacteria inoculated without NAA. The presence of auxin--whether endogenously synthesized or exogenously applied--was associated with a significant decrease in sucrose sensing, and a small but significant increase in fructose sensing. Additionally, in vitro assays demonstrated that Pa299R initiates growth more rapidly when using glucose or fructose as a sole carbon source than when using sucrose. Together, this suggests that IAA biosynthesis may function as a resource conversion strategy to enhance carbon acquisition and rapid bacterial growth in an oligotrophic, environmentally dynamic phyllosphere.

In addition to its role in carbon acquisition, we also investigated several alternative hypotheses of bacterial IAA production in the phyllosphere (Chapter Three). We demonstrated that bacterial IAA exerts no measurable impact on resistance of Pa299R to environmental stress. We also found that bacterial IAA does not affect the autofluorescence of substrate leaf epidermal cells that support large bacterial aggregates of Pa299R. Finally, we describe unsuccessful efforts to determine the role of bacterial IAA in another foliar epiphyte and compatible bean pathogen, Pseudomonas syringae pv. syringae B728a.