Xylella fastidiosa virulence in planta with copper supplementation
Xylella fastidiosa (Xf) is a gram negative, xylem-limited plant pathogenic bacteria that causes disease in many economically important crops worldwide. Copper is a widely-used antimicrobial agent on Xf hosts. While the effects of copper have been extensively studied for foliar pathogen control, it is unknown what affect it has on a xylem-colonized pathogen. Previous results from our group showed that concentrations of CuSO4 between 5-200 µM increase biofilm formation in vitro, while high concentrations (>200µM) of CuSO4 inhibited biofilm formation. In this study, we focused on in planta experiments to unveil the influence of copper in Xf-caused diseases using tobacco as a model for infection. Xf-infected and non-infected plants were watered with tap water, and water supplemented with 4mM and 8mM CuSO4. Symptom progression was observed, and sap and leaf ionome analysis was performed by inductively coupled plasm optical emission spectroscopy. Uptake of Cu was confirmed by increased concentrations of Cu in sap of plants treated with CuSO4-amended water. In independent experiments sap copper concentrations for 4 mM supplementation ranged from 10-50 µM, while 8 mM supplementation resulted in 50-100 µM Cu. In vitro this concentration range resulted in enhanced biofilm formation. The symptoms of leaf scorch in the Cu-supplemented plants showed a trend towards more severe at later timepoints. Based on the results, we proposed that the plant copper homeostasis machinery controls the level of copper in xylem preventing it from becoming elevated to a level that that would lead to bacterial inhibition. Further study will focus on how bacteria population and colonization in planta is influenced by copper.