Fig. 5.

Dynamics of the colonization of lettuce roots by B. subtilis across the whole dataset. (A) The colonization is marked by an increase in bacterial cell density in the soil further away from the root (red) within the first 26 h following inoculation, after which a maximum is reached between 30 and 34 h following inoculation. Bacterial cell density increases more persistently closer to the root surface (blue) until 30 to 34 h following inoculation and does not really reach a steady state in the analyzed time frame. Even though the bacteria total quantity seemed to reach a maximum after 26 h (green), variations in cell density along the root persisted. This indicates that migration may play a role in the later stages of the colonization of the root. (B) Intense activity at the root tip may precede colonization near or on the root surface. (Left) Example of the diagram of the colonization kinematics shows how cell density changes both with time and as a function of the position along one root. The diagram shows that densification of bacterial cell population is discrete (here, two pulses 7 h apart are recorded at ∼1 mm from the tip) and likely results from the attachment of bacteria on nongrowing tissue, since diagonal patterns indicate the constant increase in the distance from the root tip. (Middle) Overall, the distribution of bacterial cell density along the root (solid blue line) confirms that bacterial cell density concentrates in the basal region of the root (>1 mm from the root tip). (Right) On the contrary, the most intense temporal variations in bacterial cell density are observed near the root tip (dashed blue line). The activity of bacterial cells in the bulk soil (red) confirmed bacterial activity at the tip of the root is enhanced, with both the density in the bulk soil (solid red line) and the variance (dashed red line) showing a maximum in the region near the root tip. Data shown as mean ± SE, n = 6.