Fluorescence microscopy analyses of infection process of compatible and incompatible interactions between M. truncatula plants and Rm41. All images are composite images of GFP-expressing Rm41 cells (green) and root cells (red). A, Typical infection thread formed by compatible interaction between A20 and Rm41. The infection thread extends from the colonized, curled root hair to the base of the root hair cell. B, In the incompatible interaction between F83005.5 and Rm41, the bacteria can normally colonize the curled root hairs but typical infection threads cannot be detected. Occasionally, we can detect aborted, aberrant infection threads present on the F83005.5 roots. C, The nodule primordium on the A20 roots contained bacteria, while the nodule primordia on the F83005.5 roots (D) contained no bacteria despite frequent presence of bacterial colonies on the epidermal surface of the nodule primordia.