Responses of transgenic alfalfa lines carrying PSL to Rlv (p149). A, Well-developed nodule-like structures (*) on a root of alfalfa carrying the PSL gene. Scale bar = 100 μm. B, Dark-field microscopy of PSL-transgenic alfalfa root inoculated with a Gus-marked Rlv (p149). The arrows point to the numerous infection threads. Scale bar = 100 μm. C, Section through the edge of a nodule illustrating an infection thread (i.t.) in the root hair. Scale bar = 20 μm. D, Multiple threads are evident in the root hair and hypodermal cells (arrows). Scale bar = 20 μm. E, Convoluted infection thread with narrow branches protruding into the host cell (arrow). Scale bar = 10 μm. F, An infected nodule formed on a PSL-transgenic alfalfa root inoculated with Gus-marked Rlv (p149). Scale bar = 100 μm. G, Attachment assays with Gus marked strains. Attachment is stronger to the PSL transgenic roots as evidenced by the dark-blue color of the roots.