AS WEEDS GO, Arabidopsis thaliana is a rather charming specimen. On a spring day, you might see it sprouting from the cracks of a parking lot, unleashing a small riot of white flowers that give it the common name “mouse ear cress.” But its rotund leaves often bear unwelcome passengers: among them, a bacteria called Pseudomonas syringae. It sits there looking for a way into the plant, usually the stomata through which the leaf takes in water and carbon dioxide, or through a wound. That’s when things get interesting.
