Legumes are able to enter into a symbiotic relationship with nitrogen-fixing soil bacteria, collectively called rhi- zobia. The symbiosis culminates in the formation of the root nodule, which provides an optimal environment for the bacteria to fix atmospheric nitrogen for use by the plant. It was estimated that the legume-rhizobial symbi- osis can fix more than half of the amount of nitrogen produced by the chemical fertilizer industry [1]. The legume-rhizobial interaction begins with a molecu- lar dialogue between the two symbiotic partners [2]. Fla- vonoid compounds released into the rhizosphere by the legume roots attract rhizobia and induce the expression of a set of bacterial genes, known as the nod genes [3-5]. The enzymes encoded by the nod genes enable the synthe- sis and secretion of bacterial lipo-chitooligosaccharides known as nodulation (Nod) factors [2,6]. Perception of Nod factors by the cognate host receptors in turn activates a suite of host responses that are essential for accommo- dation of bacterial invasion [7-10]. One of the earliest plant responses is the curling of the root hairs which traps rhizobial bacteria within a structure called the colonized curled root hair [11,12]. It is within these trap sites that