Nod Factor Signal Transduction in the Rhizobium–Legume Symbiosis

Abstract

The symbiotic interaction between Rhizobium bacteria and most legume plants is initiated by the perception of bacterial signal molecules, the nodulation (Nod) factors, at the root hairs of the plant. This induces responses both in the root hairs, leading to infection by the bacteria, as well as at a distance in the root cortex, leading to nodule organ formation. Molecular genetic approaches have been very successful in elucidating the key components essential for this Nod factor signal transduction. Cloning of these key regulators has been possible because of the establishment of two model legumes, Lotus japonicus and Medicago truncatula, for which extensive molecular genetic tools are available. We discuss the characteristics of the identified epidermal Nod-factor-signaling components from these two legumes and position them in a genetically based signal transduction cascade. To allow a successful rhizobial symbiosis, the responses in the root hairs need to be tightly coordinated with responses in the inner root cells. This is likely achieved through secondary signals that are generated upon Nod factor perception in the epidermis and are transported to the pericycle/cortex. The recent identification of a cytokinin receptor that is essential for the cortical responses supports the involvement of secondary signals, and the possible role of cytokinin as intercellular signal is discussed.