Differential Gene Expression in the Development of Actinorhizal Root Nodules

Abstract

Actinorhizal root nodules form as a result of the interaction of actinomycetes in the genus Frankia with the appropriate host plant roots. In the actinorhizal symbiosis with Alnus, the first easily detectable sign of interaction between the plant and bacterial partners is the extensive deformation of plant root hairs. This root hair deformation which is detectable in a matter of hours has been shown not to require direct contact between bacteria and host plant (Prin, Rougier, 1987). There appears to be an as yet unidentified diffusible factor synthesized or released by frankiae in response to exposure to host plant root exudate in a manner analogous to the rhizobia/legume signaling system. Root hair deformation is followed by penetration of the root hair by Frankia filaments and the initiation of cortical cell divisions leading to prenodule formation (Berry, Sunell, 1990). The colonization of prenodule tissue is followed by the induction of a lateral root primordium in the pericycle. As the lateral root primordium grows through the infected prenodule tissue the postmeristematic cells of the now-nodule primordium are themselves infected with the microsymbiont which rapidly proliferates within host cells and begins to fix nitrogen. The nodule meristem bifurcates giving rise to the lobed nodules characteristic of Alnus.