In order to examine the conservation of transcription factors in infected cells of nitrogen-fixing root nodules, the activity of promoter::reporter gene constructs was compared in transgenic hairy root systems of different root nodule forming plant species. The promoter under examination, Cg164, was taken from the Australian tree Casuarina glauca. Cg164 encodes a homologue of a nodule infected cell-specific gene family from the closely related actinorhizal tree Alnus glutinosa. After infection of Agrobacterium rhizogenes-induced transgenic hairy root systems of the model legume Lotus japonicus with Mesorhizobium loti strain TONO, the Cg164::GUS construct led to GUS activity in the roots, in the nodule outer cortex and in the nodule vascular system, but not in the infected cells of the nodules. Hence, the cell type-specificity of expression of Cg164 was not conserved. In order to find out which factors determined infected cell-specific expression, the effects of ROS and ethylene signaling on Cg164 promoter activity were examined. Ethylene signaling was required for the expression of Cg164::GUS, while salt stress, leading to ROS production, repressed Cg164 promoter activity.