Hormone-modulated transcription factors orchestrating the root nodule symbiosis

Transcription factors play essential roles modulating gene expression during plant development and the adaptation to environmental cues through the control of morphogenetic programs. In the root nodule symbiosis between legumes and rhizobia, two coordinated morphogenetic programs are activated by the perception of bacterial signals: the organogenesis of the nodule, a lateral root organ specialized in nitrogen fixation, and the infection process that allows the bacteria to colonize the nodule. These programs are influenced by the action of phytohormones, mainly auxin, cytokinin, ethylene, gibberellin, and brassinosteroid, which act modulating the activity of different families of transcription factors. In the past years, significant advancements have been made in understanding how transcription factors of the NIN (Nodule Inception), GRAS (GIBBERELLIN-ACID INSENSITIVE (GAI), REPRESSOR of GA1 (RGA), and SCARECROW (SCR)), ERF (Ethylene Response Factor), ARF (Auxin Response Factor), LBD (Lateral Organ Boundaries Domain), and SHI/STY (SHORT INTERNODES/STYLISH) families function at different developmental stages of bacterial infection and nodule formation and differentiation. Here, we review recent advances of this hormonal-mediated modulation of transcription factors with key roles in the root nodule symbiosis and their evolutionary origin from other developmental programs, as well as their post-transcriptional regulation by small RNAs. We also provide a perspective on how epigenomic approaches can shed light on how these transcription factors influence chromatin remodeling at loci containing key symbiotic genes.