Computational inference of Rhizobium phaseoli transcriptional regulatory network predicts Transcription Factors involved in nodulation.

Growth of the common bean plant Phaseolus vulgaris is tightly linked to its symbiotic relationship with diverse rhizobial species, particularly Rhizobium phaseoli, an alphaproteobacterium that forms root nodules and provides high levels of nitrogen to the plant. Molecular cross-talk is known to happen through plant-derived metabolites, but only flavonoids have been identified as nodulation signals, which act through the activation of the NodD Transcription Factor (TF). The identification of signals that mediate nodulation via TFs can aid in the rational design of biofertilizers that promote plant-bacteria symbiosis. Here, we identified 57 TFs in the R. phaseoli genome through sequence conservation from Escherichia coli, and predicted a transcriptional regulatory network comprising 16 TFs, and 1,371 target genes. We identified the regulatory interactions relevant to nodulation via transcriptome analysis, and hypothesize that PuuR is a TF involved in nodulation, potentially acting via its known binding metabolite putrescine. Sequence and structural evidence predict a model where putrescine acts as a signaling metabolite in nodulation via the TF PuuR, and the regulation of the nodI gene.