Putrescine acts as a signaling metabolite in the transition from nodulation to nitrogen fixation inRhizobium phaseoli

Abstract

AbstractGrowth of the common bean plantPhaseolus vulgarisis tightly linked to its symbiotic relationship with diverse rhizobial species, particularlyRhizobium phaseoli, an alphaproteobacteria that forms root nodules and provides high levels of nitrogen to the plant. Molecular cross-talk is known to happen via plant-derived metabolites, but only flavonoids have been identified as signals. Flavonoids are transported inside the bacteria, where they signal the NodD regulator to elicit nodulation. Although seven other regulators are known to be involved, our knowledge of the regulatory mechanisms underlying the nodulation, and nitrogen fixation processes is limited, and the signals recognized by regulators are mostly unknown. Here, we identified 75 transcription factors inR. phaseoligenome through sequence conservation fromEscherichia coli, and assembled a transcriptional regulatory network comprising 24 regulators, and 652 target genes. We identified the interactions relevant to nodulation via gene expression, and signaled out putrescine as a signaling metabolite. We propose a model where putrescine acts as a switch on the transition from nodulation to nitrogen fixation via the dual transcription factor PuuR, and its regulation of thenodIandnifU2genes.ImportanceThis study provides new insights into the molecular cross-talk betweenPhaseolus vulgarisandRhizobium phaseoli, which is crucial for the development of alternative cropping strategies, including biopesticides and biofertilizers. In addition, we present the first transcriptional regulatory network systematically assembled forRhizobium phaseoli, which opens new avenues of research in the relationship between the transcriptional regulation and metabolism of these organisms.