MtCRE1-dependent cytokinin signaling integrates bacterial and plant cues to coordinate symbiotic nodule organogenesis in Medicago truncatula

Phytohormonal interactions are essential to regulate plant organogenesis. In response to the presence of signals from symbiotic bacteria, the Nod factors, legume roots generate a new organ: the nitrogen-fixing nodule. Analysis of mutants in the Medicago truncatula CRE1 cytokinin receptor and of the MtRR4 cytokinin primary response gene expression pattern revealed that cytokinin acts in initial cortical cell divisions and later in the transition between meristematic and differentiation zones of the mature nodule. MtCRE1 signaling is required for activation of the downstream nodulation-related transcription factors MtERN1, MtNSP2 and MtNIN, as well as to regulate expression and accumulation of PIN auxin efflux carriers. Whereas the MtCRE1 pathway is required to allow the inhibition of polar auxin transport in response to rhizobia, nodulation is still negatively regulated by the MtEIN2/SICKLE- dependent ethylene pathway in cre1 mutants. Hence, MtCRE1 signaling acts as a regulatory knob, integrating positive plant and bacterial cues to control legume nodule organogenesis.