Figure 10.

Proposed Model for the Action of Cytokinin on Auxin Transport and Accumulation during Nodule Initiation in M. truncatula.

Our data suggest a model in which cytokinin signaling mediated by the CRE1 receptor transiently activates or releases certain flavonoids (most likely naringenin and/or isoliquiritigenin) in the root, which then act as auxin export inhibitors that cause auxin (IAA) accumulation and subsequently enhance auxin response in cells that will divide to form a nodule primordium. Both transient application of synthetic auxin transport inhibitors like NPA and TIBA, or flavonoids, induce (pseudo)nodules infected by rhizobia.