microRNA172 (miR172) is a family of non-coding small RNAs that play a crucial role in flowering time of plants. However, its role in soybean is not fully understood. Soybean possesses a unique trait known as symbiotic nitrogen fixation, which enables it to meet its high demand of nitrogen for growth and yield. Our studies show that miR172c is highly and specifically induced by rhizobia in a Nod factor receptor (GmNFR)-dependent manner. GmNINa, a soybean Nodule inception essential for nodulation can directly bind to and activate the expression of miR172c. In addition, we identified an AP2 transcription repressor, NNC1 (Nodule number control1), which is a functional target gene of miR172c during nodulation.Thus, we have elucidated the signaling pathway for soybean nodulation, which revolves around the NMN (GmNINa-miR172c-NNC1) module. Moreover, miR172c together with GmNINa can switch on/off the autoregulation of nodulation (AON) signaling pathway by transcriptional regulation of GmRIC1/2, leading to systemic regulation of optimal nodule number and efficient symbiotic nitrogen fixation in soybean.
Recently, we found that the symbiotic miR172c can also act as a mobile signal moving from roots to leaves to promote flowering; while fixed nitrogen is transported from the roots to the leaves to induce local miR172c. We show that the combinations of symbiotic miR172c and local miR172c led to activation of florigen-encoding FLOWERING LOCUS T (FT) homologs (GmFT2a/5a) by repressing the flowing repressor, TARGET OF EAT1-like 4a (GmTOE4a), which allows legumes to survive and reproduce under low-nitrogen conditions. Thus, we discovered a mechanism by which symbiotic signal miR172c and miR172 induced by fixed nitrogen systemically and synergistically convey symbiotic and nutritional cues from nodules/roots to leaves to accelerate flowering of legumes. Together, our discovery has defined a legume specific flowering pathway that integrates symbiotic nodulation and flowering in legumes.