A nitrogen fixing symbiosis-specific pathway required for legume flowering.

Abstract

Symbiotic nitrogen fixation boosts legume growth and production in nitrogen-poor soils. It has long been assumed that fixed nitrogen increases reproductive success, but until now, the regulatory mechanism was unknown. Here, we report a symbiotic flowering pathway that couples symbiotic and nutrient signals to the flowering induction pathway in legumes. We show that the symbiotic microRNA-microRNA172c (miR172c) and fixed nitrogen systemically and synergistically convey symbiotic and nutritional cues from roots to leaves to promote soybean (<i>Glycine max</i>) flowering. The combinations of symbiotic miR172c and local miR172c elicited by fixed nitrogen and development in leaves activate florigen-encoding <i>FLOWERING LOCUS T</i> (<i>FT</i>) homologs (<i>GmFT2a/5a</i>) by repressing <i>TARGET OF EAT1-like 4a</i> (<i>GmTOE4a</i>). Thus, FTs trigger reproductive development, which allows legumes to survive and reproduce under low-nitrogen conditions.