The sTDIF signaling peptide modulates the root stele diameter and primary metabolism to accommodate nodulation

Legume plants form specific new organs on their root system, the nitrogen-fixing nodules, thanks to a symbiotic interaction with soil bacteria collectively named rhizobia1. Compatible rhizobia secrete signaling molecules called Nod Factors (NFs), triggering both rhizobial infection and nodule organogenesis in host roots1. Rhizobia do not only induce the formation of nodule organs, but also modulates root growth and the number of lateral roots, notably through the action of NFs2,3,4. We identified in Medicago truncatula a previously unnoticed increase of the root stele diameter occurring upon rhizobium inoculation. This symbiotic root response requires Tracheary Element Differentiation Inhibitory Factor (TDIF) signaling peptides5, and notably the MtCLE37 TDIF-encoding gene that is rapidly upregulated by rhizobium NF signals and expressed in nodules, and which was thus referred to as symbiotic TDIF (sTDIF). Indeed, the cle37/stdif mutant was not responsive to rhizobium regarding its root stele diameter increase, and had a reduced nodule number. Combined transcriptomic and metabolomic analyses revealed that stdif has a defective primary metabolism, notably affecting carbohydrates and sucrose accumulation in both roots and nodules. Remarkably, a sucrose exogenous treatment was able to rescue the rhizobium-induced stele diameter symbiotic response in stdif. This metabolic deregulation is thus instrumental in explaining the altered symbiotic response of the mutant. Overall, this study highlights a novel function of TDIF signaling peptides in legumes plants, which beyond regulating stele development, also modulate root primary metabolism adaptations that are required for symbiotic nodule development.