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

Legume plants form specific organs on their root system, the nitrogen-fixing nodules, thanks to a symbiotic interaction with soil bacteria collectively named rhizobia. Rhizobia, however, do not only induce the formation of these nodule organs but also modulate root system architecture. We identified in Medicago truncatula a previously unnoticed increase in the root stele diameter occurring upon rhizobium inoculation. This symbiotic root response, similarly observed in another crop legume, pea, occurs rapidly and locally after rhizobium inoculation, leading to an increased number of vascular cells. Interestingly, this root stele diameter symbiotic response requires tracheary element differentiation inhibitory factor (TDIF) signaling peptides and, notably, the MtCLE37 TDIF-encoding gene whose expression is increased during nodulation, thus being referred to as symbiotic nodulation TDIF (sTDIF). Indeed, a cle37/stdif mutant is not responsive to rhizobium regarding its root stele diameter increase and has a reduced nodule number. Combined transcriptomic and metabolomic analyses revealed that stdif has a defective primary metabolism, notably affecting carbohydrate/sugar accumulation in both roots and nodules. Remarkably, a sucrose or a malate exogenous treatment is 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 modulates the root primary metabolism adaptations required for symbiotic nodule development.