TDIF peptides accommodate root metabolism for symbiotic nodulation

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. In a new study published in Current Biology by the F. Frugier SILEG team, we identified in the Medicago truncatula model legume a previously unnoticed increase of 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 which 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 modulate the root primary metabolism adaptations required for symbiotic nodule development.

22/08/2025