Spatiotemporal dynamics of ethylene biosynthesis shape infection and nodule initiation in Medicago truncatula.

Ethylene is a well-established negative regulator of nodulation, yet how ethylene biosynthesis and perception are spatially coordinated during early symbiotic signaling remains unresolved. Here, we investigate the dynamics of ethylene responses in Medicago (Medicago truncatula) using transcriptomics, promoter-reporter analyses, loss-of-function approaches and a synthetic reporter. We show that the activity of the ethylene-responsive EBSn reporter shifts from inner root tissues under nonsymbiotic conditions to the outer cortex and epidermis following rhizobial inoculation, revealing a spatial reprogramming of ethylene signaling. Among the 8 Medicago 1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID SYNTHASE (ACS) genes, MtACS3 is induced in outer root cell layers upon rhizobia application, while MtACS10 is repressed in the inner cortex and pericycle, mirroring the shift in ethylene perception. Functional analysis demonstrates that MtACS10 restricts nodule initiation, whereas MtACS3 modulates the number of infection threads, prevents nodule clustering, and contributes to the radial positioning of nodule primordia. Rhizobial induced ectopic ACS expression in the root interior counteracts MtACS10 repression and blocks nodulation, highlighting the requirement for spatially confined downregulation of ethylene biosynthesis. Together, these findings establish a framework in which localized shifts in ethylene biosynthesis, mediated by distinct Medicago ACS genes, balance infection and organogenesis while co-defining the spatial limits of the root susceptible zone.