Legumes have maintained the ability to associate with rhizobia to sustain the nitrogen-fixing root nodule symbiosis (RNS). In Medicago truncatula, the Nod factor (NF)-dependent intracellular root colonization by Sinorhizobium meliloti initiates from young, growing root hairs. They form rhizobial traps by physically curling around the symbiont.¹^,² Although alterations in root hair morphology like branching and swelling have been observed in other plants in response to drug treatments³ or genetic perturbations,^4-6 full root hair curling represents a rather specific invention in legumes. The entrapment of the symbiont completes with its full enclosure in a structure called the "infection chamber" (IC),¹^,²^,⁷^,⁸ from which a tube-like membrane channel, the "infection thread" (IT), initiates.¹^,²^,⁹ All steps of rhizobium-induced root hair alterations are aided by a tip-localized cytosolic calcium gradient,¹⁰^,¹¹ global actin re-arrangements, and dense subapical fine actin bundles that are required for the delivery of Golgi-derived vesicles to the root hair tip.⁷^,^12-14 Altered actin dynamics during early responses to NFs or rhizobia have mostly been shown in mutants that are affected in the actin-related SCAR/WAVE complex.^15-18 Here, we identified a polarly localized SYMBIOTIC FORMIN 1 (SYFO1) to be required for NF-dependent alterations in membrane organization and symbiotic root hair responses. We demonstrate that SYFO1 mediates a continuum between the plasma membrane and the cell wall that is required for the onset of rhizobial infections.

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