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13 Pags.- 5 Figs. © The Author(s) 2020. Published by Oxford University Press on behalf of the Society for Experimental Biology. ; Symbiotic nitrogen fixation carried out in legume root nodules requires transition metals. These nutrients are delivered by the host plant to the endosymbiotic nitrogen-fixing bacteria living within the nodule cells, a process in which vascular transport is essential. As members of the Yellow Stripe-Like (YSL) family of metal transporters are involved in root to shoot transport, they should also be required for root to nodule metal delivery. The genome of the model legume Medicago truncatula encodes eight YSL proteins, four of them with a high degree of similarity to Arabidopsis thaliana YSLs involved in long-distance metal trafficking. Among them, MtYSL3 is a plasma membrane protein expressed by vascular cells in roots and nodules and by cortical nodule cells. Reducing the expression level of this gene had no major effect on plant physiology when assimilable nitrogen was provided in the nutrient solution. However, nodule functioning was severely impaired, with a significant reduction of nitrogen fixation capabilities. Further, iron and zinc accumulation and distribution changed. Iron was retained in the apical region of the nodule, while zinc became strongly accumulated in the nodule veins in the ysl3 mutant. These data suggest a role for MtYSL3 in vascular delivery of iron and zinc to symbiotic nitrogen fixation. ; This research was funded by a European Research Council Starting Grant (ERC-2013-StG-335284) and the Spanish State Research Agency (AEI) grant (AGL2015-65866-P) to MGG, and a AEI grant (AGL2016-75226-R) to JA and AA-F co-financed by the European Regional Development Fund (FEDER). RC-R and FJJ-P were supported by a Formación del Personal Investigador fellowship (BES2013-062674 and BES-2017-082913, respectively). IA was the recipient of a Juan de la Cierva- Formación postdoctoral fellowship from Ministerio de Ciencia, Innovación y Universidades (FJCI-2017- 33222). VE was partially funded by the Severo Ochoa Programme for Centres of Excellence in R&D from the AEI (grant SEV-2016- 0672) received by the Centro de Biotecnología y Genómica de Plantas (UPM-INIA). Development of the M. truncatula Tnt1 mutant population was, in part, funded by the National Science Foundation, USA (DBI-0703285) to KSM. AM and HK and the µXRF measurements were supported by the Ministry of Education, Youth and Sports of the Czech Republic with co-financing from the European Union (grant 'KOROLID', CZ.02.1.01/0.0/0.0/15_003/0000336) and the Czech Academy of Sciences (RVO: 60077344). ; Peer reviewed