Abstract
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.
Highlights
Iron, copper, and other transition metals are required at relatively high levels for biological nitrogen fixation,the conversion of N2 into NH4+ carried out by diazotrophic microorganisms (González-Guerrero et al, 2014, 2016)
Free-living nitrogen-fixing bacteria use a large battery of siderophores, transition metal transporters, and storage proteins to directly acquire these metals from the environment (Jurkevitch et al, 1992; Yeoman et al, 2000; Navarro-Rodríguez et al, 2019)
The paradigmatic example of a symbiotic diazotroph are the bacteria known as rhizobia.These organisms colonize the cells of legume root nodules, organs developed to provide the conditions for nitrogen fixation to occur (Downie, 2014)
Summary
Copper, and other transition metals are required at relatively high levels for biological nitrogen fixation,the conversion of N2 into NH4+ carried out by diazotrophic microorganisms (González-Guerrero et al, 2014, 2016). These elements act as cofactors of key enzymes mediating the process, such as nitrogenases that directly catalyse the reaction, cytochrome oxidases that provide energy to the reaction and control O2 levels, and many of the free radical detoxification enzymes (Appleby, 1984; Rubio et al, 2004; Rubio and Ludden, 2005). It largely remains to be determined how vascular transport occurs and which proteins are mediating it
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