Functional analysis of the OsNPF4.5 nitrate transporter reveals a conserved mycorrhizal pathway of nitrogen acquisition in plants.

Abstract

Low availability of nitrogen (N) is often a major limiting factor to crop yield in most nutrient-poor soils. Arbuscular mycorrhizal (AM) fungi are beneficial symbionts of most land plants that enhance plant nutrient uptake, particularly of phosphate. A growing number of reports point to the substantially increased N accumulation in many mycorrhizal plants; however, the contribution of AM symbiosis to plant N nutrition and the mechanisms underlying the AM-mediated N acquisition are still in the early stages of being understood. Here, we report that inoculation with AM fungus <i>Rhizophagus irregularis</i> remarkably promoted rice (<i>Oryza sativa</i>) growth and N acquisition, and about 42% of the overall N acquired by rice roots could be delivered via the symbiotic route under N-NO₃^- supply condition. Mycorrhizal colonization strongly induced expression of the putative nitrate transporter gene <i>OsNPF4.5</i> in rice roots, and its orthologs <i>ZmNPF4.5</i> in <i>Zea mays</i> and <i>SbNPF4.5</i> in <i>Sorghum bicolor</i> OsNPF4.5 is exclusively expressed in the cells containing arbuscules and displayed a low-affinity NO₃^- transport activity when expressed in <i>Xenopus laevis</i> oocytes. Moreover, knockout of <i>OsNPF4.5</i> resulted in a 45% decrease in symbiotic N uptake and a significant reduction in arbuscule incidence when NO₃^- was supplied as an N source. Based on our results, we propose that the NPF4.5 plays a key role in mycorrhizal NO₃^- acquisition, a symbiotic N uptake route that might be highly conserved in gramineous species.