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 fungusRhizophagus irregularisremarkably promoted rice (Oryza sativa) 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 geneOsNPF4.5in rice roots, and its orthologsZmNPF4.5inZea maysandSbNPF4.5inSorghum bicolor. OsNPF4.5 is exclusively expressed in the cells containing arbuscules and displayed a low-affinity NO3−transport activity when expressed inXenopus laevisoocytes. Moreover, knockout ofOsNPF4.5resulted 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.