PNAS：功能分析OsNPF4.5揭示菌根真菌介导的植物氮素吸收

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 Rhizophagus irregularis remarkably 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 gene OsNPF4.5 in rice roots, and its orthologs ZmNPF4.5 in Zea mays and SbNPF4.5 in Sorghum bicolor. OsNPF4.5 is exclusively expressed in the cells containing arbuscules and displayed a low-affinity NO₃⁻ transport activity when expressed in Xenopus laevis oocytes. Moreover, knockout of OsNPF4.5 resulted in a 45% decrease in symbiotic Nuptake 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.

arbuscular mycorrhiza | RNA sequencing | nitrate transporter | nitrogen uptake | OsNPF4.5

A model for N uptake, assimilation, and translocation in AM symbiosis. AM fungi can take up both NH₄⁺ and NO₃⁻, as well as organic N forms, such as amino acids (AAs) and small peptides (SPs), from soil solution via their extraradical mycelium (ERM). The NH₄⁺ in fungal cytoplasm can be rapidly assimilated into amino acids, mainly arginine, via the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway and translocated, probably coupled with Poly-P through the intraradical hyphae. After hydrolysis in the arbuscule, NH₄⁺ is exported from the AM fungus to the periarbuscular space (PAS) and subsequently imported, probably in the form of NH₃, into the root cell by the putative plant NH₄⁺ transporters located on the periarbuscular membrane (PAM). The NO₃⁻ absorbed by extraradical mycelium can be directly translocated into intraradical hyphae and released into the interfacial apoplast. The import of NO₃⁻ into root cell is mediated by the PAM-localized NO₃⁻ transporters, such as OsNPF4.5. NR, nitrate reductase; NiR, nitrite reductase; GS, glutamine synthetase; GOGAT, glutamate synthase; AMT, ammonium transporter, AAP, amino acid permease. Question marks and dotted lines indicate that the putative transporters or transport/metabolic processes have not yet been established.

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