Nitrate Transporter Gene Expression and Kinetics of Nitrate Uptake by Populus × canadensis 'Neva' in Relation to Arbuscular Mycorrhizal Fungi and Nitrogen Availability.

Fei Wu,Ming Tang,Na Wu,Fengru Fang,Li Li

doi:10.3389/fmicb.2020.00176

Abstract

Plants and other organisms in the ecosystem compete for the limited nitrogen (N) in the soil. Formation of a symbiotic relationship with arbuscular mycorrhizal fungi (AMF) may influence plant competitiveness for N. However, the effects of AMF on plant nitrate (NO3–) uptake capacity remain unknown. In this study, a pot experiment was conducted to investigate the effects of N application and Rhizophagus irregularis inoculation on the root absorbing area, uptake kinetics of NO3–, and the expression of NO3– transporter (NRT) genes in Populus × canadensis ‘Neva’. The results showed that R. irregularis colonized more than 70% of the roots of the poplar and increased root active absorbing area/total absorbing area. The uptake kinetics of NO3– by poplar fitted the Michaelis–Menten equation. Mycorrhizal plants had a higher maximum uptake rate (Vmax) value than non-mycorrhizal plants, indicating that R. irregularis enhanced the NO3– uptake capacity of poplar. The expression of NRTs in roots, namely, NRT1;2, NRT2;4B, NRT2;4C, NRT3;1A, NRT3;1B, and NRT3;1C, was decreased by R. irregularis under conditions of 0 and 1 mM NH4NO3. This study demonstrated that the improved NO3– uptake capacity by R. irregularis was not achieved by up-regulating the expression of NRTs in roots. The mycorrhizal pathway might repress root direct pathway in the NO3– uptake by mycorrhizal plants.

Highlights

Nitrogen (N) is an essential component of nucleic acids, proteins, and many other critical biomolecules of living organisms (Castro-Rodríguez et al, 2017)
The NO3− uptake rate is mainly determined by NO3− uptake kinetics, which can be modeled by the Michaelis–Menten equation, which denotes the absorption rate of NO3− as a function of external NO3− concentration (McMurtrie and Näsholm, 2017)
arbuscular mycorrhizal (AM) colonization structures were observed in more than 70% of the poplar roots after inoculation with R. irregularis, whereas no AM structure was observed in NM plants (Figure 1)

Summary

Introduction

Nitrogen (N) is an essential component of nucleic acids, proteins, and many other critical biomolecules of living organisms (Castro-Rodríguez et al, 2017). Microorganisms are considered to be more competitive than plants in regard to N uptake due to their unique properties, such as faster growth rates and AMF Influence Poplar Nitrate Uptake larger surface-area-to-volume ratios (Kuzyakov and Xu, 2013). Several different forms of N can be available to plant roots, such as ammonium (NH4+), nitrate (NO3−), and amino acids (Fan et al, 2017). The NO3− uptake rate is mainly determined by NO3− uptake kinetics, which can be modeled by the Michaelis–Menten equation, which denotes the absorption rate of NO3− as a function of external NO3− concentration (McMurtrie and Näsholm, 2017). Two parameters of Michaelis–Menten kinetics, the maximum uptake rate (Vmax) and the Michaelis constant (Km), can describe N absorption characteristics (York et al, 2016). The Vmax value is the maximum influx rate of NO3−, whereas the Km value is the external NO3− concentration when the absorption rate is half of Vmax and presents the affinity of transporters (York et al, 2016)

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