Targeting Nitrogen Metabolism and Transport Processes to Improve Plant Nitrogen Use Efficiency

Abstract

In agricultural cropping systems, relatively large amounts of nitrogen (N) are applied for plant growth and development, and to achieve high yields. However, with increasing N application, plant N use efficiency generally decreases, which results in losses of N into the environment and subsequently detrimental consequences for both ecosystems and human health. A strategy for reducing N input and environmental losses while maintaining or increasing plant performance is the development of crops that effectively obtain, distribute, and utilize the available N. Generally, N is acquired from the soil in the inorganic forms of nitrate or ammonium and assimilated in roots or leaves as amino acids. The amino acids may be used within the source organs, but they are also the principal N compounds transported from source to sink in support of metabolism and growth. N uptake, synthesis of amino acids, and their partitioning within sources and toward sinks, as well as N utilization within sinks represent potential bottlenecks in the effective use of N for vegetative and reproductive growth. This review addresses recent discoveries in N metabolism and transport and their relevance for improving N use efficiency under high and low N conditions.

Keywords: amino acid partitioning; crop improvement; nitrogen assimilation; nitrogen uptake and transport; nitrogen use efficiency; seed yield and quality; source and sink physiology; sustainable agriculture.

Figure 1

Importance of root N transport and metabolic processes, and xylem-phloem exchange along the transport pathway for plant biomass and yield production, and N use efficiency. Shown are ammonium (${\text{NH}}_4^{+}$), nitrate (${\text{NO}}_3^{-}$), and amino acid (AA) transporters, and AA synthesis genes that were successfully overexpressed (UPPERCASE) or repressed/knocked out (lowercase and underlined) and resulted in increased biomass (B), seed yield (Y), seed protein (SP), and/or plant N use efficiency (NUE) at varying N supply. As indicated by framed white, green, purple, or blue backgrounds, the transgenic strategies used either (1) tissue-specific gene manipulation, (2) concurrent expression of a particular gene in both root and shoot tissues (OsNRT1.1A, Wang et al., ; OsNRT1.1B, Hu et al., ; OsNRT2.1, Chen et al., 2016), (3) co-expression of two genes in root and shoot tissues (OsAMT1;2 and OsGOGAT1, Lee et al., 2020a), or (4) constitutive gene expression throughout the plant. Positive results were achieved with (a) native expression of ammonium transporters (OsAMT1-1, Ranathunge et al., ; OsAMT1;2, Lee et al., 2020a), nitrate transporters (OsNRT1.1A, Wang et al., ; OsNRT1.1B, Hu et al., ; OsNRT2.1, Chen et al., ; OsNRT2.3b, Fan et al., ; OsNPF7.3, Fang et al., ; OsNPF8.20, Fang et al., 2013), and amino acid transporters (OsAAP1, Ji et al., ; OsAAP6, Peng et al., ; GmAAP6a, Liu et al., 2020) in rice (Os), Arabidopsis (At), and soybean (Gm), (b) expression of a barley alanine aminotransferase (HvAlaAT) in canola (Good et al., 2007), rice (Shrawat et al., ; Beatty et al., 2013), sugarcane (Snyman et al., 2015), and wheat (Peña et al., 2017), and (c) by knocking out native/endogenous genes (Ataap2, Perchlik and Tegeder, ; Osaap3, Lu et al., ; Osaap5, Wang et al., 2019). Plants were grown under high (HN), moderate (MN), sufficient (SN; N supply was not specified) and low N (LN), or without N (-N). Pathways of N uptake and xylem-phloem transfer (arrows with circle), as well as N assimilation into glutamate (Glu) and glutamine (Gln), and the subsequent synthesis of other AA and their loading into the xylem are indicated. ${\text{NH}}_4^{+}$ may be transported in the xylem to the shoot, but in relatively low amounts (indicated by brackets). Arrows in xylem and phloem indicate the direction of translocation. For additional information see manuscript text.

Figure 2

Role of leaf N (re)assimilation and remobilization, amino acid (AA) synthesis and N transport processes in plant biomass, yield production, and N use efficiency. Genes expressed in mature, photosynthetically active leaves, and senescing leaves, respectively, were analyzed. Shown are genes that were successfully overexpressed (UPPERCASE) or repressed/knocked out (lowercase and underlined) and resulted in increased biomass (B), seed yield (Y), seed protein (SP), and/or plant N use efficiency (NUE) at varying N supply. As indicated by framed white, green, purple, or blue backgrounds, the transgenic strategies either used (1) tissue-specific gene manipulation, (2) concurrent expression of a particular gene in different source or sink tissues, (3) co-expression of two genes in root and shoot tissues (OsAMT1;2 and OsGOGAT1, Lee et al., 2020a), or (4) constitutive gene expression throughout the plant. Plants were grown under high (HN), moderate (MN), sufficient (SN; N supply was not specified), or low N (LN). For native/endogenous gene expression only gene names are provided, while the expression of non-native genes is indicated by the gene name and the common name of the transgenic plant species. Positive results were achieved for genes of a nitrate reductase (OsNR2, Gao et al., 2019b), glutamine synthetases (NtGS1, Oliveira et al., ; ZmGln1-3, Martin et al., ; SbGS1, Urriola and Rathore, ; bacterial GS I, Zhu et al., ; DvGS2, Zhu et al., 2014), glutamate synthases (MsNADH-GOGAT, Chichkova et al., ; OsGOGAT1, Lee et al., 2020a), asparagine synthetases (OsASN1, Lee et al., ; E. coli EcasnA, Seiffert et al., 2004), nitrate (OsNRT1.1A, Wang et al., ; (At)NC4N, a synthetic gene construct based on Arabidopsis gene sequences, Chen et al., 2020), and amino acid transporters (PsAAP1, Perchlik and Tegeder, 2017) as well as for glutamate dehydrogenases (Trichurus sp. TrGDH, Du et al., ; C. ehrenbergii CeGDH, Zhou et al., ; P. cystidiosus PcGDH, Zhou et al., ; E. cheralieri EcGDH, Tang et al., ; E. coli EcgdhA, Ameziane et al., ; Lightfoot et al., ; Aspergillus nidulans AngdhA, Egami et al., 2012) and autophagy-related proteins (AtATG8, Chen et al., ; OsATG8a, Yu et al., ; OsATG8b, Zhen et al., , Fan et al., ; OsATG8c, Zhen et al., 2019b). Pathways of N (re)assimilation, AA synthesis and conversion (AA₍₁₎ to AA₍₂₎), organelle and N metabolite degradation, as well as N phloem loading and xylem-phloem exchange (arrows with circle) are shown. Arrows in xylem and phloem indicate possible directions of translocation. For additional information see manuscript text.

Figure 3

Importance of seed N transport and metabolic processes for seed N accumulation, seed yield, and N use efficiency (NUE). Shown are genes that were successfully overexpressed and resulted in increases in seed levels of specific amino acids (AA), total soluble protein (SP), and specific high-quality proteins (QP), and positively affected shoot biomass (B), seed yield (Y), or NUE. As indicated by a framed white or green background, the transgenic strategies either used (1) seed-specific manipulation or (2) concurrent expression of a particular gene in different source or sink tissues. Plants were grown under high (HN), moderate (MN), sufficient (SN; N supply was not specified), and low N (LN). Pathways of N (re)assimilation, AA synthesis, and conversion (AA₍₁₎ to AA₍₂₎) in seed coat and embryo as well as N import into the embryo (arrows with circle) and subsequent protein synthesis are shown. Arrows in xylem and phloem indicate the direction of translocation. Positive results were achieved for embryo expression of genes encoding for AA transporters (VfAAP1, Rolletschek et al., ; Weigelt et al., ; PsAAP1, Perchlik and Tegeder, 2017), and genes related to synthesis of sulfur-rich (AtCGS, Hanafy et al., ; Song et al., ; Cohen et al., , ; AtSAT, Tabe et al., 2010) and other essential AA (EcdapA, Karachi et al., ; Zhu and Galili, ; Angelovici et al., ; CordapA, Falco et al., ; Huang et al., ; Frizzi et al., ; EcLysC, Karachi et al., , ; XbAK, Qi et al., ; OsOASA1D, Kita et al., 2010) as well as seed storage protein synthesis (BeBNA, Altenbach and Simpson, ; Altenbach et al., ; Pickardt et al., ; Saalbach et al., ; Demidov et al., ; HaSSA, Molvig et al., ; Tabe and Droux, ; Hagan et al., ; Chiaiese et al., ; SiS2SA, Lee et al., , ; At2S1, De Clercq et al., ; ZmZEIN, Dinkins et al., ; Zmγ-zein, Li et al., ; Zmβ-zein. Guo et al., ; GmA1aB1b, Takaiwa et al., ; Katsube et al., ; Momma et al., ; AhAmarantin, Rascón-Cruz et al., ; PtLRP, Liu et al., ; GhLRP, Yue et al., ; Sb401, Yu et al., ; SbgLR, Wang et al., ; OsBiP, Yasuda et al., ; Kawakatsu et al., ; OsRLRH1 and OsRLRH2, Wong et al., ; Sdα-LA, Bicar et al., 2008). Names of genes that were overexpressed are provided, however in contrast to Figures 1, 2, the transgenic plant species are not indicated as the respective genes have often been overexpressed in multiple species. See the manuscript text or listed references for more detailed information.