Wang et al. 2020b). Seven members of CLC family and five of SLAC family are
present in Arabidopsis thaliana (Wang et al. 2012).
Members of the NRT1 family are known as Low Affinity Transporters (LATs),
whereas those of NRT2 are High Affinity Transporters (HATs). Members of the
NRT1 and NRT2 gene family have well-partitioned function in uptake, transport,
and assimilation. Some of these have as specific role in uptake or in transport,
whereas some have dual roles in uptake and transport. For example, the uptake of
NO 3
À from soil is mediated by two LATs—NRT1.1, NRT1.2, and three HATs—
NRT2.1, NRT2.2, and NRT2.4, along with other transporters such as NAXT1. Each
of these transporters is present in the several cell layers of root and facilitates the
uptake from soil to outermost epidermis, via cortex, endodermis, pericycle/parenchyma, and finally into the xylem. This process involves NRT1.1, NRT1.2, NRT2.1,
and NRT2.4 in epidermis, NRT1.1 and NRT2.1 in cortical cells, only NRT1.1 in
endodermis, and finally NRT1.5 in pericycle/parenchymatous cells that are immediately adjacent to xylem. The transport of NO 3À via xylem from root-to-shoot is
performed by LATs—NRT1.5, NRT1.8, and NRT1.9. Phloem loading of nitrate
occurs with the help of NRT1.9 and NRT1.7. Two transporters—NAXT1 (Nitrate
excretion transporter 1) and NRT1.8 are involved in efflux. Other than these,
NRT1.4, NRT1.7, and NRT1.6 are involved in transport of NO 3
À from shoot to
leaves, remobilization from old to new leaves, and transport to seed for storage,
respectively (Wang and Tsay 2011; Wang et al. 2012; Fan et al. 2009). Interestingly,
the transport of NO 3À mediated by NRT1.9 in xylem and phloem is negatively
correlated (Wang and Tsay 2011). The uptake of nitrate is tightly regulated and
induced by several factors including N-starvation, light, pH, sugar, auxin (Loqué
et al. 2003; Huang et al. 1996; Liu et al. 1999; Wang et al. 2012). Similar partitioning
of function of NRT transporters and induction by several factors have also been
observed in Oryza sativa (Cai et al. 2008; Fan et al. 2016; Hu et al. 2015; Wang et al.
2018). Other transporters that are involved in nitrate transport include amino acid
transporters (AAT) such as AlaAT (Shrawat et al. 2008) and AspAT (Schultz and
Coruzzi 1995).
Transport of ammonium (NH
4+ ) is driven by AMT/MEP/Rh (Ammonium Transporter/Methylammonium Permease/mammalian Rhesus proteins) family of transmembrane proteins which are found in organisms ranging from bacteria,
cyanobacteria to animals and plants. In plants, two sub-families of AMT, viz.
AMT1 and AMT2, both having high affinity for ammonium are found (Pantoja
2012). The first ammonium transporter, AMT1.1, was isolated from Arabidopsis
thaliana and subsequent analysis identified a total of six homologues of AMT from
A. thaliana (Ninnemann et al. 1994; Gazzarrini et al. 1999; Sohlenkamp et al. 2002).
Of the six AMT homologs in A. thaliana, AMT1.1 to AMT1.5 form the AMT family
and AMT2 is similar to MEP family; members of Rh protein family are limited to
mammal in distribution (Loqué and von Wirén 2004). Till date, ammonium
transporters have been characterized from several other plant species including
Populus (Couturier et al. 2007), Lotus japonicus (D’Apuzzo et al. 2004), Triticum
(Liu et al. 2015), Lycopersicon esculentum (Ludewig et al. 2002), Tomato (Von
Wirén et al. 2000), to name a few examples.
5 Plant Roots and Mineral Nutrition: An Overview of Molecular Basis of Uptake and. . . 145
present in Arabidopsis thaliana (Wang et al. 2012).
Members of the NRT1 family are known as Low Affinity Transporters (LATs),
whereas those of NRT2 are High Affinity Transporters (HATs). Members of the
NRT1 and NRT2 gene family have well-partitioned function in uptake, transport,
and assimilation. Some of these have as specific role in uptake or in transport,
whereas some have dual roles in uptake and transport. For example, the uptake of
NO 3
À from soil is mediated by two LATs—NRT1.1, NRT1.2, and three HATs—
NRT2.1, NRT2.2, and NRT2.4, along with other transporters such as NAXT1. Each
of these transporters is present in the several cell layers of root and facilitates the
uptake from soil to outermost epidermis, via cortex, endodermis, pericycle/parenchyma, and finally into the xylem. This process involves NRT1.1, NRT1.2, NRT2.1,
and NRT2.4 in epidermis, NRT1.1 and NRT2.1 in cortical cells, only NRT1.1 in
endodermis, and finally NRT1.5 in pericycle/parenchymatous cells that are immediately adjacent to xylem. The transport of NO 3À via xylem from root-to-shoot is
performed by LATs—NRT1.5, NRT1.8, and NRT1.9. Phloem loading of nitrate
occurs with the help of NRT1.9 and NRT1.7. Two transporters—NAXT1 (Nitrate
excretion transporter 1) and NRT1.8 are involved in efflux. Other than these,
NRT1.4, NRT1.7, and NRT1.6 are involved in transport of NO 3
À from shoot to
leaves, remobilization from old to new leaves, and transport to seed for storage,
respectively (Wang and Tsay 2011; Wang et al. 2012; Fan et al. 2009). Interestingly,
the transport of NO 3À mediated by NRT1.9 in xylem and phloem is negatively
correlated (Wang and Tsay 2011). The uptake of nitrate is tightly regulated and
induced by several factors including N-starvation, light, pH, sugar, auxin (Loqué
et al. 2003; Huang et al. 1996; Liu et al. 1999; Wang et al. 2012). Similar partitioning
of function of NRT transporters and induction by several factors have also been
observed in Oryza sativa (Cai et al. 2008; Fan et al. 2016; Hu et al. 2015; Wang et al.
2018). Other transporters that are involved in nitrate transport include amino acid
transporters (AAT) such as AlaAT (Shrawat et al. 2008) and AspAT (Schultz and
Coruzzi 1995).
Transport of ammonium (NH
4+ ) is driven by AMT/MEP/Rh (Ammonium Transporter/Methylammonium Permease/mammalian Rhesus proteins) family of transmembrane proteins which are found in organisms ranging from bacteria,
cyanobacteria to animals and plants. In plants, two sub-families of AMT, viz.
AMT1 and AMT2, both having high affinity for ammonium are found (Pantoja
2012). The first ammonium transporter, AMT1.1, was isolated from Arabidopsis
thaliana and subsequent analysis identified a total of six homologues of AMT from
A. thaliana (Ninnemann et al. 1994; Gazzarrini et al. 1999; Sohlenkamp et al. 2002).
Of the six AMT homologs in A. thaliana, AMT1.1 to AMT1.5 form the AMT family
and AMT2 is similar to MEP family; members of Rh protein family are limited to
mammal in distribution (Loqué and von Wirén 2004). Till date, ammonium
transporters have been characterized from several other plant species including
Populus (Couturier et al. 2007), Lotus japonicus (D’Apuzzo et al. 2004), Triticum
(Liu et al. 2015), Lycopersicon esculentum (Ludewig et al. 2002), Tomato (Von
Wirén et al. 2000), to name a few examples.
5 Plant Roots and Mineral Nutrition: An Overview of Molecular Basis of Uptake and. . . 145