While nitrate reductase (NIA) reduces nitrate to nitrite, nitrite reductase (NIR),
glutamine synthetase (GST), and glutamate synthase (GOGAT) help in further
assimilation of N. Glutamate dehydrogenase (GDH), aspartate aminotransferase
(AspAT), and asparagine synthetase (AS) are enzymes involved in assimilation of
N and amino acid synthesis (Cheng et al. 1991; Li et al. 2017c; Bernard and Habash
2009; Wong et al. 2004).
NIN-like protein 7 (NLP7), a transcription factor belonging to RWP-RK family,
is regulated through availability of nitrate, via a nuclear retention mechanism to
further control downstream genes involved in N signaling, assimilation, and metabolism (Marchive et al. 2013). NLP7 has been identified as the major critical regulator
and can induce and repress downstream targets. It works upstream of NRT1;1 in the
presence of ammonia, and independent of NRT1.1 in its absence (Zhao et al. 2018).
Another gene NLA which encodes RING-type ubiquitin E3 ligase is known to play
role in adaptive response to N limitation via anthocyanin accumulation (Peng et al.
2007). Expression of several other transcription factors such as ZmDof1 in wheat,
OsRDD1 in rice, HY5 and bZIP in A. thaliana has been deciphered to play role in
Nitrogen uptake (Zhao et al. 2018). Members of protein kinases such as Ca
2+ CIPK,
CPK also play role in inducing primary nitrogen responsive genes (Riveras et al.
2015).
5.3.2 Phosphorus
Phosphate in soil is present in multitude of forms—organic, inorganic, and mineral
P; mostly immobile, majority of P is, therefore, unavailable for uptake by plant roots.
The inorganic form of Phosphorus, Pi, is the most accessible form for plants and
generally ranges up to 10 μM in soil. The forms of Pi available for uptake are
H 2 PO 4
À and HPO 4
2À which are dissociation products of H 3 PO 4 (Bieleski 1973;
Schachtman et al. 1998). H 3 PO 4 disassociates, in a pH dependent manner, first into
the monovalent form H 2 PO 4
À
, which then further disassociates into the divalent
form HPO 4
2À
. Studies have shown that most phosphate uptake occurs between
pH5.0 and 6.0 where H 2 PO 4
À is the predominant monovalent ionic form present.
In addition to molecular components of the host plant, arbuscular mycorrhizal
associations are known to play critical roles in mobilization of phosphate from
outside the rhizosphere, thus facilitating acquisition and uptake, an aspect that is
currently beyond the purview of this chapter (Bucher 2007).
The uptake of Phosphate is performed by transporters that have been grouped in
four distinct families—PHT1, PHT 2, PHT 3, and PHT4. Members of PHT1 are high
affinity transporters and belong to phosphate:H
+ symporter (PHS) family and have
conserved function (Muchhal et al. 1996; Smith et al. 1997; Pao et al. 1998; Hasan
et al. 2016; Gho and Jung 2019). PHT1 was initially identified as high affinity
transporter but later some members such as PHT1;2 in O. sativa (OsPT2) and
PHT1;6 in H. vulgare (HORvu-Pht1;6) were found to be low affinity phosphate
transporters. In both these plants, the low affinity transporters from PHT1 family are
suggested to be responsible for translocation and remobilization of stored Pi, rather
146
E. Bhardwaj et al.
glutamine synthetase (GST), and glutamate synthase (GOGAT) help in further
assimilation of N. Glutamate dehydrogenase (GDH), aspartate aminotransferase
(AspAT), and asparagine synthetase (AS) are enzymes involved in assimilation of
N and amino acid synthesis (Cheng et al. 1991; Li et al. 2017c; Bernard and Habash
2009; Wong et al. 2004).
NIN-like protein 7 (NLP7), a transcription factor belonging to RWP-RK family,
is regulated through availability of nitrate, via a nuclear retention mechanism to
further control downstream genes involved in N signaling, assimilation, and metabolism (Marchive et al. 2013). NLP7 has been identified as the major critical regulator
and can induce and repress downstream targets. It works upstream of NRT1;1 in the
presence of ammonia, and independent of NRT1.1 in its absence (Zhao et al. 2018).
Another gene NLA which encodes RING-type ubiquitin E3 ligase is known to play
role in adaptive response to N limitation via anthocyanin accumulation (Peng et al.
2007). Expression of several other transcription factors such as ZmDof1 in wheat,
OsRDD1 in rice, HY5 and bZIP in A. thaliana has been deciphered to play role in
Nitrogen uptake (Zhao et al. 2018). Members of protein kinases such as Ca
2+ CIPK,
CPK also play role in inducing primary nitrogen responsive genes (Riveras et al.
2015).
5.3.2 Phosphorus
Phosphate in soil is present in multitude of forms—organic, inorganic, and mineral
P; mostly immobile, majority of P is, therefore, unavailable for uptake by plant roots.
The inorganic form of Phosphorus, Pi, is the most accessible form for plants and
generally ranges up to 10 μM in soil. The forms of Pi available for uptake are
H 2 PO 4
À and HPO 4
2À which are dissociation products of H 3 PO 4 (Bieleski 1973;
Schachtman et al. 1998). H 3 PO 4 disassociates, in a pH dependent manner, first into
the monovalent form H 2 PO 4
À
, which then further disassociates into the divalent
form HPO 4
2À
. Studies have shown that most phosphate uptake occurs between
pH5.0 and 6.0 where H 2 PO 4
À is the predominant monovalent ionic form present.
In addition to molecular components of the host plant, arbuscular mycorrhizal
associations are known to play critical roles in mobilization of phosphate from
outside the rhizosphere, thus facilitating acquisition and uptake, an aspect that is
currently beyond the purview of this chapter (Bucher 2007).
The uptake of Phosphate is performed by transporters that have been grouped in
four distinct families—PHT1, PHT 2, PHT 3, and PHT4. Members of PHT1 are high
affinity transporters and belong to phosphate:H
+ symporter (PHS) family and have
conserved function (Muchhal et al. 1996; Smith et al. 1997; Pao et al. 1998; Hasan
et al. 2016; Gho and Jung 2019). PHT1 was initially identified as high affinity
transporter but later some members such as PHT1;2 in O. sativa (OsPT2) and
PHT1;6 in H. vulgare (HORvu-Pht1;6) were found to be low affinity phosphate
transporters. In both these plants, the low affinity transporters from PHT1 family are
suggested to be responsible for translocation and remobilization of stored Pi, rather
146
E. Bhardwaj et al.