From Agronomy to Molecular Genetics and Proteomics in an Effort to Improve Nitrogen Use Efficiency in Crops - Crop Improvement: New Approaches and Modern Techniques - page 346

adding NH 4 + to the medium (Pahlich 1996 ). These factors also lead to higher levels
of GDH protein in maize root tissues (Oaks 1994 ). A primary role of GDH is known
to replenish TCA cycle intermediates through their oxidation to 2-oxoglutarate.
However, in vivo, aspartate amino transferase and glutamate decarboxylase also
maintain the supply of carbon to the citric acid cycle (Kisaka et al. 2007 ). Glutamate
is deaminated to 2-oxoglutarate in isolated mitochondria; however in the presence
of amino-oxy acetate, glutamate no longer contributes to mitochondrial respiration
(Sechley et al. 1992 ). This observation indicates that GDH does not oxidize gluta-
mate. Thus, the correct in vivo role of GDH in nitrogen metabolism of higher plants
remains to be defined (Pahlich 1996 ).
Acquisition of N by Roots and Its Regulation
The nitrate uptake is the foremost step that adds to N use of any plant. The identi-
fication of genes and the proteins responsible for NO 3 - transport and distribution is
required for enhanced understanding of the mechanisms that takes place within the
plant. Nitrate transport is a proton-symport type of transporter system (Crawford
1995 ). Nitrate uptake and its release into cells are mediated by nitrate transporter(s)
located into the plasma membrane of the root. Three transporters have been identi-
fied by kinetic measurements in plant roots (Remans et al. 2006 ). These are consti-
tutive High Affinity Nitrate Transporter (cHATS), inducible High Affinity Nitrate
Transporter (iHATS) and Low Affinity Nitrate Transporter (LATS) (Okamoto et al.
2006 ; Chandna et al. 2011 ). Two of these display saturable kinetics; a low capacity
constitutive system and a high capacity inducible system (Okamoto et al. 2006 ). In
addition, a non-saturating low affinity, high capacity system becomes apparent only
at higher external NO 3 - concentration (Kronzucker et al. 1995 ; Cerezo et al. 2000 ,
2001 ). The high affinity transport system (HATS) works at low concentrations
(1 µM-1 mM). HATS transporters are constitutively expressed (cHATS) as well as
nitrate-inducible (iHATS), and are subjected to negative feedback regulation by the
products of nitrate assimilation. Both types of HATS happen to active during low N
(< 1 mM) concentration in the medium, they show up-regulation on the availability
of nitrate. Thus, constitutive HATS seems to offer a high affinity and low capacity
passage for nitrate entry in un-induced plants, a 3-fold increase in their expression
is observed on introduction of nitrate (Crawford and Glass 1998 ). Inducible HATS
are known to be induced on presence of nitrate or nitrite (Orsel et al. 2006 ). The low
affinity transport system (LATS) is known to work under high external nitrate con-
centrations i.e: > 1 mM (Glass 2003 ). NO 3 - uptake appears to increase linearly with
increasing NO 3 - concentration with no indication of saturation even at 100 mM
(Omata et al. 1989 ). The linear concentration dependence of the LATS has been
observed in a wide variety of organisms. LATS for NO 3 - in barley are referred to
as constitutive (Kronzucker et al. 1995 ). LATS which are constitutive, perhaps also
have a signalling function to play in induction of HATS and other nitrate assimila-
tory genes, which might plays a nutritional role when above a certain threshold.