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 352

decrease in enzyme activity levels. This may be because of post-translational modi-
fications (Shigeto et al. 2006 ). Till now there is no confirmations fromNiR overex-
pression in terms of improvement in NUE.
Hirel et al. ( 2005 ) signified glutamine synthetase (GS) involvement in kernel
production of maize, through QTLs for studying the GS activity in leaf showed
that the GS activity to coincide with QTLs for yield. One QTL was concurrent for
thousand kernels weight with a GS (Gln1-4) locus, and two QTLs conceded for
GS (Gln1-3) locus of thousand kernel weight and yield. This shows the positive
association between kernel yield and GS activity.In another two experiments by
Li et al. 1993 and Martin et al. 2006 identified the two cytosolic GS isoenzymes
(GS1) in maize, and their molecular and physiological properties were examined
using knockout mutation on kernel yield, thereby examining the plants grown un-
der N deficient conditions.Martin et al. 2006 observed the over expressing trans-
genic lines of Gln1-3 in leaves showed rise in kernel number, proving that the
GS1-3 isoenzyme plays an important role in regulating kernel yield under optimal
N-fertilization conditions. Yanagisawa et al. 2004 ; Coque and Gallais 2006 ; Hirel
et al. 2007 also observed the GS mutants and the GS-overexpressing lines that were
grown under N-limiting conditions and observed the reduction inkernel number
when compared to wild type. Huang et al. ( 2005 ) carried out experiments in wheat
by adding an extra GS gene and observed no overall increase in the amount of GS.
Many other studies in various crop plants like tobacco and rice employed the role
of GS and enhanced N-assimilation efficiency (Oliveria et al. 2002 ; Man 2005 ; Sun
et al. 2005 ; Cai et al. 2009 )
Transgenic over-expression and antisense technologies in alfalfa and rice plants
were engaged to alter the expression of NADH-GOGAT (Yamaya et al. 2002 ). The
studies highlighting the antisense RNA for NADH-GOGAT carried out in transgen-
ic rice plants throws some light towards the possible improvement of the transport
of N via phloem in senescing leaves. Andrews et al. 2004 , studied the expression
of NADH-GOGAT in initial growing leaf blades and spikelets, also showed the of
glutamine transported from senescing organs. Tabuchi et al. 2007 showed changes
in various nitrogenous metabolites and decreased leaf protein, rubisco activity and
nitrate contents in Barley mutants having reduced Fd-GOGAT. Genes does emerge
to be good candidates that can be employed for improving NUE, but the use de-
pends on crop and cropping conditions (Shrawat and Good 2008 ).
Improving NUE Through Manipulation of Signalling
Targets
The failure to improve NUE through over expression of candidate genes in trans-
genic plants involved in nitrate and ammonia assimilation concluded that that meta-
bolic flux of these pathways are being controlled by regulatory switches outside
these pathways. Yet, the exact mechanism involved in nitrate signalling still needs