Agriculture Reference
In-Depth Information
developing tissues or in storage organs to fi nally ensure plant vegetative
or sexual reproduction.
15.5 IMPROVEMENT OF NITROGEN UTILIZATION USING
GENETICALLY MODIFIED CROPS
Nitrate reduction is rarely limiting for optimal grain yield or biomass
production. In contrast, this is not the case for the ammonia assimilato-
ry pathway [153]. For example the work of Fuentes et al. [154] showed
that, in tobacco, overexpression of a gene encoding cytosolic glutamine
synthetase (GS1) from alfalfa, causes an increase in photosynthesis and
growth under a low N fertilization regime. These results suggest that the
transgenic tobacco plants overexpresing GS1 are able to utilize N more ef-
ficiently under N stress conditions. Interestingly, Oliveira et al. [155] also
showed that in tobacco, the overexpression of a gene encoding a pea GS1
lead to increased biomass production both under limiting and non-limiting
N feeding conditions.
By overexpressing a pine GS1 gene in poplar, Jing et al. [156] and Man
et al. [157] observed that the transgenic trees, which were older than fi ve
years exhibited a 41%, increase in growth rate, whereas the other pheno-
typic characteristics of the genetically modifi ed plants remained similar.
In wheat, the overexpression of a gene for GS1 from French bean led
to an increase in grain yield (grain weight in particular) and therefore
of NUE, which was estimated to be about 20% [158]. However, to our
knowledge there has been no further development of this interesting study,
either because of the diffi culty of fi eld testing in Europe or because this
testing is currently being performed in the private sector. Similar work
was conducted in maize consisting in the overexpression of a native gene
encoding GS1 (
Gln1-3
) of maize. Grain yield (mainly grain number) of
the maize transgenic plants grown under greenhouse conditions was in-
creased by about 30%. However, grain N content and biomass production
of the transgenic plants were not modifi ed at maturity [159]. More re-
cently, transgenic rice lines overexpressing GS1 showed improved harvest
index, N harvest index and N utilization effi ciency. However, these lines
