Agriculture Reference
In-Depth Information
did not exhibit higher NUE under N-limiting conditions compared to non-
limiting N conditions [160].
In other species, the overexpression of GS1 had a rather negative im-
pact on growth and yield of the plant. For example, overexpression of a
GS1gene from tobacco in the legume birds foot trefoil (
Lotus cornicula-
tus
L.) grown on nitrate led to an acceleration of senescence, which was
apparently detrimental to the overall plant developmental process [161].
When the transgenic
L. corniculatus
plants were grown under symbiotic
N-fi xing conditions an increase in plant biomass production was unex-
pectedly observed. However, the physiological mechanisms involved in
this increase remain unknown [86].
In rape (canola), the overexpression of a gene encoding the enzyme
alanine aminotransferase (AlaAT) from barley, directed by a rape root-
specifi c promoter, led to a dramatic increase in biomass production and
seed yield [162]. Improvement of plant productivity was only observed
under low N fertilization conditions and was attributed to a higher fl ux of
nitrate, associated or induced by a decrease in the content of glutamine and
glutamate in the stem. In the fi eld when the applied N fertilizer rate was
reduced by 40%, the agronomic performance of the transgenic rapeseed
plants overexpressing AlaAT was similar to that of untransformed control
plants grown under higher optimal N fertilizer rates.
Overexpression of the same gene in rice led to increased biomass pro-
duction and N content of stems [163]. Unlike in rapeseed, there was an
increase of glutamine and asparagine content both in the stems and in the
roots. The genetically modifi ed rice plants had a fi ner, denser and more
branched root system, which was presumably more favorable for the ab-
sorption of N. This result indicates that genetic modifi cation targeted to
improve N utilization effi ciency also had an impact on plant development,
although the effect of AlaAT overexpression was variable from one spe-
cies to another in terms of both plant growth and metabolic activity.
There are a few other examples of successful genetic modifi cation of
N metabolism using either structural or putative regulatory genes. When
the bacterial enzyme glutamate dehydrogenase (GDH A) from
E. coli
was
constitutively overexpressed in tobacco, biomass production of the trans-
genic plants was increased by about 10-15%. In addition to the increase in
