Agriculture Reference
In-Depth Information
biomass production GDHA overexpressors had more leaves and their free
amino acid content was higher, suggesting that both N metabolism and C
metabolism were modifi ed [164]. The transgenic tobacco plants were also
more tolerant to water stress.
In rice, overexpression of a gene of unknown function OsENOD93-1,
a N-responsive gene identifi ed following genome-wide gene expression
profi ling, led to an increase in grain yield, of 13-14% and 19-23% under
limiting and non-limiting N nutrition conditions respectively [165]. When
a gene encoding NAD(H)-dependent GOGAT from alfalfa was constitu-
tively expressed in tobacco, a signifi cant increase in biomass production
was observed [166]. Overexpression of the native NAD(H)-dependent
GOGAT in rice led to an increase in grain weight [167,168]. These results
suggest that the GOGAT enzyme plays a major role with respect to organic
N management and is used either for growth or for grain production de-
pending on the species examined.
There are fewer studies in which the importance of regulatory genes
has been clearly demonstrated [169]. When a Dof1 gene encoding a tran-
scription factor from maize was overexpressed in Arabidopsis ( Arabidop-
sis thaliana L.), an increase in amino acid content and of N uptake was
observed, especially when plants were grown at a low level of N supply. In
addition, the transgenic plants produced more biomass under low N supply
and they did not exhibit symptoms of N defi ciency in comparison to the
untransformed control plants, which developed much earlier symptoms of
senescence. When the Dof 1 gene was overexpressed in potato, transgenic
plants accumulated more amino acids especially glutamine and glutamate
[169]. These two sets of experiments suggest that this gene could be used
to improve the uptake and utilization of N in several species. Thus, over-
expressing regulatory genes rather than structural genes, such as genes
encoding GS, GOGAT or AlaAT appears to be an interesting alternative to
improve plant NUE and overall plant growth and development in a more
stable and balanced way across species.
When vegetable crops such as lettuce or spinach are grown under green-
house conditions they can accumulate substantial amounts of nitrate in the
leaf cell vacuoles. The threshold of nitrate accumulation often exceeds the
limits permitted by law, even when N fertilization is reduced because min-
eralization of soil organic matter always provides a surplus of nitrate to the
 
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