Agriculture Reference
In-Depth Information
plant [170]. In human food, when nitrate is absorbed in excess, its reduc-
tion to nitrite during digestion can oxidize hemoglobin, causing a kind of
anemia. Moreover, nitrites can be converted to carcinogenic nitrosamines
[12,13]. Conventional methods of selection have led to the development
of varieties able to reduce the absorbed nitrate more effi ciently instead of
storing it, but these varieties are not able to completely eliminate any risk
of toxic accumulation. Studies were therefore undertaken to limit nitrate
accumulation by increasing the capacity of a plant to reduce nitrate by
increasing nitrate reductase (NR) activity in genetically modifi ed plants,
by overexpressing a gene that allows the deregulation of the synthesis of
the enzyme [171]. In tobacco a 50% reduction in leaf nitrate content was
observed after introduction of the native structural NR gene (
Nia2
) placed
under the control of the 35S strong constitutive promoter. Using the same
approach, encouraging results were obtained in a variety of potato [172]
that showed a 95% decrease in the amount nitrate in the tubers. In another
variety of potato, the transgenic plants showed a marked improvement
in biomass production, especially in tubers, with still lower amounts of
nitrate. The more effective reduction of nitrate probably allowed a better
allocation of N to the photosynthetic apparatus and to enzymes involved in
C metabolism, which was demonstrated by higher leaf chlorophyll content
in the transgenic potato plants [173].
In lettuce transformed with the same 35S-
Nia2
construct, a problem of
post-transcriptional regulation of the NR enzyme was encountered [174].
The transgenic lettuce accumulated 21% less nitrate after 22 days. Howev-
er, the nitrate content was only 4% lower in 84 days-old transgenic plants.
The hypothesis that the strength of the 35S promoter decreases during
plant ageing was put forward, suggesting that a way to maintain NR activ-
ity at a high level regardless of plant age needs to be found. Such a strategy
to reduce the nitrate content in vegetable crops requires further research
before the use of the
Nia2
transgene can be effi ciently mastered.
Although we do not have any clear information from the private sector
about the recent development and commercialization of transgenic plants
modifi ed for NUE, it seems to be likely that crops overexpressing the en-
zymes AlaAT and GS1 will be commercially released within the next fi ve
years, following extensive validation of their function under different fi eld
trial conditions and using different genetic backgrounds.
