The assimilation of ammonium into organic nitrogen catalyzed by the enzyme
glutamine synthetase (GS; EC 6.3.1.2) has been suggested to be the limiting
step for plant nitrogen utilization (H-M. Lam et al. 1995, Plant Cell 7. 8
87-898). We have developed a molecular approach to increase glutamine produ
ction in transgenic poplar by the overexpression of a conifer GS gene. A ch
imeric construct consisting of the cauliflower mosaic virus 35S promoter fu
sed to pine cytosolic GS cDNA and nopaline synthetase polyadenylation regio
n was transferred into pBin19 for transformation of a hybrid poplar clone (
INRA 7171-B4, Populus tremula x P. alba) via Agrobacterium tumefaciens. Tra
nsformed poplar lines were selected by their ability to grow on selective m
edium containing kanamycin. The presence of the introduced gene in the popl
ar genome was verified by Southern blotting and polymerase chain reaction a
nalysis. Transgene expression was detected in all selected poplar lines at
the mRNA level. The detection of the corresponding polypeptide (41 kDa) and
increased GS activity in the transgenics suggest that pine transcripts are
correctly processed by the angiosperm translational machinery and that GSI
subunits are assembled in functional holoenzymes. Expression of the pine G
S1 gene in poplar was associated with an increase in the levels of total so
luble protein and an increase in chlorophyll content in leaves of transform
ed trees. Furthermore, the mean net growth in height of GS-overexpressing c
lones was significantly greater than that of non-transformed controls, rang
ing from a 76% increase in height at 2 months to a 21.3% increase at 6 mont
hs. Our results suggest that the efficiency of nitrogen utilization may be
engineered in trees by genetic manipulation of glutamine biosynthesis.