An alternative way to increase plant productivity through the use of nitrogen fertilizers is to improve the efficiency of nitrogen utilization via genetic engineering. The effects of overexpression of pine glutamine synthetase (GS) gene and nitrogen availability on growth and leaf pigment levels of two Betula species were studied. Untransformed and transgenic plants of downy birch (B. pubescens) and silver birch (B. pendula) were grown under open-air conditions at three nitrogen regimes (0, 1, or 10 mM) for one growing season. The transfer of the GS1a gene led to a significant increase in the height of only two transgenic lines of nine B. pubescens, but three of five B. pendula transgenic lines were higher than the controls. In general, nitrogen supply reduced the positive effect of the GS gene on the growth of transgenic birch plants. No differences in leaf pigment levels between control and transgenic plants were found. Nitrogen fertilization increased leaf chlorophyll content in untransformed plants but its effect on most of the transgenic lines was insignificant. The results suggest that birch plants carrying the GS gene use nitrogen more efficiently, especially when growing in nitrogen deficient soil. Transgenic lines were less responsive to nitrogen supply in comparison to wild-type plants.
Changing expression of a single gene by introducing mutations or transformation often affects expression of other genes, which results in the modification of the plant phenotype. We have obtained aspen plants with reduced expression levels of 4CL gene (4-coumarate-CoA ligase). Change in qualitative composition and 11-23% reduced lignin content were observed in the wood of the greenhouse plants. In the study of rhizogenesis in vitro it was found that the micro shoots of transgenic plants formed a greater number of adventive roots than in the control, 30%. Studying the transgenic aspen plants grown in a greenhouse (3 months) and passed the semi-natural conditions trial (4 months) has revealed 5-10% reduction in the lignin content, alteration of the phenotype – decrease in biometrical values (height and diameter of stem). Analysis of RNA from the plants grown under semi-natural conditions indicated changed expression levels of monolignol biosynthesis genes, 4CL, CCR1 (cinnamoyl-CoA reductase), CAD6 (cinnamyl-alcohol dehydrogenase), and CCoAOMT (caffeoyl-CoA O-methyltransferase). For the lines PtXVI4CL9a and PtXIII4CL2c grown under semi-natural conditions, weight of the root system was 1.5-2 fold reduced in the transgenic plants as compared to the control. The conductive: feeding root ratio was increased. Morphology of the root system was changed.
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