Poplar trees for phytoremediation of high levels of nitrate and applications in bioenergy

Abstract

The utilization of high amounts of nitrate fertilizers for crop yield leads to nitrate pollution of ground and surface waters. In this study, we report the assimilation and utilization of nitrate luxuriant levels, 20 times more than the highest N fertilizer application in Europe, by transgenic poplars overexpressing a cytosolic glutamine synthetase (GS1). In comparison with the wild-type controls, transgenic plants grown under high N levels exhibited increased biomass (171.6%) and accumulated higher levels of proteins, chlorophylls and total sugars such as glucose, fructose and sucrose. These plants also exhibited greater nitrogen-use efficiency particularly in young leaves, suggesting that they are able to translocate most of the resources to the above-ground part of the plant to produce biomass. The transgenic poplar transcriptome was greatly affected in response to N availability with 1237 genes differentially regulated in high N, while only 632 genes were differentially expressed in untransformed plants. Many of these genes are essential in the adaptation and response against N excess and include those involved in photosynthesis, cell wall formation and phenylpropanoid biosynthesis. Cellulose production in the transgenic plants was fivefold higher than in control plants, indicating that transgenic poplars represent a potential feedstock for applications in bioenergy. In conclusion, our results show that GS transgenic poplars can be used not only for improving growth and biomass production but also as an important resource for potential phytoremediation of nitrate pollution.

Keywords: Populus; bioenergy; biomass; glutamine synthetase; nitrate pollution; transgenic trees.

Figure 1

Phenotypes of 3‐month‐old untransformed controls (WT) and transgenic poplars irrigated with a solution containing adequate (10 mm) or high (50 mm) nitrate concentration.

Figure 2

Spatial distribution of total N, total C, NutE and NupE in WT and transgenic poplars growing at different nitrate levels. Open bars correspond to poplars irrigated with a solution containing 10 mm of nitrate. Closed bars correspond to poplars irrigated with a solution containing 50 mm of nitrate. Values are means ± SD of three independent plant samples. Different letters indicate significant differences between samples at P < 0.001. The same notation is applicable to legends of Figures 3, 7 and 8.

Figure 3

Spatial distribution of protein and chlorophyll contents in WT and transgenic poplars growing at different nitrate levels.

Figure 4

Changes in the transcriptome of WT and transgenic poplars grown at different nitrate levels. Venn diagram depicting the overlap between genes differentially expressed in 10 mm vs 50 mm WT, 10 mm vs 50 mm Transgenics, 50 mm WT vs 50 mm Transgenics and 10 mm WT vs 10 mm Transgenics.

Figure 5

Mapman representation of a metabolism response overview. (a) Comparison of WT plants growing at 10 and 50 mm nitrate. (b) Comparison of transgenic plants growing at 10 and 50 mm nitrate. Blue scale indicates up‐regulated genes at 50 mm. Red scale indicates down‐regulated genes at 50 mm.

Figure 6

Mapman representation of a stress response overview. (a) Comparison of WT plants growing at 10 and 50 mm nitrate. (b) Comparison of transgenic plants growing at 10 and 50 mm nitrate. Blue scale indicates up‐regulated genes at 50 mm. Red scale indicates down‐regulated genes at 50 mm.

Figure 7

Spatial distribution of GS transcripts in WT and transgenic poplars growing at different nitrate levels.

Figure 8

Spatial distribution of carbohydrates and lignin in WT and transgenic poplars growing at different nitrate levels.