Repercussion of mesophyll-specific overexpression of a soybean cytosolic glutamine synthetase gene in alfalfa (Medicago sativa L.) and tobacco (Nicotiana tabaccum L.)

Glutamine synthetase (GS) plays a central role in plant nitrogen metabolism. Plant GS occurs as a number of isoenzymes present in either the cytosol (GS₁) or chloroplast/plastid (GS₂). There are several reports of improved performance in transgenic plants overexpressing GS₁ transgenes driven by the constitutive CaMV35S promoter. Improvement has been attributed to the GS₁ transgene product functioning to enhance re-assimilation of NH₄⁺ released by photorespiration or protein degradation. In this paper, alfalfa and tobacco transformants expressing a soybean gene driven by a photosynthetic cell-specific promoter have been compared to transformants with the same transgene driven by the stronger CaMV35S promoter. The two classes of alfalfa and tobacco transformants showed differences in the level of GS₁ transcript and GS₁ protein accumulation, but the difference in the total GS activity was small. The discrepancy in the transgene expression level and GS activity has been attributed to posttranslational regulation at the level of holoprotein stability. Both classes of transformants exhibited similar level of improvement in soluble protein and in the rates of photosynthesis and photorespiration. The data supports the hypothesis that GS₁ made in the mesophyll cells is involved in the re-assimilation of NH₄⁺ released via photorespiration and/or protein degradation.