Expression of a bacterial asparagine synthetase gene in oilseed rape (Brassica napus) and its effect on traits related to nitrogen efficiency

The investigation and improvement of nitrogen efficiency in oilseed rape (Brassica napus L.) are important issues in rapeseed breeding. The objective of this study was to modify ammonium assimilation in transgenic rapeseed plants through the expression of the Escherichia coli asparagine synthetase (AsnA, E.C. 6.3.1.1) gene under the control of the cauliflower mosaic virus (CaMV) 35S promoter, and to study its influence on amino acid composition in leaves and on seed traits related to nitrogen efficiency. In regenerated transgenic plants, the 37 kDa AsnA protein was detected by Western blot analysis, but was lacking in untransformed control plants of cv. Drakkar. In the transformants, in vitro asparagine synthetase activities ranged from 105 to 185 nmol asparagine mg(-1) protein h(-1), whereas, in untransformed control plants, only negligible asparagine synthetase activities of up to 5 nmol asparagine mg(-1) protein h(-1) were found. Despite these significant activities, no changes in the amino acid composition in the leaves or in the phloem of transgenic plants were detectable. In a pot experiment, two transgenic lines expressing the prokaryotic asparagine synthetase clearly performed inferiorly to control plants at limiting nitrogen (N) fertilizer supply. Although the seed N content was increased, the seed yield and the seed N yield were reduced, which was interpreted as an increased nitrate assimilation leading, at limiting N supply, to a reduced seed yield and seed N yield. At high N fertilizer supply, the differences were less pronounced for one transgenic line, whereas the other showed a higher seed N yield and an improved nitrogen harvest index. The results show that the expression of the E. coli asnA gene in oilseed rape could be of advantage at high N supply, but not at limiting N fertilizer supply.