Legumes obtain a substantial portion of their nitrogen (N) from symbiotic N2 fixation in root nodules. The glutamine synthetase (GS, EC 220.127.116.11)/glutamate synthase (GOGAT) cycle is responsible for the initial N assimilation. This report describes the analysis of a transgenic alfalfa (Medicago sativa L.) line containing an antisense NADH-GOGAT (EC 18.104.22.168) under the control of the nodule-enhanced aspartate aminotransferase (AAT-2) promoter. In one transgenic line, NADH-GOGAT enzyme activity was reduced to approximately 50%, with a corresponding reduction in protein and mRNA. The transcript abundance for cytosolic GS, ferredoxin-dependent GOGAT (EC 22.214.171.124), AAT-2 (EC 126.96.36.199), asparagine synthase (EC 188.8.131.52), and phosphoenolpyruvate carboxylase (PEPC, EC 184.108.40.206) were unaffected, as were enzyme activities for AAT, PEPC and GS. Antisense NADH-GOGAT plants grown under symbiotic conditions were moderately chlorotic and reduced in growth and N content, even though symbiotic N2 fixation was not significantly reduced. The addition of nitrate relieved the chlorosis and restored growth and N content. Surprisingly, the antisense NADH-GOGAT plants were male sterile resulting from inviable pollen. A reduction in NADH-GOGAT enzyme activity and transcript abundance in the antisense plants was measured during the early stages of flower development. Inheritance of the transgene was stable and resulted in progeny with a range of NADH-GOGAT activity. These data indicate that NADH-GOGAT plays a critical role in the assimilation of symbiotically fixed N and during pollen development.
Bibliographical noteFunding Information:
We thank Drs Michael Russelle and Joanne Lamb for their technical assistance. This work was supported by National Science Foundation Grant No. IBN-9206890. This paper is a joint contribution from the Plant Science Research Unit, USDA, Agricultural Research Service and Minnesota Agricultural Experiment Station (Paper No. 981-13-0105 Scientific Journal Series). Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products or vendor.
- Amino acid
- Nitrogen assimilation