Poster: Metabolic Engineering

Abs # 1020: Repercussions of organ-specific modulation of GS activity in alfalfa and Lotus japonicus.

Presenter:	Seger, Mark , segerm@hotmail.com
Authors	Seger, Mark  (A)   Potenza, Carol  (A)   Bagga, Suman  (A)   Moguel, Salvador  (A)   Ortega, Jose L (A)   Sengupta-Gopalan, Champa  (A)
Affiliations:	(A): New Mexico State University

Glutamine synthetase (GS) plays a central role in the assimilation of nitrogen and our approach towards improving nitrogen use efficiency has been to modulate the levels of the GS enzyme. Plant GS occurs as a number of isoenzyme forms and are located either in the cytosol (GS₁) or plastid (GS₂). Gene constructs consisting of different GS₁ gene driven by the constitutive CaMV 35S promoter were introduced into Lotus japonicus and alfalfa. L. japonicus plants transformed with the GS₁ gene construct showed the presence of the GS₁ transgene transcript in all the organs including the nodules. Moreover, these transformants showed increased GS activity and GS₁ polypeptide in all the organs, and they exhibited increased growth and protein content. The alfalfa transformants with the GS₁ gene construct, while showing high levels of the transcript for the transgene in the leaves, showed no trace of the transcript in the nodules. Furthermore, the transformants showed a dramatic drop in the level of the GS₁ transcript when the plants were grown with NO3- compared to plants that were not fed with NO3-. The results suggest regulation of GS₁ activity at the level of transcript turnover and that NO3- or an assimilation product may trigger GS₁ transcript destabilization in alfalfa. To understand the basis for this discrepancy in response to the constitutive over-expression of GS₁ between alfalfa and Lotus, experiments are in progress to follow the expression pattern of both the endogenous and GS₁ transgene transcript in Lotus grown under different nitrogen regimes. Experiments are also in progress to modulate GS₁ activity in an organ specific manner in both plants with the goal to understand the basis for improved performance of Lotus plants overexpressing GS₁ constitutively.