DIFFERENTIAL EXPRESSION WITHIN THE GLUTAMINE-SYNTHETASE GENE FAMILY OF THE MODEL LEGUME MEDICAGO-TRUNCATULA

The glutamine synthetase (GS) gene family of Medicago truncatula Gaert n. contains three genes related to cytosolic GS (MtGSa, MtGSb, and MtG Sc), although one of these (MTGSc) appears not to be expressed. Sequen ce analysis suggests that the genes are more highly conserved interspe cifically rather than intraspecifically: MtGSa and MtGSb are more simi lar to their homologs in Medicago sativa and Pisum sativum than to eac h other. Studies in which gene-specific probes are used show that both MtGSa and MtGSb are induced during symbiotic root nodule development, although not coordinately. MtGSa is the most highly expressed GS gene in nodules but is also expressed to lower extents in a variety of oth er organs. MtGSb shows higher levels of expression in roots and the ph otosynthetic cotyledons of seedlings than in nodules or other organs. In roots, both genes are expressed in the absence of an exogenous nitr ogen source. However the addition of nitrate leads to a short-term, 2- to 3-fold increase in the abundance of both mRNAs, and the addition o f ammonium leads to a 2-fold increase in MtGSb mRNA. The nitrogen supp ly, therefore, influences the expression of the two genes in roots, bu t it is clearly not the major effector of their expression. In the dis cussion section, the expression of the GS gene family of the model leg ume M. truncatula is compared to those of other leguminous plants.