Expression of glutamine-synthetase genes in cotyledons of germinating Phaseolus vulgaris L

In the legume Phaseolus vulgaris L., glutamine synthetase (GS; EC.6.3.1.2.) is encoded by four actively transcribed genes, gln-α, gln-β, gln-γ and gln-δ. We have studied the expression of these genes in cotyledons during seed germination and have studied the effect of light and nitrate on this process. An RNase-protection method, used to detect the abundances of GS mRNAs, revealed that the four GS genes are differentially expressed in the germinating cotyledons. The gln-α. mRNA was present in dry seeds and was the most abundant GS mRNA during early stages of germination. The gln-β and gln-γ mRNAs were first detectable 2 d after sowing and their abundances differed in light- and dark-grown cotyledons at later stages of germination. The gln-δ mRNA (which encodes the plastid-located GS) was detectable only in light-grown cotyledons, at a low abundance. A nitrate supply of 2 mM had only a minor effect on the expression of the GS genes. Western immunodetection and ion-exchange high-performance liquid chromatography demonstrated that the α polypeptide and isoenzyme were present in extracts of dry seeds and represented the major GS products at 2 d and 4 d. Both the β and γ polypeptides appeared at the 2-d stage. The role of differential GS gene expression in controlling cotyledonary GS activity is discussed.