Regulation of glutamine synthetase genes in leaves of Phaseolus vulgaris

Publisher Website
Google Scholar
Abstract
Glutamine synthetase (GS) activity increased over three-fold in developing primary leaves of Phaseolus vulgaris L. This increase was shown to be the result of differential expression of three members of the GS gene family: gln-α and gln-β, which encode cytosolic GS polypeptides, and gln-δ, which encodes the chloroplast-located GS. The gln-δ gene was the most highly expressed GS gene and was regulated in a complex manner with two different transcripts accumulating differentially during leaf development. This gene was expressed weakly in the dark and was induced strongly by lingt; this induction was shown not to be an indirect effect of photorespiration. In the long term, gln-δ showed increased expression in photorespiring compared with non-photorespiring leaves. However, in the short term, there was no induction of gln-δ following transfer of plants to photorespiratory conditions. These results suggest that regulation of gln-δ by photorespiration was the result of indirect, long-term effects on cellular metabolism. In general, in all these experiments, analysis of cytosolic versus chloroplastic GS polypeptides and of the GS isoenzyme profiles showed the same pattern of changes in abundance as that observed for the mRNAs suggesting that regulation of GS gene expression occurred primarily at the mRNA level. However, it was noteworthy that the δ isoenzyme remained at a high abundance in older leaves, grown in both light and dark, despite a decrease in abundance of gln-δ mRNA.