Down-regulation of specific members of the glutamine synthetase gene family in alfalfa by antisense RNA technology

Glutamine synthetase (GS) catalyzes the ATP-dependent condensation of NH3 with glutamate to produce glutamine. In plants GS is an octameric enzyme and is located either in the cytoplasm (GS(1)) or in the chloroplast (GS(2)). Two distinct classes of GSI genes with unique 3'-untranslated region (3'UTR) have been identified in alfalfa. We have demonstrated that the two classes exhibit differential expression pattern in the different plant organs suggesting different functional roles for the different isozymes. To determine the functional significance of the two classes of GS(1) genes in alfalfa, we have utilized antisense gene constructs aimed specifically at the 3'UTR of the two GS(1) genes and introduced them individually into alfalfa. Our data show that the gene constructs are effective in lowering the corresponding transcript level very effectively though there were organ-specific differences in the level of reduction. No transcript corresponding to the antisense gene construct was detected in any of the alfalfa transformants though they accumulated to significant levels in transgenic tobacco containing the same construct. This suggests that the antisense transcript was not stable in the presence of the homologous target sequence. Transgenic alfalfa with up to 80% reduction in the transcript level corresponding to each gene class, however, showed no reduction in GS activity or GS(1) polypeptide level. The results suggest that GS(1) mRNA levels are not rate-limiting for GS(1) polypeptide synthesis and that GS(1) levels are controlled both at the transcriptional and translational/post-translational level.