Patterns of expression of the genes for glutamine synthetase isoforms during somatic and zygotic embryogenesis in carrot.

Three cDNA clones encoding isoforms of carrot glutamine synthetase (GS) were isolated and used as probes for analysis of the patterns of expression of the genes for GS isoforms during somatic embryogenesis and seed development in carrot. Transcripts corresponding to two of the cDNAs, CGS102 and CGS201, accumulated in both somatic embryos and developing seeds in the same manner. Their levels were high at the early stage of embryogenesis but decreased at the late stage. This pattern of expression is similar to the pattern of changes in GS activity observed during somatic embryogenesis. In contrast, expression of the transcript for another GS isoform detected with CGS103 cDNA was observed at the late stage of seed development and in senesced leaves but not in somatic embryos or young leaves. We also analyzed the levels of the transcripts in somatic embryos that had been cultured in media with either ammonium ions or glutamine as the nitrogen source. The amounts of the CGS102 and CGS201 transcripts fell when glutamine was supplied in the medium. These results indicated that GS activity was regulated at the transcriptional level and that the pattern of expression of the genes for GS during somatic embryogenesis reflected that during zygotic embryogenesis. It is possible that somatic embryogenesis and zygotic embryogenesis have common regulatory systems with respect to nitrogen metabolism.