Spatial and temporal expression of two cytosolic glutamine synthetase genes in Scots pine: functional implications on nitrogen metabolism during early stages of conifer development

Ammonium assimilation during the initial stages of Scots pine growth involv es two cytosolic glutamine synthetase (GS, EC: 6.3.1.2) isoenzymes encoded by separate genes, GS1a and GS1b. GS1a was most exclusively expressed in ph otosynthetic tissues of the seedling whereas GS1b was expressed ubiquitousl y showing higher levels in non-photosynthetic tissues such as root and hypo cotyl. Temporal expression analysis has shown that when germination starts GS1b is the predominant form in the embryo, however, its relative abundance in the tissue decreased in the postgerminative stages when green cotyledon s are developed. In contrast GS1a was present at a low level in the embryo but its abundance increased markedly during germination and seedling growth . These data suggest that GS1a and GS1b genes display different and non-red undant roles in the nitrogen metabolism of conifers. The precise localizati on of individual transcripts by in situ hybridization strongly supports thi s possibility. GS1 gene products are mainly expressed in different cellular types: GS1a in chlorophylic parenchyma and GS1b in the vascular bundles of all tissues examined in the seedling. Our data support that glutamine bios ynthesis in pine seedlings follows a different pattern related to angiosper ms involving two cytosolic GS proteins: one of them a typical cytosolic GS which may be involved in the generation of glutamine for N transport and a second cytosolic GS generating amino donors for the biosynthesis of major N compounds in photosynthetic tissues, a closer role to angiosperm chloropla stic GS. The results are discussed with regard to recent studies on N mobil ization and metabolism during the initial stages of conifer development.