2 DIFFERENT MODES OF EARLY DEVELOPMENT AND NITROGEN ASSIMILATION IN GYMNOSPERM SEEDLINGS

Light-independent chloroplast development and expression of genes enco ding chloroplast proteins occur in many but not all species of gymnosp erms. Early development in maritime pine (Pines pinaster) seedlings wa s strongly light-independent, whereas Ginkgo biloba seedlings exhibite d a typical angiosperm-like morphogenesis with differentiated patterns in light and dark. In pine, chloroplast polypeptides were undetectabl e in the seed embryo and accumulated in cotyledons of both light-and d ark-grown plants in good correlation with light-independent chlorophyl l synthesis. In contrast, chlorophyll and chloroplast proteins were on ly detected in light-grown ginkgo. Pine cytosolic glutamine synthetase (GS) and ferredoxin glutamate synthase (Fd-GOGAT) were present at low levels in the seeds and accumulated at comparable amounts in light-an d dark-grown seedlings. Fd-GOGAT was also barely detectable in the see ds of ginkgo and only accumulated in green plants with mature chloropl asts. In G. biloba seeds and etiolated plants only cytosolic GS was id entified, while in light-grown seedlings this molecular form was prese nt at low abundance and choroplastic GS was the predominant isoenzyme. The above results have been confirmed by immunolocalization of GS pro tein in pine and ginkgo plantlets. In pine, GS was present in the peri pheral cytoplasm of mesophyll cells and also in the phloem region of t he vascular bundle. Immunocytochemical analysis showed that the labell ing of mesophyll and phloem cells was only cytoplasmic. In developing ginkgo, GS antigens were present in the chloroplasts of mesophyll pare nchyma cells of leaflets and green cotyledons. In contrast, a weak lab elling of GS was observed in the parenchyma and phloem cells of non-gr een cotyledons enclosed in the seed coat. Taking all this into account , our data indicate the existence of two different modes of GS and GOG AT regulation in gymnosperms in close correlation with the differentia l response of plants to light. Furthermore, the results suggest that g lutamine and glutamate biosynthesis is confined to the chloroplast of mesophyll cells in species with light-dependent chloroplast, developme nt whereas compartmentation would be required in species with light-in dependent plastid development.