OCCURRENCE AND PHYLOGENETIC SIGNIFICANCE OF GLUCOSE-UTILIZATION BY CHAROPHYCEAN ALGAE - GLUCOSE ENHANCEMENT OF GROWTH IN COLEOCHAETE ORBICULARIS

Two critical innovations had a profound influence upon the evolutionar y history of plants: the nutritionally dependent embryo and apoplastic phloem loading processes. Both depend upon the ability of the plant c ell membrane to transport sugars. The evolutionary origin of sugar tra nsport by plants is, therefore, of special phylogenetic importance. Re cent evidence suggests that hexoses such as glucose are the main form of sugar transported apoplastically across the placental junction betw een gametophyte and sporophyte of the moss Polytrichum (Renault et al. , 1992, Plant Physiology 100: 1815-1822). There is also considerable e vidence that hexose transport may be involved in apoplastic phloem loa ding in Arabidopsis and other flowering plants. Results of numerous mo lecular, biochemical, immunofluorescence, and ultrastructural studies indicate that bryophytes and charophycean algae are related to the anc estors of vascular plants. This report demonstrates that the charophyt e Coleochaete orbicularis exhibits enhanced growth in the presence of glucose under conditions in which inorganic carbon sources are limitin g. Computer image analysis was used to demonstrate that nonaerated cul tures of C. orbicularis grown for 7 weeks in an inorganic medium suppl emented with 1% glucose produce as much as 13.6 times the biomass of c lonal cultures grown for the same length of time in the same medium wi thout glucose. Furthermore, addition of 1% glucose to nonaerated cultu res yielded chlorophyll a concentration 20 times higher after 4 weeks growth than cultures grown under the same conditions without added glu cose. This and other evidence derived from comparative growth measurem ents strongly suggest that Coleochaete takes up (or loads) glucose. Ex perimental results also suggest that hexose uptake by charophytes (suc h as Coleochaete) that typically grow in low alkalinity waters might p rovide a supplementary source of organic carbon when dissolved carbon dioxide levels are low. This capability could have served as an evolut ionary precursor to hexose import by plant embryos, as well as leptom and phloem loading in bryophytes and vascular plants.