END-PRODUCT CONTROL OF CARBON METABOLISM IN CULTURE-CROWN SUGAR-BEET PLANTS - MOLECULAR AND PHYSIOLOGICAL EVIDENCE ON ACCELERATED LEAF DEVELOPMENT AND ENHANCED GENE-EXPRESSION

Sugar beet (Beta vulgaris L.) seedlings were grown on media containing 90 to 300 mM sucrose or glucose. Compared to controls, sugar-grown pl ants had higher growth rate, photosynthesis, and leaf sugar levels. Th e steady-state level of transcripts increased significantly for the sm all subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisc o) (rbcS) and the cytosolic fructose-1,6-bisphosphatase and moderately for the Rubisco targe subunit (rbcL). The transcript level of sucrose phosphate synthase remained unchanged. Fructose-1,6-bisphosphatase an d Rubisco activities did not change in the presence of sugars, but tha t of sucrose phosphate synthase increased (44 and 90% under selective and nonselective assay conditions, respectively). Accelerated leaf dev elopment was indicated by (a) autoradiograms of leaves that showed tha t sucrose loading occurred earlier, (b) export capacity that also occu rred earlier but, after about 2 weeks, differences were not detectable , and (c) sucrose synthase activity that declined significantly. Sever al conclusions emerged: (a) response was nonosmotic and gene and sugar specific, (b) sugars caused accelerated leaf development and sink-to- source transition, (c) enhanced gene expression was due to advanced le af development, and (d) whereas Rubisco and cytosolic fructose-1,6-bis phosphatase genes were sugar repressed in mature leaves of greenhouse- grown plants, they were unaffected in mature, culture-grown leaves. To our knowledge, these data provide the first evidence in higher plants that, depending on the physiological/developmental context of leaves, sugars lead to differential regulation of the same gene.