EXPRESSION DYNAMICS OF GENES ENCODING KEY CARBON METABOLISM ENZYMES DURING SINK TO SOURCE TRANSITION OF DEVELOPING LEAVES

During ontogeny, leaves undergo transition from net importers of photoassimilates (sink) to net exporters (source). Results of broadly-based experiments are presented showing metabolic, physiological and molecular changes which occur during developmental transition of sugarbeet (Beta vulgaris L.) leaves. Changes in the expression of genes encoding key enzymes of carbon metabolism; the cytosolic fructose-1,6-bisphosphatase (FBPase), sucrose-phosphate synthase (SPS) and ribulose-1,5-bisphosphatase (Rubisco) as well as several related metabolic parameters were monitored during the course of leaf development. Six leaf stages were used according to their sink/source status (leaf 1, youngest) as well as a single ''transitional'' leaf in which the base, middle and the tip function as sink, transitional and source, respectively. In this leaf, FBPase and Rubisco transcripts increased as source status was attained in each section. Photosynthetic rates and chlorophyll content increased as leaves expanded, reaching their maximum in leaves 3, 4 and 5, but declined in leaf 6. Significant changes were also observed in leaf sucrose, glucose and fructose. Sucrose was highest in leaves 3, 4 and 5. Leaves 1, 2, and 3 had high levels of hexose. In general, FBPase, SPS and Rubisco activity, protein and transcript levels were low in immature leaves, reached maximal levels in leaves 3, 4 and/or 5 and declined or remained stable in leaf 6. While transcripts for FBPase, SPS and Rubisco changed as transition progressed, that of the cytosolic aldolase, (a non-regulatory enzyme in sucrose pathway) did not. The coordinated pattern of gene expression correlated highly with leaf photosynthetic rates and sucrose content indicating molecular coordination of sucrose synthesis and photosynthesis. The data provide first evidence on the regulation of expression of genes encoding the two regulatory enzymes of sucrose biosynthesis in developing leaves and on the molecular coordination of some of the regulatory enzymes of carbon metabolism during leaf transition.