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 photo assimilates (sink) to net exporters (source). Results of broadly-based experiments are presented showing metabolic, physiological and molecu lar 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-bisphosph atase (FBPase), sucrose-phosphate synthase (SPS) and ribulose-1,5-bisp hosphatase (Rubisco) as well as several related metabolic parameters w ere monitored during the course of leaf development. Six leaf stages w ere used according to their sink/source status (leaf 1, youngest) as w ell as a single ''transitional'' leaf in which the base, middle and th e tip function as sink, transitional and source, respectively. In this leaf, FBPase and Rubisco transcripts increased as source status was a ttained in each section. Photosynthetic rates and chlorophyll content increased as leaves expanded, reaching their maximum in leaves 3, 4 an d 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, FBP ase, 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 cytosoli c aldolase, (a non-regulatory enzyme in sucrose pathway) did not. The coordinated pattern of gene expression correlated highly with leaf pho tosynthetic rates and sucrose content indicating molecular coordinatio n of sucrose synthesis and photosynthesis. The data provide first evid ence on the regulation of expression of genes encoding the two regulat ory enzymes of sucrose biosynthesis in developing leaves and on the mo lecular coordination of some of the regulatory enzymes of carbon metab olism during leaf transition.