MOLECULAR CHARACTERIZATION AND FUNCTIONAL-ANALYSIS OF SUCROSE-CLEAVING ENZYMES IN CARROT (DAUCUS-CAROTA L)

The amount of carbon transported into storage organs of crop plants to a large degree determines crop yield. The role of sucrose-cleaving en zymes in this process is not clear and it is the main goal of our work to tackle this question. Sucrose cleavage is catalysed either by inve rtase or sucrose synthase both of which exist in several isoforms with different subcellular locations. Carrot (Daucus carota L.) contains t hree major isoenzymes of acid invertase, which either accumulate as so luble polypeptides in the vacuole (isoenzymes I and II) or are ionical ly bound to the cell wall. Carrot sucrose synthase is thought to be a cytoplasmic enzyme encoded by two genes, cDNA clones have been isolate d and characterized for cell wall invertase, for isoenzymes I and II o f vacuolar invertase, and for sucrose. Gene-specific fragments of thes e clones used to determine the steady-state levels of transcripts in t he prominent sink and source organs of developing carrot plants. The e xpression patterns of each gene were different and were organ- and dev elopment-specific. Developing tap roots contained only transcripts for isoenzyme II of vacuolar invertase and sucrose synthase. The source/s ink balance of these plants was manipulated and only the expression of these two genes was markedly altered, indicating their importance in sucrose partitioning. Based on these carrot plants with developing tap roots in which results, a model is proposed for sucrose partitioning sucrose synthase regulates sucrose utilization, whereas isoenzyme II o f vacuolar invertase controls sucrose storage and sugar composition.