This manuscript has been published online, prior to printing.Once the issue is complete and page numbers have been assigned, the citation will change accordingly. transported as sucrose from sugar-exporting (source) organs, such as leaves, to sugar-importing (sink) organs, such as tubers, seeds or storage roots. The coordinated modulation of gene expression in source and sink organs is to a large extent choreographed by the sugar status in the cells (reviewed in ref. 5 and refs. therein). In general, low sugar levels promote photosynthesis and mobilization of energy reserves, whereas high sugar levels stimulate growth and storage of starch and other carbohydrates. Development of sink organs is orchestrated by the coordinated activities of a large number of genes that encode metabolic and regulatory enzymes, as well as other proteins. Starch synthesis is catalyzed by an enzymatic machinery containing ADP-glucose pyrophosphorylase (AGPase), starch synthases (SS), starch branching enzymes (SBE), and starch debranching enzymes (DBE). Most, if not all, of these enzymes exist in two or more isoforms (reviewed in refs. 6-8 for reviews on starch biosynthesis). Cassava (Manihot esculenta Crantz L.) is a perennial starch crop of major global importance, particularly in the Developing World. 9,10 Starch is synthesized and deposited in the underground tuberous storage roots, which can measure up to 1 m in length and over 10 cm in diameter. The storage root is a non-reproductive organ and can accumulate close to 85% of its total dry weight as starch. During development of the cassava storage root, expression of the SBEII and SBEI genes, encoding, respectively, SBEII and SBEI, gradually increases from 90 to 360 days after planting (d.a.p.). 11 Accumulation of SBEII and SBEI transcripts in 360 d.a.p. plants was also found to exhibit a short-term fluctuation with a period of one day. In the work presented here, we followed the induction of SBE expression in isolated cassava storage root discs by sugars, sugar analogs and abscisic acid (ABA). Our results establish the presence of an endogenous semidian (12-h) oscillator in the storage root cells at the level of hexokinase (HXK). They also suggest that sugar and ABA signaling proceeds via independent pathways, and that the sugar signaling cascade is activated by the entry of sucrose into the cell and relies on HXK activity. Starch branching enzyme (SBE) activity in the cassava storage root exhibited a diurnal fluctuation, dictated by a transcriptional oscillation of the corresponding SBE genes. The peak of SBE activity coincided with the onset of sucrose accumulation in the storage, and we conclude that the oscillatory mechanism keeps the starch synthetic apparatus in the storage root sink in tune with the flux of sucrose from the photosynthetic source. When storage roots were uncoupled from the source, SBE expression could be effectively induced by exogenous sucrose. Turanose, a sucrose isomer that cannot be metabolized by plants, mimicked the effect of sucrose, demonstrating tha...