Tissue-specific expression of two genes for sucrose synthase in carrot (Daucus carota L.)

Sucrose synthase, which cleaves sucrose in the presence of uridine diphosphate (UDP) into UDP-glucose and fructose, is thought to be a key determinant of sink strength of heterotrophic plant organs. To determine the roles of the enzyme in carrot, we characterized carrot sucrose synthase at the molecular level. Two genes (Susy*Dc1 and Susy*Dc2) were isolated. The deduced amino acid sequences are 87% identical. However, the sequences upstream of the translation initiation codons are markedly different, as are the expression patterns of the two genes. Susy*Dc2 was exclusively expressed in flowers. Transcripts for Susy*Dc1 were found in stems, in roots at different developmental stages, and in flower buds, flowers and maturing seeds, with the highest levels in strong utilization sinks for sucrose such as growing stems and tap root tips. Expression of Susy*Dc1 was regulated by anaerobiosis but not by sugars or acetate. The carrot sucrose synthase protein is partly membrane-associated and this insoluble form may be directly involved in cellulose biosynthesis. Tap roots of the carrot cultivar used accumulated starch in the vicinity of the vascular bundles, which correlated with high sucrose synthase transcript levels. This finding suggests that soluble sucrose synthase in tap roots channels sucrose towards starch biosynthesis. Starch accumulation appears to be transient and may be involved in sucrose partitioning to developing tap roots.