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

Sucrose synthase, which cleaves sucrose in the presence of uridine diphosph ate (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 molec ular level. Two genes (Susy *Dc1 and Susy*Dc2) were isolated. The deduced a mino acid sequences are 87% identical. However, the sequences upstream of t he translation initiation codons are markedly different, as are the express ion patterns of the two genes. Susy*Dc2 was exclusively expressed in flower s. Transcripts for Susy*Dc1 were found in stems, in roots at different deve lopmental stages, and in flower buds, flowers and maturing seeds, with the highest levels in strong utilization sinks for sucrose such as growing stem s and tap root tips. Expression of Susy*Dc1 was regulated by anaerobiosis b ut not by sugars or acetate. The carrot sucrose synthase protein is partly membrane-associated and this insoluble form may be directly involved in cel lulose biosynthesis. Tap roots of the carrot cultivar used accumulated star ch in the vicinity of the vascular bundles, which correlated with high sucr ose 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 partiti oning to developing tap roots.