Sturm A, Lienhard S, Schatt S, Hardegger M
Friedrich Miescher-Institut, Basel, Switzerland.
Plant Mol Biol. 1999 Jan;39(2):349-60. doi: 10.1023/a:1006199003756.
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 (SusyDc1 and SusyDc2) 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. SusyDc2 was exclusively expressed in flowers. Transcripts for SusyDc1 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.
蔗糖合酶在尿苷二磷酸（UDP）存在的情况下将蔗糖裂解为UDP-葡萄糖和果糖，被认为是异养植物器官库强度的关键决定因素。为了确定该酶在胡萝卜中的作用，我们在分子水平上对胡萝卜蔗糖合酶进行了表征。分离出了两个基因（SusyDc1和SusyDc2）。推导的氨基酸序列有87%的同源性。然而，翻译起始密码子上游的序列明显不同，这两个基因的表达模式也是如此。SusyDc2仅在花中表达。在茎、不同发育阶段的根、花芽、花和成熟种子中发现了SusyDc1的转录本，在蔗糖强利用库如生长中的茎和主根根尖中水平最高。Susy*Dc1的表达受厌氧调节，但不受糖或乙酸盐调节。胡萝卜蔗糖合酶蛋白部分与膜相关，这种不溶性形式可能直接参与纤维素生物合成。所用胡萝卜品种的主根在维管束附近积累淀粉，这与高蔗糖合酶转录水平相关。这一发现表明，主根中的可溶性蔗糖合酶将蔗糖导向淀粉生物合成。淀粉积累似乎是短暂的，可能参与蔗糖向发育中的主根的分配。
