ROLES FOR SUCROSE TRANSPORTERS DURING TUBEROUS ROOT DEVELOPMENT IN SWEET POTATO

为了研究蔗糖转运蛋白在甘薯中的作用，我克隆了三个与植物蔗糖转运蛋白同源蛋白的cDNAs(IbSUT1、IbSUT2和IbSUT3)。这些基因与马铃薯、拟南芥和蚕豆的蔗糖转运蛋白在核苷酸水平上有60%~72%的同源性，在氨基酸水平上有58%~77%的同源性。根据基因组Southern杂交分析结果，甘薯蔗糖转运蛋白基因似乎构成了一个小基因家族。IbSUT1、IbSUT2和IbSUT3的转录本大小分别约为3.1kb、2.4kb和1.6kb。IbSUT1、IbSUT2和IbSUT3具有明显不同的表达模式。IbSUT1在所有器官中的表达数量大致相同。IbSUT2在叶柄和幼根中的RNA水平至少是叶片、匍匐茎和块茎中的两倍。IbSUT3在叶柄中的转录水平最高，在其他器官中的转录水平较低约30%。在块根发育过程中，IbSUT1基因在根中的表达水平基本相同，而IbSUT2和IbSUT3的基因表达水平则继续下降。这些结果表明，甘薯至少有三种蔗糖转运蛋白，IbSUT1是块根发育所必需的。

To study roles of sucrose transporters in sweet potato, I cloned three cDNAs (IbSUT1, IbSUT2 and IbSUT3) for proteins with homologies to plant sucrose transporters. The cDNAs showed 60 to 72% identity at the nucleotide level and 58 to 77% identity at the amino acid level to sucrose transporters from potato, Arabidopsis and fava bean. Based on the result of genomic Southern blot analysis, the sucrose transporter gene appears to constitute a small gene family in sweet potato. The size of transcripts of IbSUT1, IbSUT2 and IbSUT3 was about 3.1kb, 2.4kb and 1.6kb, respectively. IbSUT1, IbSUT2 and IbSUT3 had markedly different expression patterns. IbSUT1 was expressed in approximately equal amounts in all organs. RNA levels of IbSUT2 in petioles and young roots were at least two times higher than those in leaf blades, stolons and tuberous roots. The highest transcript level for IbSUT3 was found in petioles, and about 30% lower levels at the other organs. Approximately equal levels of mRNA for IbSUT1 were found in roots during tuberous root development, whereas mRNA levels of IbSUT2 and IbSUT3 continued to decrease. These results indicate sweet potato has at least three kinds of sucrose transporters and IbSUT1 is required for tuberous root development.