BACKGROUND Sucrose (Suc), as the precursor molecule for rubber biosynthesis in Hevea brasiliensis, is transported via phloem-mediated long-distance transport from leaves to laticifers in trunk bark, where latex (cytoplasm of laticifers) is tapped for rubber. In our previous report, six Suc transporter (SUT) genes have been cloned in Hevea tree, among which HbSUT3 is verified to play an active role in Suc loading to the laticifers. In this study, another latex-abundant SUT isoform, HbSUT5, with expressions only inferior to HbSUT3 was characterized especially for its roles in latex production. RESULTS Both phylogenetic analysis and subcellular localization identify HbSUT5 as a tonoplast-localized SUT protein under the SUT4-clade (=type III). Suc uptake assay in baker's yeast reveals HbSUT5 to be a typical Suc-H+ symporter, but its high affinity for Suc (Km = 2.03 mM at pH 5.5) and the similar efficiency in transporting both Suc and maltose making it a peculiar SUT under the SUT4-clade. At the transcript level, HbSUT5 is abundantly and preferentially expressed in Hevea barks. The transcripts of HbSUT5 are conspicuously decreased both in Hevea latex and bark by two yield-stimulating treatments of tapping and ethephon, the patterns of which are contrary to HbSUT3. Under the ethephon treatment, the Suc level in latex cytosol decreases significantly, but that in latex lutoids (polydispersed vacuoles) changes little, suggesting a role of the decreased HbSUT5 expression in Suc compartmentalization in the lutoids and thus enhancing the Suc sink strength in laticifers. CONCLUSIONS Our findings provide insights into the roles of a vacuolar sucrose transporter, HbSUT5, in Suc exchange between lutoids and cytosol in rubber-producing laticifers.

中文翻译：

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来自巴西橡胶树的液泡蔗糖转运蛋白HbSUT5的特征：通过调节树皮和胶乳中细胞内蔗糖的转运参与乳胶生产。

背景技术蔗糖（Suc），作为巴西橡胶树橡胶生物合成的前体分子，是通过韧皮部介导的长距离运输从叶片运输到树干皮中的胶乳，其中胶乳（胶乳的细胞质）被用于橡胶。在我们之前的报告中，已经在橡胶树中克隆了六个Suc转运蛋白（SUT）基因，其中HbSUT3已被证明在Suc装载到胶乳虫中发挥积极作用。在这项研究中，另一种乳胶丰富的SUT亚型，即HbSUT5，其表达仅次于HbSUT3，其特征尤其在于其在乳胶生产中的作用。结果系统发育分析和亚细胞定位均确定HbSUT5为SUT4进化枝（= III型）下液泡膜定位的SUT蛋白。面包酵母中的Suc摄取测定显示HbSUT5是典型的Suc-H +同向转运体，但是它对Suc的亲和力很高（​​在pH 5.5时Km = 2.03 mM），并且在运输Suc和麦芽糖方面具有相似的效率，因此在SUT4包壳下成为特殊的SUT。在转录本水平上，HbSUT5在橡胶树皮中大量表达并优先表达。在橡胶树皮和树皮中，通过敲击和乙烯利的两种增产刺激处理，HbSUT5的转录显着降低，其模式与HbSUT3相反。在乙烯利处理下，乳胶胞浆中的Suc水平显着降低，但乳胶黄体（多分散液泡）中的Suc水平变化不大，表明HbSUT5表达的降低在黄体中的Suc分隔中发挥了作用，从而增强了乳胶石中Suc的沉陷强度。结论我们的发现为液泡蔗糖转运蛋白HbSUT5的作用提供了见解，