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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.

bmc plant biology

Characterization of a vacuolar sucrose transporter, HbSUT5, from Hevea brasiliensis: involvement in latex production through regulation of intracellular sucrose transport in the bark and laticifers