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
Abstract 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. Suc transporters (SUTs) play important roles during various steps of Suc transport in higher plants. Results: In our previous report, six SUT genes have been cloned in Hevea tree, among which HbSUT3 has been 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. Both phylogenetic analysis and subcellular localization identify HbSUT5 as a SUT4-clade (=type III) vacuolar membrane SUT, suggesting its potential participation in Suc exchange between lutoids (polydispersed microvacuoles) and cytosol in latex. Suc uptake assay in yeast identifies HbSUT5 as a typical Suc-H+ symporter, but the high affinity of HbSUT5 for Suc (Km = 2.03 mM at pH 5.5) and its similar efficiency in transporting maltose making it a peculiar SUT under the SUT4-clade. At the transcript level, HbSUT5 is abundantly and preferentially expressed in Hevea barks. It is contrary to HbSUT3 that the transcripts of HbSUT5 are obviously decreased both in Hevea latex and bark during the treatments of tapping and ethephon, indicating it counteracts the yield-stimulating effects of two treatments. Conclusions: A vacuolar sucrose transporter, HbSUT5, may play an important role in Suc exchange between lutoids (polydispersed vacuoles) and latex in laticifers. It is better to understand that the whole HbSUT family regulate and control Suc accumulation in laticifers, influencing rubber yield formation in Hevea.