Abstract
Sucrose transporters are essential membrane proteins for the distribution of photoassimilates in higher plants. In Solanaceous species the proteins of all known sucrose transporters are co-localized in enucleate sieve elements and un- dergo permanent turnover. The mRNA of the sucrose transporter StSUT1 is localized in both, sieve elements and compan- ion cells. Sucrose transporter mRNAs have been detected in the phloem sap of several species. Here, we analyzed the mo- bility of sucrose transporter transcripts in grafted plants and between host and parasitic plants. Phloem-mobility was found when a c-myc tagged SUT1-fusion construct without untranslated regions (UTRs) was expressed under the CaMV 35S promoter. We conclude that neither 3'- nor 5' -UTRs are essential for mRNA transport through plasmodesmata. Cycloheximide, which inhibits translation, has also effects on SUT transcript stability. Whereas SUT1 transcripts are de- stabilized when translation is inhibited, SUT2 and SUT4 transcripts accumulate up to 4fold under these conditions. Inhibi- tor studies revealed post-transcriptional regulation of SUT2 and SUT4 transcript accumulation. A model is proposed ex- plaining the coordination of SUT expression in Solanaceae.
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