Phloem Mobility and Stability of Sucrose Transporter Transcripts

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.