SummarySucrose transporters of higher plants belong to a large gene family. At least four different sucrose transporters are known in Solanaceous plants, although their function remains to be elucidated in detail. The isolation of LeSUT1 and LeSUT2 from Lycopersicon esculentum has been described earlier. Whereas SUT1 is supposed to be the main phloem loader of sucrose in Solanaceae, the role of SUT2 remains a matter of debate. A transgenic approach was taken to evaluate the potential functions of SUT2/SUC3 proteins in sucrose transport or sensing. Expression of LeSUT1 and LeSUT2 was inhibited independently in transgenic tomato plants, using the antisense technique, in order to analyse their specific functions. Although the phloem-specific inhibition of LeSUT1 antisense plants showed a phenotype consistent with an essential role in phloem loading, constitutive LeSUT2 antisense inhibition exclusively affected tomato fruit and seed development. Neither LeSUT1, nor the LeSUT2 antisense plants were able to produce normal tomato fruits; however, it is likely that independent mechanisms underlie these phenomena. While phloem loading was blocked in LeSUT1 antisense plants, the fertility of fruits was reduced in LeSUT2 antisense plants. A detailed physiological analysis of these plants established a role for SUT2 in pollen tube growth and thus assigned a physiological role for SUT2.
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 undergo permanent turnover. The mRNA of the sucrose transporter StSUT1 is localized in both, sieve elements and companion cells. Sucrose transporter mRNAs have been detected in the phloem sap of several species. Here, we analyzed the mobility 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 destabilized when translation is inhibited, SUT2 and SUT4 transcripts accumulate up to 4fold under these conditions. Inhibitor studies revealed post-transcriptional regulation of SUT2 and SUT4 transcript accumulation. A model is proposed explaining the coordination of SUT expression in Solanaceae.
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