Sucrose transporter regulation at the transcriptional, post-transcriptional and post-translational level

Sucrose transporters are crucial to carbon partitioning in higher plants. With their role in loading of sucrose into the phloem they control sucrose distribution throughout the whole plant and drive the osmotic flow system in the phloem. Recently, first insight was obtained on the coordination of sucrose transporter action with plant growth and development. The analysis of transgenic plants with reduced or enhanced expression of sucrose transporters helped to elucidate their physiological function and regulation in detail and connections to light and hormone signalling pathways were discovered. Whereas members of the SUT1 subfamily of sucrose transporters seem to be tightly controlled at the transcriptional and post-translational level in solanaceous plants, other family members show primarily post-transcriptional control of their mRNA stability. Post-translational regulation of sucrose transporters might be affected by direct protein-protein interactions or by recycling of sucrose transporters at the plasma membrane. A model is proposed showing cell-to-cell movement of both the SUT1 mRNA as well as the SUT1 protein via the desmotubule connecting companion cells where transcription of sucrose transporters occurs, and the neighbouring sieve elements. We provide an overview over sucrose transporter regulation in Solanum species at the transcriptional, post-transcriptional and post-translational level with emphasis on the many old and new questions surrounding the topic and how they could be answered.