Introns control expression of sucrose transporter LeSUT1 in trichomes, companion cells and in guard cells

In solanaceous plants such as tomato and tobacco, the sucrose transporter SUT1 is crucial for phloem loading. Using GUS as a reporter, the promoter and other regulatory cis elements required for the tomato LeSUT1 expression were analyzed by heterologous expression of translational chimeric constructs in tobacco. Although LeSUT1 is highly expressed at the RNA level, GUS expression under the control of a 1.8 kb LeSUT1 promoter resulted in few plants expressing GUS. In GUS-positive transformants, expression levels were low and limited to leaf phloem. Increasing or decreasing the length of LeSUT1 promoter did not lead to a significant increase in positive transformants or higher expression levels. Translational fusion of GUS to the LeSUT1 C-terminus in a construct containing all exons and introns and the 3'-UTR led to a higher number of positive transformants and many plants with high GUS activity. LeSUT1 expression was detected in ab- and adaxial phloem companion cells, trichomes and guard cells. The role of individual introns in LeSUT1 expression was further analyzed by placing each LeSUT1 intron into the 5'-UTR within the 2.3 kb LeSUT1 promoter construct. Results showed remarkable functions for the three introns for SUT1 expression in trichomes, guard cells and phloem cells. Intron 3 is responsible for expression in trichomes, whereas intron 2 is necessary for expression in companion cells and guard cells. The combination of all introns is required for the full expression pattern in phloem, guard cells and trichomes.