The promoter of the Arabidopsis thaliana SUC2 sucrose-H+ symporter gene directs expression of beta-glucuronidase to the phloem: evidence for phloem loading and unloading by SUC2.

The Arabidopsis thaliana (L.) Heynh. SUC2 gene encodes a plasma-membrane sucrose-H+ symporter. The DNA sequence of the SUC2 promoter has been determined. Using a translational fusion of this promoter to the N-terminus of beta-glucuronidase (GUS) and the GUS histochemical assay, the tissue specificity of the SUC2 promoter was studied in Arabidopsis plants transformed with this fusion construct. The SUC2 promoter directed expression of GUS activity with high specificity to the phloem of all green tissues of Arabidopsis such as rosette leaves, stems, and sepals. During leaf development the expression of SUC2-GUS activity was first seen in the tips of young rosette leaves. In older leaves and during their concomitant sink/source transition, expression proceeded from the tips to the bases of the leaves, indicating that expression of the SUC2 sucrose-H+ symporter is tightly coupled to the source-strength of Arabidopsis leaves. Expression of SUC2-GUS activity was also seen, however, in sink tissues such as roots and developing Arabidopsis pods, suggesting that the product of the SUC2 gene might not only be important for phloem loading, but also for phloem unloading. A possible regulatory effect of carbohydrates (glucose and sucrose) on the activity of the SUC2 promoter was studied and excluded, both in excised leaves and young seedlings of transgenic Arabidopsis plants. The overall pattern of SUC2-GUS expression correlated well with that of the Arabidopsis thaliana AHA3 plasma-membrane H(+)-ATPase which is also expressed in the phloem and most likely represents the primary pump generating the energy for secondary active transporters such as SUC2.