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

Abstract

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.

Interspecific RNA Interference of SHOOT MERISTEMLESS-Like Disrupts Cuscuta pentagona Plant Parasitism

“…However, in that study, the nature of the host parasite interactions was not described and no transgene specific siRNAs were found in the parasite. The transgenic tobacco plants expressed RNAi directed against the dodder STM and KNAT1-3 genes using the vascular-specific SUCROSE-PROTON SYMPORTER2 (SUC2) promoter (Truernit and Sauer, 1995). Dodder growing on plants transformed with this construct showed no downregulation of the KNAT1-3 homolog.…”

“…The siRNAs that mediate RNA silencing have been shown to traffic in the phloem and effect silencing away from the site of transgene expression. The Arabidopsis SUC2 promoter expresses specifically in phloem companion cells and maintains vasculature-specific expression when used to drive reporter genes in a variety of species, including tobacco (Truernit and Sauer, 1995;Roberts et al, 2001;Zhao et al, 2004). In Arabidopsis, the SUC2 promoter has been used to drive an RNAi construct silencing the Arabidopsis homolog of the tobacco SULFUR (SUL) mRNA (Dunoyer et al, 2010).…”

Genetic Control of Carbon Partitioning in Grasses: Roles of Sucrose Transporters and Tie-dyed Loci in Phloem Loading

“…In Arabidopsis, the best characterized group 2 SUT shown to function in phloem loading is AtSUC2. AtSUC2 RNA and protein are expressed in the companion cells of minor veins in a pattern reflective of the sink-to-source transition in leaves (Truernit and Sauer, 1995;Stadler and Sauer, 1996;Wright et al, 2003). AtSUC2 has a biochemical affinity for Suc consistent with a role in phloem loading from the apoplast (Chandran et al, 2003).…”

Characterization of Source- and Sink-Specific Sucrose/H+ Symporters from Carrot

“…Subsequently, these clones were used as probes for the heterologous cloning of Suc transporters from other plant species such as Arabidopsis , Plantago major , and castor bean (Weig and Komor, 1996). Most of these clones were predominantly expressed in the vascular system (Truernit and Sauer, 1995) and seem to be involved in loading Suc into the phloem (Riesmeier et al, 1994;Stadler and Sauer, 1996). Only two cDNA clones were isolated that were preferentially expressed in sink organs: a flowerspecific transporter of P. major (Gahrtz et al, 1996), and a seed-specific transporter of fava beans (Weber et al, 1997).…”