Sucrose-mediated transcriptional regulation of sucrose symporter activity in the phloem

Abstract

A proton-sucrose symporter mediates the key step in carbon export from leaves of most plants. Sucrose transport activity and steady-state mRNA levels of BvSUT1, a sugar beet leaf sucrose symporter, are negatively regulated specifically by sucrose. Results reported here show that BvSUT1 mRNA was localized to companion cells of the leaf's vascular system, which supports its role in the systemic distribution of photoassimilate. Immunoblot analysis showed that decreased transport activity was caused by a reduction in the abundance of symporter protein. RNA gel blot analysis of the leaf symporter revealed that message levels also declined, and nuclear run-on experiments demonstrated that this was the result of decreased transcription. Further analysis showed that symporter protein and message are both degraded rapidly. Taken together, these data show that phloem loading is regulated by means of sucrose-mediated changes in transcription of a phloemspecific sucrose symporter gene in a regulatory system that may play a pivotal role in balancing photosynthetic activity with resource utilization. P lants are photoautotrophic organisms composed of both heterotrophic (sink) and autotrophic (source) tissues. Resources acquired via photosynthesis in source leaves are loaded into the phloem of the plant's elaborate vascular system and distributed among the sink tissues in a process called ''assimilate partitioning''. Because up to 80% of all fixed carbon is exported to sinks (1), regulation of assimilate partitioning is vital to plant growth and development, and it can have dramatic affects on crop yield and productivity (2). Long-distance transport in the phloem cells of the plant vascular system is mediated by a positive hydrostatic pressure difference between the source and sink tissues that drives mass flow of solution. Positive pressure in the leaf phloem results from hyperaccumulation of an osmotically active solute. In most plants this solute is sucrose, which is loaded into the phloem by an electrogenic secondary active proton-sucrose symporter (3-5). Recently, the sucrose transport activity of a sugar beet leaf proton-sucrose symporter (BvSUT1) was found to be negatively regulated by sucrose and not by hexoses or changes in osmotic potential (6), but the mechanism and implications of this regulatory pathway remained unresolved. Here we report that sucrose transport activity in the phloem is regulated via sucrose-mediated changes in transcription of the proton-sucrose symporter gene in a regulatory system that plays a pivotal role in balancing photosynthetic activity with resource utilization. Materials and MethodsTissue Fixation and Embedding. Leaf tissue from mature sugar beets (Beta vulgaris Linnaeus) was fixed on ice for 6 h under vacuum in 4% wt/vol paraformaldehyde in potassium phosphate buffer (pH 7.4). Tissue was dehydrated through a 10% stepgraded ethanol series beginning at 20% and ending at 90% and allowed to incubate overnight at 4°C. After three 100% ethanol washes, the tissue was substituted with tertiary...

Expression of Sucrose Transporter cDNAs Specifically in Companion Cells Enhances Phloem Loading and Long-Distance Transport of Sucrose but Leads to an Inhibition of Growth and the Perception of a Phosphate Limitation

“…A construct in which CoYMVp was fused to AtSUC2 complementary DNA (cDNA) was shown to rescue the Atsuc2-4 -/-knockout mutation (Srivastava et al, 2009b). Both BvSUT1 from sugar beet (Beta vulgaris; Vaughn et al, 2002) and AtSUC2 (Osuna et al, 2007) are suppressed by high Suc levels. We previously suggested that the modified fine-tuning of exotic promoters, such as CoYMVp, compared to AtSUC2p, could benefit plant growth by maintaining high AtSUC2 expression under conditions where it is otherwise repressed (Srivastava et al, 2009b).…”

Linking phloem function to structure: Analysis with a coupled xylem–phloem transport model

“…For Linking powered by eXtyles example strong feedbacks between sugar concentration and sugar loading at the source have been found to exist (e.g. Vaughn et al, 2002). A steady state model approach cannot explicitly incorporate these feedbacks.…”

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

“…Biochemical studies have shown Suc transport activity for numerous dicot as well as monocot SUTs (for recent reviews, see Aoki et al, 2003;Kü hn, 2003;Lalonde et al, 2004;Sauer, 2007). In multiple plants, it has been shown that SUT RNA and/or protein abundance is regulated by Suc (Chiou and Bush, 1998;Aoki et al, 1999;Matsukura et al, 2000;Vaughn et al, 2002;Ransom-Hodgkins et al, 2003), but the genes that control SUT expression have not been identified.…”